The world health report

Chapter 2


What are risks to health?

Risk can mean different things to different people, as summarized in Box 2.1. The two most common meanings will be used in this report -- risk as a probability of an adverse outcome, or a factor that raises this probability.

Box 2.1 What does risk mean?

  • Risk can mean a probability, for example, the answer to the question: "What is the risk of getting HIV/AIDS from an infected needle?"
  • Risk can mean a factor that raises the probability of an adverse outcome. For example, major risks to child health include malnutrition, unsafe water and indoor air pollution.
  • Risk can mean a consequence. For example, what is the risk from driving while drunk? (answer: being in a car crash).
  • Risk can mean a potential adversity or threat. For example, is there risk in riding a motorcycle?

In this report, the first two meanings are used. Risk is defined as a probability of an adverse health outcome, or a factor that raises this probability. Other important risk-related definitions are outlined below.

  • Prevalence of risk -- the proportion of the population who are exposed to a particular risk. For example, the prevalence of smoking might be 25% in a particular population.
  • Relative risk -- the likelihood of an adverse health outcome in people exposed to a particular risk, compared with people who are not exposed. For example, if people who smoke for a certain time are, on average, 15 times more likely to develop lung cancer than those who do not smoke, their relative risk is 15.
  • Hazard -- an inherent property, for example of a chemical, that provides the potential for harm.
  • Population attributable risk -- the proportion of disease in a population that results from a particular risk to health.
  • Attributable burden -- the proportion of current disease or injury burden that results from past exposure.
  • Avoidable burden -- the proportion of future disease or injury burden that is avoidable if current and future exposure levels are reduced to those specified by some alternative, or counterfactual, distribution.

Sources: (1,2).