CHAPTER TWO 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 wellas high-risk individuals in strategies for risk reductio to find theright balance between the to approaches. Risk assessment has emerged in recent years from its roots in the study of environmental problems, and th 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
Defining and Assessing Risks to Health 7 CHAPTER TWO efining and ssessing isks to ealth 7 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.
DEFINING AND ASSESSING RISKS TO HEALTH WHAT ARE RISKS TO HEALTH? 2 isk 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 WHY FOCUS ON RISKS TO HEALTH? Focusing on risks to health is key to preventing disease and injury. The most emotive ind 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 as 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 typicall ery limited, cre-
Defining and Assessing Risks to Health 9 2 DEFINING AND ASSESSING RISKS TO HEALTH WHAT ARE RISKS TO HEALTH? isk 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. 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, cre-
The World Health Report 2002 nting 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: com mon, major threats to health are usually not reported beca luse thev are area 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, ost-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 sev eral decades, this interest has intensified and has also begun to include many new perspec tives.The field of risk analysis has grown rapidly, focusing on the identification, qu aton 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 vay 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 pol mutant or amount of exposure Risk characterization combines the exposure and dose-response assessments to alculate 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 nese 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 omparing the burden of disease and injury resulting from different risks. The work pre
10 The World Health Report 2002 ating 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 pre-
Defining and Assessing Risks to Health Box 2.1 What does risk mean? Risk can mean a probability, for example, the answer to the question: Relative risk- the likelihood of an adverse health outcome in people What is the risk of getting HIV/AIDS from an infected needle? exposed to aparticular risk, compared with people who are not exposed Risk can mean a factor that raises the probability of an adverse outcome. For example, if people who smoke for a certain time are, on average, 15 For example, major risks to child health include malnutrition, unsafe water times more likely to develop lung cancer than those who do not smoke, and indoor air pollution. their relative risk is 15 Risk can mean a consequence. For example, what is the risk from driving Hazard-an inherent property, for example of a chemical, that provides hile drunk? (answer: being in a car crash) the potential for harm. Risk can mean a potential adversity or threat. For example, is there risk in Population attributable risk-the proportion of disease in a popula riding a motorcycle? tion that results from a particular risk to health. In this report, the first two meanings are used Risk is defined as a probabil- Attributable burden-the proportion of current disease or injury bur- ity of an adverse health outcome, or a factor that raises this probability. Other.Avoidable burden-the proportion of future disease or injury burden important risk-related definitions are outlined below Prevalence of risk-the proportion of the population who are exposed at is avoidable if current and future exposure levels are reduced to to a particular risk. For example, the prevalence of smoking might be those specified by some altemative, or counterfactual, distribution 25% in a particular population. Sources: (1, 2) sented 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 esti mates 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 ar 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. KEY GOALS OF GLOBAL RISK ASSESSMENT An effective risk assessment must have a well-defined scope, which in tum depends on the purpose of the analysis. For example, an evaluation of emissions from a particular industrial facility is likely to concentrate on their health effects on local populations. In contrast, a project to set national environmental priorities may be much broader in scope, covering such factors as emissions of greenhouse gases and ozone-depleting substances Some trade-offs will inevitably be required. Governments and ministries of health oversee
Defining and Assessing Risks to Health 11 sented 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. KEY GOALS OF GLOBAL RISK ASSESSMENT An effective risk assessment must have a well-defined scope, which in turn depends on the purpose of the analysis. For example, an evaluation of emissions from a particular industrial facility is likely to concentrate on their health effects on local populations. In contrast, a project to set national environmental priorities may be much broader in scope, covering such factors as emissions of greenhouse gases and ozone-depleting substances. Some trade-offs will inevitably be required. Governments and ministries of health oversee 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)
The World Health Report 2002 overall population health and so, at the broadest level, need information from risk assess- ments that are comprehensive as well as being reliable, relevant and timely. Because the range of risks to health is almost limitless, it is essential for govemments to have a quanti tative approach to gauging their importance. Risks need to be defined and studied compre hensively irrespective of factors such as their place in a causal chain or the methods used (from the disciplines of the physical, natural, health, and social sciences) for their analysis. The following sections outline some of the different dimensions that should be considered. STANDARDIZED COMPARISONS AND COMMON OUTCOME MEASURES Ideally, the impact of eachrisk factor should be assessed in terms of a"common currency that incorporates loss of quality of life as well as loss of life years. The principal metric used this report is the daLy (disability-adjusted life year)-one DALY being equal to the loss ne healthy life year(13) a key initial question when assessing the impact of a risk to health is to ask"compared to what? " This report employs an explicit counterfactual approach, in which current distributions of risk factors are compared with some alternative, or counterfactual, distribu tion of exposure. Many different counterfactuals are potentially of interest To enhance com parability across risk factors, the basis for the results in Chapter 4 is the theoretical min risk distribution, that is exposure levels that would yield the lowest population risk(for example, no tobacco use by any members of a population). For the analysis of the costs and effects of interventions to reduce risk in Chapter 5, a related counterfactual is used-based on the burden that would exist in the absence of relevant interventions risk factor distri butions that are plausible, feasible and cost-effective will lie somewhere between the cur- rent risk factor levels and the related theoretical minimum. The envisaged shift from current to counterfactual scenarios has been termed the distributional transition(see Figure 2.1) In many instances, the counterfactual of most relevance will involve small to moderate distributional transitions(for example, 10%, 20%or 30%), as these are most likely to be feasible and cost-effective. These estimates are also less susceptible to the influence of arbitrary choices of theoretical minima, and are likely to be the most reliable, as the dose- response is often least certain at low exposure levels Figure 2. 1 Example of distributional transitions for blood pressure and for tobacco smoking Tobacco smoking Current level After 10%6 distributional transition e After 20%o distributional transition After 30%6 distributional transition 160170 Systolic blood pressure(mmHg Cigarettes per day
12 The World Health Report 2002 overall population health and so, at the broadest level, need information from risk assessments that are comprehensive as well as being reliable, relevant and timely. Because the range of risks to health is almost limitless, it is essential for governments to have a quantitative approach to gauging their importance. Risks need to be defined and studied comprehensively irrespective of factors such as their place in a causal chain or the methods used (from the disciplines of the physical, natural, health, and social sciences) for their analysis. The following sections outline some of the different dimensions that should be considered. STANDARDIZED COMPARISONS AND COMMON OUTCOME MEASURES Ideally, the impact of each risk factor should be assessed in terms of a “common currency” that incorporates loss of quality of life as well as loss of life years. The principal metric used in this report is the DALY (disability-adjusted life year) – one DALY being equal to the loss of one healthy life year (13). A key initial question when assessing the impact of a risk to health is to ask “compared to what?” This report employs an explicit counterfactual approach, in which current distributions of risk factors are compared with some alternative, or counterfactual, distribution of exposure. Many different counterfactuals are potentially of interest. To enhance comparability across risk factors, the basis for the results in Chapter 4 is the theoretical minimum risk distribution, that is exposure levels that would yield the lowest population risk (for example, no tobacco use by any members of a population). For the analysis of the costs and effects of interventions to reduce risk in Chapter 5, a related counterfactual is used – based on the burden that would exist in the absence of relevant interventions. Risk factor distributions that are plausible, feasible and cost-effective will lie somewhere between the current risk factor levels and the related theoretical minimum. The envisaged shift from current to counterfactual scenarios has been termed the distributional transition (see Figure 2.1). In many instances, the counterfactual of most relevance will involve small to moderate distributional transitions (for example, 10%, 20% or 30%), as these are most likely to be feasible and cost-effective. These estimates are also less susceptible to the influence of arbitrary choices of theoretical minima, and are likely to be the most reliable, as the dose– response is often least certain at low exposure levels. Blood pressure 0 4 5 6 1 2 3 7 8 90 110 150 170 100 120 140 160 Systolic blood pressure (mmHg) Population (%) Theoretical miminum Distributional transitions Current level Tobacco smoking 0 20 40 60 80 100 0 1-14 15-24 25-34 35+ Cigarettes per day Population (%) Current level After 10% distributional transition After 20% distributional transition After 30% distributional transition Theoretical minimum Figure 2.1 Example of distributional transitions for blood pressure and for tobacco smoking 130