Disability-adjusted life year (DALY) losses
The various hazards that can be present in water, excreta, wastewaters and polluted surface waters can have very different health outcomes. Some outcomes are mild (e.g., diarrhea,), while others can be severe (cholera, haemolytic uraemic syndrome associated with E. coli O157, or cancer); some are acute (diarrhea), while others are delayed (infectious hepatitis, cancer); some especially relate to certain age ranges and groups (skeletal fluorosis in older adults often arises from long-term exposure to high levels of fluoride in childhood; infection with hepatitis E virus has a very high mortality rate among pregnant women). In addition, any one hazard may cause multiple effects (e.g., gastroenteritis, Guillain-Barré syndrome, reactive arthritis and mortality associated with Campylobacter).
In order to support public health priority setting a common metric is required that can be applied to all types of hazard and takes into account different health outcomes including probabilities, severities and durations of effects. Disability-adjusted Life Years (DALYs) provides this metric. The basic principle of the DALY is to weight each health impact in terms of severity within the range of 0 for good health to 1 for death. The weighting is then multiplied by duration of the effect and the number of people affected. In the case of death duration is regarded as the years lost in relation to normal life expectancy (taken as 70 years). Using this approach a mild diarrhea with a severity weighting of 0.1 and lasting for 7 days results in a DALY loss of [(0.1 × 7)/365] – i.e., 0.002; while death resulting in a loss of 30 years of life equates to a DALY loss of 30. [Strictly speaking DALY losses have units of years, but generally they are just expressed as a number.]
Hence, DALYs = YLL (years of life lost) + YLD (years lived with a disability or illness) − in this context ‘disability’ refers to conditions that detract from good health.
Calculation of DALY losses
Infection with rotavirus (in developed countries), for example, causes:
● mild diarrhea (severity rating of 0.1) lasting 7 days in 97.5% of cases
● severe diarrhea (severity rating of 0.23) lasting 7 days in 2.5% of cases
● the death (severity rating of 1) of very young children in 0.015% of cases