Disability-adjusted life year
Disability-adjusted life years are a method for combining different health impacts such as mortality and morbidity into a single common metric. The DALY is one of the most commonly used integrated health measures and was first introduced by Murray and Lopez (1996) in collaboration with World Health Organization and the Worldbank in an attempt to introduce morbidity in mortality-based health discussions.
DALYs incorporate three important factors of health: loss of life expectancy due to premature mortality, combined with the duration of living in a deteriorated health state, standardized to the severity of the deteriorated health state. Some DALY calculations also use age weights and time preferences. Age weights indicate the relative importance of healthy life at different ages, for example, a rise of importance from birth until age 25 and decline thereafter. Time preference compares the value of health gains today to the value attached to health gains in the future. In economic theory, the latter is assumed to be lower than the former. However, for ethical reasons, we have chosen not to make a difference between elder and younger people and for sustainability and durability reasons, we have not discounted health benefits in the future. For more information on these and other discussion points: see paragraph 6.4.
In general, DALYs can be calculated using the equations below:
DALY = AB * D * S AB = AR * P * F AR = (RR’-1)/RR’ RR’ = ((RR-1) * C) + 1
AB: Attributable Burden; the number of people in a certain health state as a result of exposure to the (environmental) factor that is being analyzed, not corrected for comorbidity.
D: Duration of the health state; for morbidity, prevalence numbers have been used and therefore duration is one year (except for hospital visits, for which the mean duration of the specific hospital visit has been used). For mortality, the duration of time lost due to premature mortality is calculated using standard expected years of life lost with model life-tables.
S: Severity; the reduction in capacity due to morbidity is measured using severity weights. A weight factor, varying from 0 (healthy) to 1 (death), is determined by experts (clinicians, researchers, etc).
AR: Attributive Risk; risk of getting a specific disease as a result of exposure to a certain (environmental) factor.
P: Base prevalence for morbidity; number of deaths for mortality
F: Fraction of the population exposed to the (environmental) factor under investigation (for air pollution, this fraction is set to 1, meaning that everybody is exposed to a certain degree)
RR’: Adjusted Relative Risk
RR: Relative Risk
C: Concentration of the environmental factor, expressed in the unit of the Relative Risk
Source: Knol, A.B. en Staatsen, B.A.M. (2005). Trends in the environmental burden of disease in the Netherlands, 1980-2020. Rapport 500029001, RIVM, Bilthoven. Downloadable at http://www.rivm.nl/bibliotheek/rapporten/500029001.html