Quantitative assessment of environmental health impacts at population level
Environmental Burden of Disease is one methodology for quantitatively assessing environmental health impacts at the population level in terms of deaths, disability adjusted life years (DALYs), or occasionally, number of cases. Other indirect measures can be used to estimate health impacts, for example the number of hospital admissions.
Objectives
To provide a quantitative estimate of the number of people within a defined population with a health impact that is attributable to an environmental exposure.
Methods
Quantitative assessments of health impacts are based on combining exposure data with exposure-response information. Such assessments require (i) the compilation of exposure data, (ii) a systematic review of evidence from epidemiology and other scientific disciplines concerning the association between environmental factors and human health, and (iii) the combination of exposure and exposure -response.
Quantitative assessments include the following steps:
- Specify the health risk to be addressed in the impact assessment.
- Specify the measure of exposure and the range of exposure to be considered.
- Estimate the population exposure distribution.
- Select appropriate health outcome(s).
- Select exposure-response relationship in the population of interest, from the scientific literature or available guidance.
- Combine exposure and exposure-response relationship data for each population group under consideration. (e.g. by age and gender).
- Calculate the attributable fraction of disease(s) multiplied by the health statistics associated with the disease(s) under study.
- Quantify uncertainty of the estimate (range of potential effect).
Examples
WHO has recently undertaken an assessment of the amount of disease caused by 26 risk factors in 14 WHO-defined epidemiological regions of the world, by age and gender group. As an example, the results for the number of deaths from six selected environmental risk factors for the group of countries Indonesia, Sri Lanka, Thailand are presented in the graph below.
In estimating burden of disease associated with the risk factor lead, for example, the following steps were carried out:
- Assessment of the exposure distribution in the population, in terms of blood lead level.
- Selection of associated health effects and diseases: loss of IQ points, mild mental retardation, increased blood pressure, cardiovascular diseases and anaemia.
- Estimation of exposure-response relationships through meta-analysis of existing epidemiological studies after assessment of their validity and relevance.
- Estimation of attributable cases for mild mental retardation and anaemia , directly from the exposure-response relationships.
- Estimation of the fraction of cardiovascular disease cases attributable to the lead risk factor from the exposure-response relationships. Multiplication of the total number of cardiovascular disease cases in the population by the fraction of disease attributable to lead, yielding the estimated number of disease cases attributable to the lead risk factor in the study population.
Activities and applications
WHO has developed evidence-based methods for quantitative assessments of health impacts at population level, useful not only globally and regionally, but also at country and city level. The methodology has been applied by WHO to the 14 WHO-defined epidemiological regions of the world. Guidance is also available for country level burden of disease assessments for various risk factors.
Relevance to environment and health linkages and policy setting
These assessments provide a means of estimating the number of people who can be expected to become ill as a consequence of an environmental exposure within a population. Together with associated cost -effectiveness analysis strategies , this method offers quantifiable evidence to the decision-maker seeking to define priorities within a socio-economic and political context. Such an assessment also provides the basis for other economic evaluations.
See also
References
WHO, World Health Report 2002: Reducing Risks, Promoting Healthy life, WHO, Geneva, 2002, (http://www.who.int/whr/).
Prüss A, Corvalán C, Pastides H, De Hollander AEM. “ Methodologic Considerations in Estimating Burden of Disease from Environmental Risk Factors at National and Global levels”, International Journal of Occupational and Environmental Health, 2001, 7:58-67. (http://www.who.int/peh/burden/methods.htm). (This reference also presents a review of quantitative assessments of health impacts at population level that have been performed up to 1999)
WHO, Series on Assessment of Environmental Burden of Disease at National and Local level , WHO, Geneva, 2006. WHO, Environmental Burden of Disease Web Page: http://www.who.int/peh/burden/burdenindex.htm).