Defining and assessing risks to health.

This chapter offers a detailed explanation of the report's approach to health risks. It argues that while much scientific effort and most health resources today are directed towards treating disease, rather than preventing it, focusing on risks to health is the key to prevention. Such risks do not occur in isolation, so both proximal and distal causes of adverse health outcomes need to be considered. Population-based strategies aim to make healthy behaviour a social norm, thus lowering risk in the entire population. Small shifts in some risks in the population can translate into major public health benefits. Therefore this chapter strongly advocates the assessment of population-wide risks as well as high-risk individuals in strategies for risk reduction. The key challenge is to find the right balance between the two approaches. Risk assessment has emerged in recent years from its roots in the study of environmental problems, and the steps generally involved in environmental risk assessment can be adapted to apply more specifically to the analysis of health risks. This chapter explains the benefits of comparing different risks to health and defines and explains risk assessment.

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).

WHY FOCUS ON RISKS TO HEALTH?

Focusing on risks to health is key to preventing disease and injury. The most emotive and tangible images in health are of people suffering from disease, but preventing disease and injury occurring in the first place requires systematic assessment and reduction of their causes. Much scientific effort and most health resources are directed towards treating disease--the "rule of rescue" still dominates (3). Data on disease or injury outcomes, such as death or hospitalization, tend to focus on the need for palliative or curative services. In contrast, assessments of burden resulting from risk factors will estimate the potential of prevention. One notable exception concerns communicable diseases, since treating infected individuals can prevent further spread of infection, and hence treatment can be a method of prevention in itself.

Even when the focus is on causes as well as disease outcomes, much scientific activity has been directed to assessing whether a risk exists at all. Does electromagnetic frequency radiation cause leukaemia? Do certain infections predispose to heart attacks? These assessments are usually accompanied by estimates of how much higher the risk is in individuals who are exposed compared with those who are not. It has been much less common to assess impact at a population level by asking "of all the disease burden in this population, how much could be caused by this risk?"

Many factors are relevant in prioritizing strategies to reduce risks to health: the extent of the threat posed by different risk factors, the availability of cost-effective interventions, and societal values and preferences are particularly important. These factors are also key for research priorities--if major threats exist without cost-effective solutions, then these must be placed high on the agenda for research. Governments are also likely to place particular value on ensuring their main efforts focus on the largest threats to health in their countries. Reliable, comparable and locally relevant information on the size of different risks to health is therefore crucial to prioritization, especially for governments setting broad directions for health policy and research. However, such information has typically been very limited, creating a gap in which interest groups may seek either to downplay or to overestimate some risks. In addition, there is an inherent imbalance in media information about risks: common, major threats to health are usually not reported because they are already known, whereas rare or unusual threats to health are highly newsworthy.

Stewardship is one of the key functions of government, necessitating a broad overview, a long-term horizon and an evidence-based approach, and requiring information from reliable, comparable assessments of the magnitude of different major risks to health. This report helps to redress the dearth of such information. The report recognizes that risk analysis is a political enterprise as well as a scientific one, and that public perception of risk also plays a role in risk analysis, bringing issues of values, process, power and trust into the picture. The roles and contributions of risk assessment, communication, risk management, cost-effectiveness and policy development form the focus of the report.

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 …