The world health report

Chapter 2

Development of risk assessment

People have been interested in risks to health throughout history. During the past several decades, this interest has intensified and has also begun to include many new perspectives. The field of risk analysis has grown rapidly, focusing on the identification, quantification and characterization of threats to human health and the environment -- a set of activities broadly called risk assessment.

While clearly there has been very long interest in comparing risks posed by different threats to health, formal frameworks have been developed only relatively recently. Risk assessment has its roots in the environmental sector, where it was developed as a systematic way of comparing environmental problems that pose different types and degrees of health risk. Such environmental risk assessment exercises generally comprise four elements.

  • Hazard identification identifies the types of health effect that can be caused, based on toxicological data from laboratory or epidemiological studies: for example, chemical X causes liver damage.
  • Exposure assessment combines data on the distribution and concentrations of pollution in the environment with information on behaviour and physiology to estimate the amount of pollutant to which humans are exposed. Biomarkers have been used to gauge levels of some exposures, such as lead and dioxin.
  • Dose--response assessment relates the probability of a health effect to the dose of pollutant or amount of exposure.
  • Risk characterization combines the exposure and dose--response assessments to calculate the estimated health risks, such as the number of people predicted to experience a particular disease, for a particular population. This typically includes estimation and communication of uncertainties.

Environmental risk assessments of likely health effects, together with consideration of costs, technical feasibility and other factors, can be used to set priorities for environmental management. Environmental risk assessment has analogies to the strategies developed in epidemiology for assessing population attributable risks, that is, the proportion of disease in a population that results from a particular hazard. A more general approach based on these frameworks can be extended to many other areas. A key part of this report outlines such methods and provides an illustrative analysis of burden caused by a variety of different risks to health.

Risk assessment can be defined here as a systematic approach to estimating and comparing the burden of disease and injury resulting from different risks. The work presented in this report builds on several similar estimates conducted in recent years. The first global estimates of disease and injury burden attributable to a set of different risk factors were reported in the initial round of the global burden of disease study (4,5). These estimates add to the many others made for selected risk factors in specific populations, for example, tobacco (6), alcohol and other drugs (7), environmental factors (8), blood pressure (9), and selected risk factors for certain regions (10,12).

In the first round of the global burden of disease study, risk factors were assessed that were either exposures in the environment (for example, unsafe water), human behaviour (for example, tobacco smoking) or physiological states (for example, hypertension). However, in such early risk assessments, there was a lack of comparability between different risk factor assessments arising, in part, from a lack of standard comparison groups and different degrees of reliability in assessing risk factors. Also, the relevance of varying time lags between exposure and outcome -- for example, short for alcohol and injuries and long for smoking and cancer -- was not captured. A key aim of this analysis is therefore to increase comparability between the estimates of the impact of different risk factors and characterize the timing of these impacts.

Risk assessment estimates burden of disease resulting from different risk factors, each of which may be altered by many different strategies; it can provide an overall picture of the relative roles of different risks to human health. Specific strategies for identifying the appropriate sets of interventions, and the crucial roles of cost-effectiveness analyses in choosing from among them, are outlined in Chapter 5.