DALYs are the answer, but what was the question?

Health impact assessment is increasingly used in the development of environmental and public health policies and regulations. It commonly involves the identification of environmental hazards, and the quantification of the expected Burden of Disease (BoD). This (environmental) disease burden can be expressed in a variety of ways. WHO and others increasingly use the DALY: Disability Adjusted Life Years, encapsulating the amount of healthy life years, a summary measure which is the focus of this paper. After selecting a set of endpoints with sufficient evidence for a relation with the risk factors under study, the expected environmental disease burden in a population can be quantified by combining population-density data with concentration distributions of a relevant exposure indicator (e.g. concentration), information on exposure-response relationships, and estimates of the duration and severity of diseases. DALYs can be used to compare and prioritize dissimilar environmental health problems, by expressing them in one unit. Such comparisons can alternatively be made on the basis of the number of people exposed, or with a certain disease, but this does not take the duration or severity of the diseases and related disability and quality of life effects into account. Despite increased use, it is still under debate whether, when and where the use of DALYs is suitable. This paper outlines the DALY method, the rationale behind it, gives some examples of its application and addresses some of the challenges we are confronted with when applying this approach. We conclude that the DALY approach is typically more suited for larger scale issues, and should never be the sole base for priority setting. Local assessments struggle with a lot of uncertainties at the cost of discriminating power. DALYs are only useful for comparing different environmental factors or different and incomparable health states. Communication of uncertainties and assumptions is paramount and finally the answer about when and where applicable can only be given in relation to underlying policy questions.