DALY is a measure of the reduction in a population’s quality of life and loss of years of healthy life.It is found by first calculating the number of people with a health impactand multiplying this by the duration of the impact and a weighting factorfor the loss of quality of life from the health impact.
The weighting factor ranges from 0 (the value assigned to full health) to 1 (the value assigned to death and the health status deemed to be equivalent). In this way disease and death can be expressed as a number, making it possible to make rough comparisons between various problems (such as air pollution and noise nuisance) in terms of the duration, number and severity of their health impacts.
DALYs provide a basis for classifying and prioritising policy programmes and measures in terms of their impacts on public health.
DALYs are particularly useful for relative comparisons of total health impacts. If the differences between alternatives, environmental risk factors and the impacts of measures are relatively large, alternatives and the efficacy of measures and the priorities relating to addressing environmental risk factors can be shown.
Usually, a not too large absolute value is assigned to the calculated DALYs because there are sometimes large uncertainties in the calculation. The result, the number of DALYs, is also difficult to interpret. It is important to take care to communicate this.
A disadvantage is that DALYs can only be calculated for the health impacts for which sufficient information is available on the relationship between exposure and health impacts and the weighting factor.
If a social cost-benefit analysis is drawn up for a plan or project, the use of DALYs in an environmental assessment report make it possible to also provide quantitative information on health impacts in that analysis. The so-called OEI Guidelines (on the impacts of infrastructure) contain a method to determine the costs and benefits of alternatives on the basis of the calculated DALYs.