Chapter 10 Identifying Hazards

Identifying Hazards and Communicating Risks

After reading this chapter, you will be able to:

Linking these topics to the Medical Council exam objectives, especially sections 78-6 and 78-8.

Thinking outside the box

Julie Richards consults Dr. Rao because she is concerned about her eyes which have been red and irritated ever since she spent a day cleaning out the basement last week. She wonders if it is related to radon gas. She heard radon is a problem in the area and that it accumulates in basements. Dr. Rao is well aware that the environment can cause health problems. For him, the notion of environment includes the natural one, the built or man-made one and the social environment.

Glancing through his records, he notices that this is not the first time someone in the Richards family has experienced such problems. Seven years ago, when Paul injured his neck at work, Dr. Rao asked him about injuries sustained by other people at his work, and also about other people who were off sick. At the time, Dr. Rao made a note that an unusually high number of people who worked in the mine were consulting him for reasons ranging from trivial upper respiratory tract infections to myocardial infarction. A few weeks after Paul was injured the first round of lay-offs at the mine was announced. It was then that Dr. Rao wondered if the work environment might be a problem.

Identifying Environmental Problems

Environmental hazards (see Definition box below: Hazards and risks) cause a wide range of diseases and environmental factors are involved in the aetiology of virtually every illness. Although a high index of suspicion is essential for diagnosis, once diagnosed, environmentally-induced diseases often respond to simple environmental solutions. Moreover, many patients worry about elements of their environment that have little or no effect on health, so physicians need to be able to distinguish those that are hazardous from those that are not. To diagnose an illness as predominantly environmental in origin, physicians have to include a thorough environmental assessment in their history-taking. Clues that an illness is caused by an environmental factor include:

  • The patient suspects it
  • The pattern of illness is atypical; for example, the patient is not in the usual age group, the usual risk factors are absent, the course of the illness is unusual, the symptoms do not respond to the usual treatments
  • The temporal pattern of the illness suggests it (for example, symptoms improve when the patient goes on holiday and worsen on returning home, or they worsen when the patient is at work and improve at home)
  • There is no obvious other cause for the illness
  • The signs and symptoms suggest specific toxins, such as lead or mercury poisoning.

Hazards and risks

Hazard is the inherent capability of an agent or a situation to have an adverse effect. A hazard is a factor or exposure that may adversely affect health: the thing that may do damage.

Risk is the probability that a health effect (damage) will occur.

Threat refers to how severe will be the damage that may occur.

Health risk = hazard x exposure x susceptibility.

For there to be a risk, a susceptible individual or population (the receiver) must be exposed (via a path) to a hazard (the source–see Figure 10.1). If there is no exposure, there is no risk. If there is exposure, but no susceptibility, there is no risk. The same idea is used in fire prevention: use a non-flammable material (i.e., reduce susceptibility) or reduce its exposure to heat (block the path), or remove the source of heat.

Once an environmental cause is suspected, the physician should take a detailed environmental history to identify all hazards the patient may have been exposed to, as shown in the box “Taking an environmental history”. All of the patient’s activities and environments should be explored. The chronology of events, patient’s proximity to the presumed source, and whether or not other people are affected should be explored for evidence that supports or discounts the hypothesis of an environmental cause (i.e., establishing Time, Place, and Person).

Once the physician has established that the problem is likely related to the environment, he should ensure that other people exposed are protected. He can call the public health service for advice on how to manage health problems linked to the environment. Indeed, some provinces require physicians to notify the public health department of particular environmental illnesses or possible outbreaks. In the case of a hazard that is likely to affect the population at large, the public health service is responsible for controlling the problem. However, even when the problem is community wide, the public health service may only be able to educate and inform. At times, public health services advise on the safety of projects proposed by industry or governments.

The patient’s work health and safety board (see Chapter 12, Occupational health service) is responsible in the case of a work hazard. In an emergency, as with a case of carbon monoxide poisoning, public security services (fire and police) would coordinate control efforts.

Taking an environmental history1

Ask about the following topics (CH2OPD2 is a useful mnemonic):

  • Community or neighbourhood sources of hazard: pollution; industry; waste storage
  • Home: year of construction, renovations; materials used in construction and decoration; moulds; garden and house plants; use of cleaning products, pesticides, herbicides
  • Hobbies and leisure: exposure to chemicals, heavy metals, dusts, or micro-organisms
  • Occupation: current and previous occupations; work with known hazards; air quality
  • Personal habits: hygiene products; smoking
  • Diet: sources of food and water; cooking methods; food fads
  • Drugs: prescription, non-prescription, and alternative medications; health practices.

If a scanning question reveals a possible hazard, ask detailed questions to find out as much as possible about the nature and level of the hazard and then check Time, Place and Person:

  • Time: When did symptoms begin? When did the exposure begin? When do symptoms get worse? When do they improve?
  • Place: Where is the patient when symptoms get worse? Where is the likely hazard? What is the channel through which the hazard reaches the patient?
  • Person: Does anyone else have similar symptoms? Who? When? Where?

More than one piece in the puzzle

Ask questions about all the environments that the patient inhabits—even if the patient suspects one in particular or if your first few scanning questions point to a specific environment. If you stop enquiring as soon as you find your first hazard, you might miss others to which the patient is exposed. The real cause of the patient’s problems may not be the first your history uncovers.

Julie’s eye trouble

Dr. Rao asks Julie a bit more about her activities around the time her eye problems began. She says she was cleaning out the basement, which was very dusty. She also used a cleaning product that she hadn’t used before and because it was cold outside she didn’t open the window. The day before cleaning the basement she had gone for a walk; it had been a windy day and some dust had blown into her eyes.

Right now her eyes are normal. Dr. Rao advises Julie to make sure that there is adequate ventilation when using a cleaning product. He also suggests that she have a look at the Canada Mortgage and Housing website for advice on home maintenance. As this is not the first time that a patient has asked about radon gas, Dr. Rao asks his practice manager to contact the local public health department for information on radon that he could pass on to his patients.

Reducing Risk

The three main activities in addressing health problems possibly linked to environmental hazards are risk assessment, risk management, and risk communication. The clinician addresses these when discussing environmental disease with a patient; a public health officer applies them in responding to community wide problems.

Risk assessment

Risk assessment involves a series of steps in evaluating the likelihood of occurrence and probable severity of health effects due to a hazard. Various agencies assess hazards, including occupational health agencies, environmental protection agencies, and public health authorities. Clinicians confronted with a case of disease with possible environmental links might use some of the following risk assessment steps to arrive at a diagnosis.

Steps in risk assessment for clinicians2

  1. Hazard identification: Are environmental hazards involved? What are they? How likely are people to be exposed and what will the effects be?
  2. Exposure assessment: Is the patient’s exposure to the hazard sufficient to cause these symptoms? Which population groups are likely to be at most risk?
  3. Risk characterization: Given the patient’s level of exposure, is the hazard likely to cause these types of symptoms in this type of patient? What are the dose response curves for different routes of exposure (inhalation, ingestion, etc.)?
  4. Risk estimation: How much has the hazard contributed to the patient’s condition? What is the risk from this hazard for this specific population?

Risk at a population level

In public health practice, risk assessment may start before any exposure happens. The four steps of public health risk assessment are as follows:

  1. Hazard identification: Is this a possible cause of harm? How? How likely are people to be exposed to the hazard and what will the effects be?
  2. Risk characterization: What are the dose response curves in different routes of exposures (inhalation, ingestion etc.)?
  3. Exposure assessment: How likely are different sectors of the population to be exposed to the hazard, at what level of exposure? Which population groups are likely to be at most risk from it (age, sex, genetic endowment, behaviours etc.)?
  4. Risk estimation: What is the risk of this hazard in this physical state to this specific population that is exposed in this manner?

Risk assessment, step 1: Hazard identification

The first step identifies the agents possibly responsible for the problem. Each hazard is defined by its ability to cause adverse effect, under what conditions and to which population groups. Hazards are classified as biological, chemical, physical, ergonomic, psychosocial, or related to safety. Table 10.1 gives examples of the various types of hazard and their possible health effects.

Table 10.1: Examples of the various types of environmental agents and their associated health effects

Type of hazard Examples Health effects
Biological Bacteria, viruses Specific syndromes associated with different agents e.g., salmonella food poisoning, hepatitis A, infection with Methicillin resistant staphylococcus (MRSA)
  Moulds Allergies, cancers
  Animals Allergies, zoonoses
Chemical Heavy metals (the risk may depend on the physical state) Specific syndromes e.g., lead poisoning, mercury poisoning
  Benzene Acute myeloid leukemia with prolonged exposure
  Carbon monoxide Asphyxiation
  Asbestos Asbestosis, carcinoma of the lung, mesothelioma
Physical Noise Hearing loss
  Radiation DNA damage leading to cancers
  Ultraviolet light Skin damage, vision loss
  Temperature extremes Hypo- or hyperthermia, frost bite
  Kinetic energy Falls and collisions leading to bone and soft tissue injury
Ergonomic Poorly designed work station Back pain
  Physically repetitive activity Repetitive strain injuries
Psychosocial Job stress Non-specific physical and psychological manifestations
  Poor social support Psychological problems
Difficulty in coping

Hazards can be found almost everywhere in the environment, including the air, water, soil, and in food. Hazards may affect health directly, or indirectly when they produce a change that puts a person or population at risk, for instance climate change. Table 10.2 lists some of the common hazards.

Table 10.2: Examples of hazards in the environment

Hazards in: Examples
Air Carbon monoxide
Particulate matter
Water Fecal contamination
E. Coli
Blue-green algae
Soil Heavy metals
Petroleum by-products
Food Listeria
Mercury in fish

Indoor hazards that cause health problems include household chemical products, carbon monoxide, radon, mould, lead, and consumer products (cosmetics, perfumes, hygiene products). Second hand smoke is still a problem in some households.

The clinical picture may indicate what kind of hazard to search for. Airborne irritants can cause itchy or sore eyes, runny nose or coughing. Irritants that come into direct contact with the skin can cause dermatitis. Allergens can cause numerous manifestations of allergy including dermatitis, asthma, sneezing. Asphyxiants cause different respiratory problems depending on the type of asphyxiant. For instance carbon monoxide blocks oxygen transport. Other gases can accumulate in high concentrations displacing oxygen in the air. Certain ingested or absorbed substances may damage organ systems in pathognomonic ways. For instance, severe lead poisoning causes neurological changes, abdominal pain, or anaemia, whereas poisoning with mercury typically produces tremors, among other neurological symptoms. Mesothelioma is linked to exposure to asbestos.

The effects of a hazard may be delayed, sometimes for years. This is especially true of cancer-causing agents. Identifying this type of hazard and evaluating claims about it can be difficult. Furthermore people can be exposed to more than one hazard, each contributing to their health problem. For instance, in the case of a person who works in construction and who smokes, it can be difficult to assess the relative contributions of his occupation and his smoking to his chronic lung disease.

If an outbreak of a disease of environmental origin is suspected, public health officials will collect initial information on the possible sources. Once a case definition (see CASE DEFINITION in Glossary) is established, cases are sought and information is collected on when and where they were exposed, when they started showing effects, and demographic characteristics and predisposing factors (Time, Place and Person). Information from all cases is then collated to produce a picture of the distribution of cases in time and place (see Chapter 11: Detection and control of outbreaks).

Risk assessment, step 2: Risk characterization

This step describes the potential health effects of a hazard and answers the clinical question of whether the identified hazard could have caused this patient’s symptoms. As far as possible, effects on molecular, biochemical, cellular and organ systems are described. A chemical hazard may only cause health problems when in a specific form: ingested elemental liquid mercury is virtually non-toxic, however mercury vapour is toxic, as are all mercury organic and inorganic compounds. The route of entry into the body may also be an important determinant of the damage caused: there is no convincing evidence that ingested asbestos is toxic, whereas inhaled asbestos causes asbestosis, mesothelioma and other cancers.3  Furthermore, a person’s response to a hazard is mediated by the factors that influence the hazard’s toxicokinetics and toxicodynamics (See Definitions: Kinetics and dynamics). The individual’s genetic makeup and metabolic state as well as environmental factors moderate the toxicokinetics and toxicodynamics, so susceptibility to damage by a hazard varies from one person to another; malaria offers an example.

Differences in susceptibility

Malaria is an example of a hazard from which the risk is modified by the genetic makeup of at-risk people. The sickle cell trait confers some resistance to malaria, and this could explain why the trait has persisted in African populations. One study showed that, compared to children without the trait, those with the sickle cell trait had a relative risk of all cause mortality of 0.45 (95% CI 0.24–0.84) from the ages of 2 to 16 months.4 This is the peak age for severe malaria. There was no difference in mortality before the age of 2 months, probably because of maternal immunity, or after the age of 16 months, probably because those who survive to 16 months develop some immunity from repeated exposures to small infectious loads.

Alcohol is not fat soluble. If two people of the same weight take the same amount of alcohol, the one with more body fat will have a higher blood level of alcohol because the alcohol that person took is dissolved in proportionally less water. This is one of the reasons that women, who generally have more body fat than men, tend to achieve higher blood alcohol levels for a given amount of alcohol. The other reason, of course, is that women tend to be smaller overall than men.

Kinetics and dynamics5

Toxicokinetics = the activity or fate of toxins in the body over a period of time, including the processes of absorption, distribution, localization in tissues, biotransformation, and excretion.

Kinetics comes from the Greek word that means movement. Cinema (moving pictures) and kinesiology are derived from the same word.

Toxicodynamics = the study of the biochemical and physiological effects of toxins and the mechanisms of their actions, including the correlation of actions and effect of toxins with their chemical structure. It includes the effects of a toxin on the actions of other toxins.

Dynamics comes from the Greek word meaning force or power.

Toxicokinetics and toxicodynamics are analogous to pharmacokinetics and pharmacodynamics.

A low dose of some substances may be beneficial for a person, yet a higher dose is toxic. For instance, fat soluble vitamins, such as A and D, are essential for health, but too much of either is toxic. Sunlight in small doses increases the production of Vitamin D, while in high doses it can cause skin cancer. Some hazards, such as heat and noise, must reach a threshold level before damage occurs. Others, including many cancer-causing agents, are presumed to cause damage even at the lowest measurable levels. The effects of some agents, such as X-rays, are cumulative over a lifetime, while other agents, such as alcohol, allow the body to recuperate somewhat during temporary breaks in exposure.


Hormesis refers to a biphasic dose response to an environmental agent characterized by stimulation or a beneficial effect at moderate doses and an inhibitory or toxic effect at high doses.

Toxicokinetics of stress

Toxicokinetics and the varying dose-impact of different toxins have a parallel in the relationship between psychological stress and resulting strain or distress. Personality mediates the impact of psychological stress on the resulting strain, producing characteristic stress-strain responses. In toxicology, different substances have differing exposure-damage curves somewhat reminiscent of the stress-strain curves described by Young’s modulus

For information on hazardous substances, clinicians can look to the scientific literature, check with the local public health department or call the toxicology or poison centre. If the probable source of the hazard is an industrial product, information is likely to be available via the Workplace Hazardous Materials Information System (WHMIS), which sets out the labelling requirements for hazards, see Table 10.3. The product label also indicates if a ‘Material Safety Data Sheet’ is available for the product. This sheet contains further details about the hazard, how to handle it safely and what to do in an emergency.

Table 10.3: The Workplace Hazardous Materials Information System Symbols 6

Class A - Compressed Gas Class A – Compressed Gas Contents under pressure. Cylinder may explode or burst when heated, dropped or damaged.
Class B - Flammable and Combustible Material Class B – Flammable and Combustible Material May catch fire when exposed to heat, spark or flame. May burst into flames.
Class C - Oxidizing Material Class C – Oxidizing Material May cause fire or explosion when in contact with wood, fuels or other combustible material.
Class D, Poisonous and Infectious Material, Division 1: Immediate and serious toxic effects Class D, Poisonous and Infectious Material, Division 1: Immediate and serious toxic effects Poisonous substance. A single exposure may be fatal or cause serious or permanent damage to health.
Class D, Poisonous and Infectious Material, Division 2: Other toxic effects Class D, Poisonous and Infectious Material, Division 2: Other toxic effects Poisonous substance. May cause irritation. Repeated exposure may cause cancer, birth defects, or other permanent damage.
Class D, Poisonous and Infectious Material, Division 3: Biohazardous infectious materials Class D, Poisonous and Infectious Material, Division 3: Biohazardous infectious materials May cause disease or serious illness. Drastic exposures may result in death.
Class E - Corrosive Material Class E – Corrosive Material Can cause burns to eyes, skin or respiratory system.
Class F - Dangerously Reactive Material Class F – Dangerously Reactive Material May react violently, causing explosion, fire or release of toxic gases, when exposed to light, heat, vibration or extreme temperatures.


Risk assessment, step 3: Exposure assessment

Exposure assessment is the step that quantifies the exposure of a person or population to a hazard. The levels of some hazards can be directly measured, either in the environment or in the people exposed. More often, however, exposure must be estimated from a careful history of the patient’s activities, as well as an inspection of the environment (see box on thalidomide). Details of activities, working practices, and processes during which a person is likely to be exposed must be examined to determine how and how much the person was exposed to the hazard.

When the problem may have been caused by an agent that produces delayed effects, such as a carcinogen, the exposure history should trace exposures back twenty years or more. Occupational disease can occur in retired people; in workers, it can be caused by previous employment.

Thalidomide and phocomelia7

Thalidomide was developed as a sedative and anticonvulsant drug. As a sedative, it was remarkable because it was almost impossible to die from an overdose; in fact, no LD50 could be established. It was first marketed as a “harmless” sedative in Germany in 1957, and it became the drug of choice for many conditions, including morning sickness in early pregnancy. At the time, drugs were thought not to cross the placenta and, therefore, not to affect the foetus. However, by 1960, geneticists and paediatricians began to see children with unusual limb abnormalities that are now known as phocomelia (seal flippers). By 1961, it was noted that the drug could cause peripheral neuritis in people taking it. There were also reports of phocomelia in babies whose mothers had taken thalidomide in early pregnancy and scientists were beginning to suspect that thalidomide was causing the abnormalities. By the end of 1961, thalidomide had been taken off the market in the UK. In Canada, the drug continued to be sold until March 1962, although physicians were warned not to use it in pregnant women.

Recently, thalidomide has gained a new lease on life. In 2005, it was found effective in treating weight loss and cachexia associated with various cancers, as well as in slowing the growth of myeloma cells.


Risk assessment, Step 4: Risk estimation

Risk estimation quantifies the likelihood that a hazard will affect a specific person or population; it also estimates the size or severity of the effect. In this step, the information obtained during the previous steps is summarized and collated. The epidemiological triad (see Chapter 2, Figure 2.8) can be used to assemble the information on what the hazard is, where it comes from, how the environment allows it to come into contact with the host, and the host’s susceptibility to the hazard in order to arrive at a conclusion on who is at what level of risk. Table 10.4 illustrates what needs to be considered in this step.

Table 10.4: Examples of various host, agent, and environmental risk factors for selected categories of health problems

Problem type Health problem Host Agent Environment
Infectious disease Hepatitis C virus infection among people who inject drugs Co infection with HIV RNA virus (Flaviviridae family) Lack of sterile drug preparation and injection equipment (e.g., syringes)
Environmental health problem Asthma Genetic susceptibility Allergen Carpets; pets in household; inefficient ventilation
Occupational health problem Back injuries in manufacturing facilities Posture Mechanical forces Lack of equipment necessitates human lifting.

The risks for an individual and for a population should be balanced against the costs and risks of intervening. In individuals it might be a choice between the risk of asthma and the loss of a beloved pet or, if the risk is work-related, it could be a choice between staying healthy or staying employed. In populations, the options can affect a number of sectors and lead to political discussion. For instance, a population may be at risk from pollutants emanating from a factory. It might, however, be at greater risk from the socioeconomic fall-out from closing the factory. Once the levels of risks due to the different hazards are documented, possibilities for risk reduction can be explored.

Risk and the precautionary principle8

The four steps of risk assessment described in the text fall into a quantitative risk assessment paradigm. They can be used to assess probable risks and benefits, allowing people to choose the option with least risk. However, the paradigm has been criticised for not taking into account the complexities and uncertainties of risks and risk assessment. The precautionary principle expresses the “better be safe than sorry” approach. It is generally used in situations where there is risk of severe, immediate, and irreversible damage to people or to the environment. A criticism of this approach is that it tends to be based on the assessment of only one option and may ignore the risks and benefits of the others.


Occupational Hazards

Most people spend a considerable part of their life at work, and if hazards are present the workers’ exposure to them is likely to be prolonged. In some occupations, specific hazardous agents must be used, putting workers at risk of occupational diseases associated with them. Physicians should ask patients four simple questions: “What sort of work do you do?” “What sorts of work have you done?” “Do you think your work may be a cause of your problem?” And “What job are you going back to?”

Occupational injuries and diseases

The vocabulary of work-related health problems is governed by provincial legislation on work health and safety and workers’ compensation. As a result, the precise definitions of key words may vary. However, the underlying concepts remain the same.
An occupational injury must arise out of employment and in the course of employment, and includes one of the following

  • a willful and intentional act, not being the act of the worker who suffers the accident
  • a chance event or incident occasioned by a physical or natural cause
  • a disablement caused by an occupational disease
  • a disablement or disabling condition caused by employment.

An occupational injury may be due to an occupational disease or a work accident (even though the word accident is used in legislation, according to many experts in the prevention of work injury it is to be eschewed; although most accidents are preventable, accident includes the notion of unpredictability).

Injury is used in its broad sense to mean any kind of harm occurring to the worker. It can also refer to aggravation of pre-existing condition.
Occupational disease: a disease peculiar to or characteristic of an industrial process, trade, or occupation, or a disease that arises out of and in the course of employment.

The Mad Hatter

The Mad Hatter in Alice in Wonderland illustrates mercury poisoning as an occupational disease. At the time that the book was written, mercury was used in the process of felting, one of the steps in making a hat our of wool. The Hatter exhibits the neurological effects of mercury exposure: irritability, excitability and anxiety.

Common, non-specific work-related diseases include dermatitis, asthma, and musculoskeletal disorders. It is also accepted that mental health can be influenced by stressful conditions at work.

Table 10.5: Some examples of occupational disease

Condition Agent Example of occupations at risk
Berylliosis Beryllium Aerospace industry
Byssinosis Cotton dust (numerous agents) Cotton industry
Farmer’s lung Mould in hay Farming
Asbestosis, Mesothelioma Asbestos Demolition work; ship-building
Hepatitis A Hepatitis A virus Sewer workers
Silicosis Silica dust Stone workers
Lymphoma Pentachlorophenol
Silica dust
Lumber yard workers
Foundry workers; stone cutters
Foundry workers; demolition

Injuries do not occur randomly; groups at particular risk of occupational injury include:

    • Young workers lacking in work experience or safety training—particularly those in temporary or summer jobs;
    • Workers obliged to work long hours at a fast pace. This could include piece-workers or those working on production lines;
    • Workers given work responsibility without the authority or control necessary to meet that responsibility; and
    • Workers in certain high-risk industries such as construction and forestry.

According to Canadian federal and provincial laws, specific services must be provided for the protection of workers (see Chapter 12, Occupational Health Services) and for compensating those who are harmed as a result of work.

Risk Management

The source-path-receiver model, derived from energy engineering, is used to propose ways of controlling risk. The source is the equipment or process that is directly responsible for a hazard. The hazard could be a form of energy, such as acoustic, thermal, etc., or it could be a substance, such as toxic fumes or dusts. The path is the conveying medium (air, water, etc.), while the receiver is the human being, the worker. In contrast to the epidemiological model, which allows for bidirectional influences, the source-path-receiver model is unidirectional.

Figure 10.1: Source-path-receiver: a model to identify ways of reducing environmental risks
Figure 10.1: Source-path-receiver: a model to identify ways of reducing environmental risks

As complete elimination of risk is generally impossible, risk management aims to reduce the risk from the hazard without causing another problem. Strategies to reduce risk can target the source of the hazard, the path or the receiver, whether the risk involves a person or a population. Strategies can be educational or advisory, technical or engineered, economic or regulatory. As illustrated in Figure 10.1, strategies aimed at the source can modify or substitute the hazard (for example, replacing asbestos with other materials), or enclose it if it cannot be removed. Legislative strategies can be used to reduce the emission of hazardous pollutants.

Modifications to the path include environmental changes; for example, closing windows to prevent smog from entering the home; using bed nets to prevent transmission of malaria, or a film of environmentally safe soap or oil on standing water to drown mosquito larvae.

Finally, strategies can alter individual and population susceptibility to the risks of hazards. Examples include immunization against infectious disease, ensuring the nutritional status of disadvantaged children, and counselling for behaviour change. Personal safety equipment can reduce exposure. For example, power line workers use special tents in winter to reduce exposure to cold, and airport apron workers use ear protectors to prevent noise damage. Doctors may also promote other safety measures: for instance safety harnesses, helmets and steel-capped boots for construction workers and miners.

Nurse Jennings interviews Mr. White

Shortly after Nurse Jennings counselled David on his health behaviours in Chapter 8, she happened to meet Mr. White, a contractor for whom David occasionally works. She began by asking Mr. White what he thought of his workers’ safety habits and if there had been many injuries on his work sites….

A First Nations environmental hazard

In October 2005, 450 people from the Kashechewan First Nation were evacuated from their community because of problems with their drinking water. The community had been under a “boil water” advisory for two years. For five years there had been continuing problems with the supply; there was persistent E. Coli contamination and the chlorine used to reduce this was aggravating skin problems. The Ontario Ministry of the Environment found several problems with the water quality monitoring and with the treatment of drinking water. There were also a number of problems with the sewage system, including the fact that the sewage system outlet was upstream of the drinking water intake, which increased the risk of drinking water contamination.9

This incident prompted calls for improved standards for the drinking water supply in First Nations communities. It was noted that the regulatory framework in First Nations communities did not ensure that water quality was as high as that in other Canadian communities. By 2008, no new legislation on water quality had been developed.10  In 2016, concerns over water quality still remained (see

Haddon’s matrix and injury prevention

Dr. William Haddon combined the epidemiological triad (see Chapter 2, Figure 2.8) with a time dimension to create Haddon’s matrix, which sets out the factors that determine the level of risk associated with a hazard and the severity of its effects, in order to identify modifiable risk factors for injury.11 It is commonly applied to the analysis of road traffic collisions, as illustrated in Table 10.6. In this example the host is the person injured (driver or other), while the agent is the equipment that determines how much energy (mechanical or thermal) is transmitted to the host. The environment refers to the physical and social environments in which the injury occurs. The time axis distinguishes factors that operate before the injury (e.g., recent snow had made the road slippery) from those that operate at the moment of injury (e.g., the driver was wearing a seat belt). Other factors operate following the collision (e.g., bystanders knew CPR).

Table 10.6: Illustration of Haddon’s matrix applied to a motor vehicle collision

Host Agent Environment
Physical Social
Pre-event Driver in a hurry Car recently serviced Road design Enforcement of speed limits
Event Wearing seat belt Air bags working Icy patch Number of people standing nearby who may get hurt
Post-event Has cell phone to call for help Tendency for car to catch on fire Emergency vehicle access Level of assistance provided by bystanders

The matrix is now being used in other situations, including the assessment of medical errors. An advantage of the matrix is that it gives the full picture of the problem instead of focussing blame on individuals; it leads to a more constructive consideration of ways to reduce this type of error. Table 10.7 illustrates a situation in which potassium was used instead of saline for dissolving a drug that was then administered intravenously, causing severe cardiac arrhythmia and leading to the collapse of the patient.

Table 10.7: Illustration of Haddon’s matrix applied to an incident of erroneous intravenous administration of potassium

Agent Environment
Physical Social
Pre-event Unwell, co-morbidity Inexperienced nurse;
medications packaged in similar containers
Only one drawer in which to store both potassium and saline Rushed doctor preoccupied by another patient;
senior nurse on break
Event Too sick to enquire what medication he is being given Nurse hurriedly mixes medication;
monitors patient’s pulse while injecting
Night time;
ward lighting low, patient’s bed lamp not working.
Nurse being called to other patients;
must hurry
Post-event Immediate signs of arrhythmia Calls cardiac team Cardiac trolley was left at the far end of the corridor after an earlier call Cardiac team responds quickly

 As a way to enhance patient safety, completing the matrix identifies hazards as well as protective factors. The protective factors, such as nurses monitoring patients while they administer injections, calling promptly for help, and quick response of the cardiac team should be reinforced, while the risk factors—storage and packaging of solutions, poor lighting, inadequate supervision, and harassment by other team members—can be rectified. (To prevent this type of mistake, most hospitals do not store potassium on the ward.)

Active versus passive interventions

Interventions to reduce risks are categorised according to the participation required of the person at risk. Active interventions rely on continued compliance for their effectiveness, whereas passive interventions are applied once only, or else their effects last for some time. In general, passive interventions are more effective because people do not have to remember to use them. For instance, administration of influenza vaccine (passive) confers immunity for the whole influenza season, whereas hand-washing (active) must be repeated several times a day. An airbag (passive), once installed in a car, stays there until it is needed, whereas seat belts (active) must be put on every time. Even in well-managed workplaces, people’s compliance with the use of personal safety equipment can be poor. For example, ear protectors can be hot and uncomfortable, and may mask auditory danger signals, so that reducing environmental noise levels is preferable, where possible. Safety harnesses can impede mobility, but in some situations they can be replaced by guard rails or safety cages.

Risk Communication

Once a risk has been identified and quantified and methods of reducing it have been found, the information has to be communicated to people at risk to allow them to understand their risk and take steps to mitigate it. Communication is the exchange of information; good communication is the exchange of information in a way that the recipient understands what the sender intends (Figure 10.2). A model of communication derived from communication technology offers a useful basis for thinking about interpersonal communication. This model distinguishes six elements in the communication process: the message, the messenger, the encoding, the channel, the decoding, and the recipient.

Figure 10.2: The communication process showing the relationships between the elements of communication
Figure 10.2: The communication process showing the relationships between the elements of communication

The message

How risk is perceived depends on factors beyond the simple statistical level of risk; perception varies according to the nature of the hazard (the exposure) and according to its possible effects (the outcome). Factors that increase the perception of danger are listed in Table 10.8.

Table 10.8: Factors that increase public perception of danger12, 13
Exposure characteristics
  • Involuntary
  • Not under personal control
  • Unnatural (e.g., terrorist attack)
  • Unfamiliar
  • Mismatch between risk and benefit (people who suffer the consequences experience no gain from the activity)
  • No trust in institutions involved
  • Media attention
Outcome characteristics
  • Catastrophic (instead of chronic)
  • Affects children or future generations
  • Unknown or uncertain outcome
  • Affects identifiable people, not statistics
  • Dreaded outcomes (e.g., cancer)
  • Immediate (vs. delayed)
  • Irreversible
  • Media attention

These characteristics can transform a minor statistical risk into a major perceived risk. For example, in the U.S., the risk of death per mile travelled in a car is about 10 times the risk of death per mile travelled in a commercial airplane, but people are appalled when they hear about an airplane crash. News of a car crash has much less effect. From Table 10.8, the difference in reaction is due to the unfamiliarity of the event and the fact that it is beyond the control of the traveller. Furthermore, the death toll from an airline crash is generally in the hundreds and so they are seen as catastrophic, receiving a great deal of media attention. By contrast, road traffic fatalities are familiar, under the control of road users, and each only affects a few people.

Table 10.8 also offers some insight into why some people may worry about environmental risks and yet refuse to change their own high-risk lifestyles. For example, smokers can be anxious about small environmental risks that they cannot control, while continuing to ignore the great risk of smoking.

Finally, the message may be obscured by uncertainty. Clinicians should be as precise as possible when giving information:

  • Words can be ambiguous and their meaning can vary: terms such as high or low risk are unclear. Clinicians should make certain that their patients understand the precise meaning of the possible outcome and the course of action suggested.
  • If numbers are used instead of words, be aware of the biases inherent in a positive versus a negative framing of statistics (see the next section).
  • The definition of the risk may lack specificity. The risk should always include a time frame. A 10% chance of death from a lung cancer over a lifetime is not the same as a 10% chance in the next five years. A 20% chance of loss of function could be interpreted as a 20% reduction in function, or that 20% of people will have complete loss of function, or that 20% of people will have some loss of function.

Framing the message

In relation to communication and decision-making, framing refers to the way in which information on risk is presented. Crucially, large changes in preferences can be caused by minor variations in the words used in presenting a choice.14

Emphasis on gain versus emphasis on loss

People tend to be averse to loss and will do more to avert a perceived loss than they will to achieve a gain. See Nerd’s corners: Treatment A or B and Glass half full.

The default option

The option that is presented as the usual choice is more likely to be chosen than one presented as the alternative.

Numbers versus proportions

Data expressed as proportions tend to be seen as relatively benign, whereas data expressed as a frequency tend to engage people much more.

Framing the information

Treatment A or B?
A well-known psychology experiment contrasts the framing of two options for tackling an imaginary disease. Six hundred people are affected by a fatal illness, and we must choose between two forms of treatment:14

  • Treatment A will save 200 people.
  • Treatment B has a one-third possibility of saving all 600 people, and a two-thirds probability of saving no one.

Purely mathematically, the options are equivalent—saving 200 versus a one-third chance of saving 600. But most people opt for Treatment A because the certainty of saving 200 lives overrides the risk of losing 600. However, things change when Treatment A is expressed as a loss:

  • Treatment A will allow 400 people to die.
  • Treatment B has a one-third possibility of saving everyone and a two-thirds probability that all 600 will die.

Most people now opt for Treatment B because the one-third possibility of saving everyone is more attractive than the certainty of losing 400: loss aversion affects the choice.
The way a choice is formulated is called the ‘frame’ and, to ensure informed choice, a clinician should communicate the information using a number of alternative frames.

Glass half full or glass half empty
Emily and Ian are psychology students. In the last exam, both answered all the questions: Emily got 74% correct while Ian got 26% wrong. Which is the better student? It is usually found that positive framing leads to positive feelings and negative framing leads to negative ones, so Emily is usually judged to be better than Ian. In discussing the risk of Alzheimer’s disease, dwelling on the 8% of people over sixty-five years old who have it makes the situation seem worse than focusing on the 92% of people who don’t.

What is the risk?
In one experiment, the case of a mentally disturbed patient was presented to physicians. They were told that 20 out of 100 patients similar to this one were likely to commit an act of violence. They were then asked if they would discharge the patient, and 41% said they would not. A similar group of physicians was presented with the same scenario, but told that the patient had a 20% chance of committing an act of violence: only 21% of these physician refused discharge.15

The messenger

People respond more to the attitude of the messenger than to his or her status as a professional or authority. People tend to disregard information given by a recognized expert if he shows a lack of caring or empathy (Figure 10.3). Therefore, to get a message across to a patient, a clinician needs to show a caring attitude rather than trying to impress with science.

Figure 10.3: Personal qualities of the messenger and their relative effect on how the message is received
Figure 10.3: Personal qualities of the messenger and their relative effect on how the message is received

The recipient

The recipient is an active participant in communication. The recipient’s prior knowledge, beliefs, attitudes and experience affect his understanding of the message (see Nerd’s corner People and their perceptions). When communicating with patients, a clinician needs to assess the patient’s state and adapt the message accordingly.

People and their perceptions

General disposition: Optimistic people tend to feel at low risk. Pessimistic people, and those who are anxious or depressed, tend to overestimate risk. Nonetheless, defence mechanisms that reduce feelings of threat may lead them to deny the risk entirely.

Affective forecasting: People tend to be unrealistically pessimistic about how they will cope with situations they have not experienced. A person who, while still healthy, declares that she would prefer not to be resuscitated if it seems likely that survival might result in serious disability, may have a change of attitude once the disability occurs when she realizes that she is better able to cope than she thought; this is also called response shift.

Perception of threat: Most people feel that they have a lower-than-average chance of getting a severe illness. This perception is more pronounced when the health problem is seen as controllable, is likely to occur in the distant future, and occurs in a type of person that the patient considers different from himself.

Confirmatory bias: People are more inclined to retain information that supports their prior beliefs than dissonant information. Even the most objective researcher has a tendency to focus on information that supports his hypothesis. A patient who believes his risk of cancer is low may minimise the significance of cancer symptoms. Clinicians are inclined to retain their preliminary diagnoses, even in the presence of contrary evidence.

Reduction of vulnerability: The need to feel invulnerable can make people deny or forget information about personal risk. When told of the risk, people may counter by questioning the validity or reliability the information. A patient receiving bad news is more likely to request a second opinion than a patient receiving good news. People also tend to find contrary examples to corroborate their denial, so smokers remember their grandfather who smoked twenty cigarettes a day and died in full health at the age of eighty-five.

The channel

The usual channel for clinical communication is the spoken word: clinician and patient talk to each other. However, words can be supplemented with visual aids, such as posters, leaflets, and, occasionally, videos. The recipient must have access to the channel of communication: written materials may not reach people who have difficulty reading.

Encoding and decoding

Information must be coded before it can be passed on. For communication to be successful, the messenger and the recipient must share a common understanding of the code. Even though a clinician speaks the same language as the patient, differences in socio-economic milieu, education and experience can limit their shared understanding of terms. When a physician discusses an appendectomy with a patient, the physician is talking about a routine procedure that the patient is likely to recover from in a few days. The patient, on the other hand, is hearing about an alarming and unique experience that is likely to be painful, will leave a permanent scar, and will disrupt life for, at the very least, several days.

When communicating with their patients, clinicians should:

  • Use words and concepts that their patients can understand;
  • Remember that clinicians are familiar with medical conditions and procedures, whereas their patients are not and that the patient’s perception is clouded by apprehension;
  • Modify their tone and body language to conform to the common code, bearing in mind that gestures may mean different things in different cultures. In North America, a Greek who shakes his head may be asking a question, not saying “no,” while an Italian gesturing for someone to come closer may appear to be waving goodbye. Some people feel that it is polite to look an interlocutor in the eye, others find it threatening.

Risk information is often presented as numbers or graphs. Most people can grasp the information in these forms, but there are a number of points to remember when using numbers to communicate information:

  • People don’t come in halves
    People are more at ease with whole numbers, so fractions and decimal places should be avoided. Few estimates of risk or of therapeutic benefit are so precise that they merit decimals.
  • Numerator and denominator
    People tend to focus on the numerator and ignore the denominator. A disease that afflicts ten people in a hundred tends to be seen as less common than one that afflicts a hundred people in a thousand. When discussing a single risk with a patient it can be helpful to express it in several ways: 10%, or one in ten, or ten in a hundred. However, when asking people to compare risks of different outcomes, one should keep the denominator constant. It is difficult for most people to understand the difference between one in five risk of one outcome and a 25% risk of another outcome. They will find it easier if a 20% risk is compared to a 25% one or if a one in five risk is compared with a one in four risk.
  • Relative and absolute risk
    Relative risks may be particularly difficult to interpret because people rarely know the context. Although hormone replacement therapy doubles the risk of breast cancer, it causes only eight additional cases of breast cancer in 10,000 woman-years. For an individual woman, doubling the risk does not greatly increase her absolute risk because the baseline risk is so small, whereas, on a population level, a doubling of the risk may be significant. It is advisable to use only absolute risks when communicating with individuals, because proportional changes may often obscure a lack of substantive importance.16 (See Chapter 5 for a discussion of relative and absolute risk).

When discussing risk with a patient, the information should be communicated in different ways, using both positive and negative framing, to help the patient arrive at a fully informed decision.

An effective message contains more than simple information; it also implies what the recipient should do with the information. For example, after discussing the risks and benefits of exercise with a patient, a clinician should conclude by relating this information to the patient’s personal situation and making it clear that the patient should take more exercise.

Self-test Questions

1. A patient is complaining of a skin rash, which you think looks like contact dermatitis. What should you question her about?

Contact dermatitis, along with many other skin diseases, is generally not due to intrinsic factors, but is an environmental disease. Questions should cover all spheres of the patient’s life, following the mnemonic CH2OPD2:

  • Community acquired? In this case an unlikely source although contact with plants, or use of herbicides or insecticides in the neighbourhood should be considered
  • Home: consider contact with garden or house plants; use of cleaning products, pesticides, herbicides, construction or decorative materials
  • Hobbies and leisure: exposure to chemicals, dusts, or micro-organisms, or to particular clothing necessary for the activity – wet suits for surfing, gloves for gardening – depending on which part of the body is affected
  • Occupation: contact dermatitis is common in occupational settings. You may need to ask about previous occupations; don’t forget voluntary work; work with known hazards; the protective clothing that is used for protection, such as latex gloves
  • Personal habits: hygiene products including scents, creams and lotions, source of water used for washing; products used for washing clothes and bedding.
  • Diet: an unlikely source of contact dermatitis
  • Drugs: prescription, non-prescription, and alternative medications, particularly medications applied to the skin – nicotine or hormone patches.

You should question the patient particularly about changes in the period prior to the onset of symptoms. Where you find a possible source, make sure you get precise information about the period of exposure, how great the exposure is, the variation of symptoms in relation to episodes of exposure, whether other people were exposed and if they have similar symptoms.

2. Name and briefly describe the stages of risk assessment.

Hazard identification: explore the environment to identify possible sources of hazard. In the clinical setting, a good history usually identifies possible sources.

Risk characterization: describe the effects of the hazard. The local public health department will usually be able to advise on the effects of different hazards and about populations at particular risk.Exposure assessment: describe the duration of exposure and the levels of the hazard. In the  clinical setting a good history should include precise details on the level of hazard to which the patient is exposed, the length of time of an exposure, how often the exposure occurs, and since when has this been happening, in other words to gauge the dose to which the patient is exposed. Tests may be available to measure the dose the patient has had. It may also be possible to measure the level of hazard in the patient’s environment.

Risk estimation: integrate and analyse the information from the three previous steps to judge how much the hazard contributes to patient’s problem.

3. List the psychological influences on interpretation of quantitative data.

In relation to the person who is at risk

General disposition: optimistic people believe themselves to be less at risk than pessimistic people.Confirmatory bias: people pay more attention to information that confirms what they already believe.

Affective forecasting: people find it difficult to assess how they will react in future possible situations.

In relation to the hazard

Perception of threat: people generally believe that they are at less than the average risk, particularly when they have control over their exposure to the hazard.

Reduction of vulnerability: people wish not to feel vulnerable to a perceived threat and may therefore deny their risk.

In relation to how the risk is communicated

Loss aversion: people will avoid a choice that is presented as a loss.

Default option: people will choose the option that they think is the usual or ‘normal’ one.

Number versus proportion and numerator versus denominator: people pay attention to whole numbers that can represent whole people. They can misunderstand proportions, and pay little attention to denominators.

Glass half full or glass half empty: people will focus on the number that is presented, not on the one that is understood. If a disease incidence is presented as such, they will magnify the risk of becoming ill, rather than the risk of staying well.

Relative vs absolute risk: people interpret a high relative risk as almost equivalent to a high absolute risk, even if the underlying absolute risk in the ‘high risk’ group is small.

4. A noisy machine is causing some workers to complain of deafness. What could you suggest to alleviate the problem?

Using the source-path-receiver model,

Source – modify or re-design the machine to reduce the noise at its source, use a different type of machine or process that doesn’t produce noise, relocate it away from the workers or enclose it to confine the noise produced.

Path – use acoustic barriers to absorb or block the noise reaching the workers.

Receiver – enclose or relocate the workers away from the machine, supply workers with ear protectors and ensure that exposure to the noise in infrequent and for very short periods.


Reflection Questions

1.   A 42-year-old female patient living a kilometre away from overhead, high tension power lines is very worried of the lines’ effect on her health. How do you approach this case?
2.   You find her risk from the high tension lines is negligible. How do you reassure her?
3.   The same patient is not very worried about smoking ten cigarettes a day. How do you explain this?


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