Box 3.1Disability-adjusted life years

The various hazards that can be present in water can have very different health outcomes. Some outcomes are mild (e.g. diarrhoea), whereas others can be severe (e.g. cholera, haemolytic uraemic syndrome associated with Escherichia coli O157 or cancer). Some are acute (e.g. diarrhoea), whereas others are delayed (e.g. infectious hepatitis or cancer). Some especially relate to certain age ranges and groups (e.g. 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 duration of effects. The disability-adjusted life year (DALY) 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. Using this approach, a mild diarrhoea with a severity weighting of 0.1 and lasting for 7 days results in a DALY of 0.002, whereas death resulting in a loss of 30 years of life equates to a DALY of 30.

Hence, DALY = YLL (years of life lost) + YLD (years lived with a disability or illness). In this context, disability refers to a condition that detracts from good health.

For example, infection with rotavirus (in developed countries) causes:

  • mild diarrhoea (severity rating of 0.1) lasting 7 days in 97.5% of cases;
  • severe diarrhoea (severity rating of 0.23) lasting 7 days in 2.5% of cases;
  • rare deaths of very young children in 0.015% of cases.
The DALY per case can then be calculated as follows:
  • DALY = (0.1 × 7/365 × 0.975) + (0.23 × 7/365 × 0.025) + (1 × 70 × 0.00015)
    = 0.0019 + 0.0001 + 0.0105
    = 0.0125

Infection with Cryptosporidium can cause watery diarrhoea (severity weighting of 0.067) lasting for 7 days with extremely rare deaths in 0.0001% of cases. This equates to a DALY per case of 0.0015.

Further information on the use of DALYs in establishing health-based targets is included in the supporting document Quantifying public health risk in the WHO Guidelines for drinking-water quality (Annex 1).

From: 3, Health-based targets

Cover of Guidelines for drinking-water quality
Guidelines for drinking-water quality: Fourth edition incorporating the first and second addenda [Internet].
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