To estimate health benefits in monetary terms, a standard technique, used by the World Health Organization (WHO), is the valuation of disability-adjusted life-years (DALY). The DALY is an economic parameter which extends the concept of potential years of life lost due to premature death to also include equivalent years of “healthy” life lost due to poor health or disability (WHO, 2008). The DALY combines in one measure the time lived with a disability, and the time lost due to premature mortality, both of which are plausible outcomes of contaminant exposure.
DALYs for a disease or health condition are calculated as the sum of the years of life lost due to premature mortality (YLL) in the population and the years lost due to disability (YLD) for incident cases of the health condition. In other words,
DALY = YLL + YLD
The years of life lost (YLL) correspond to the number of deaths multiplied by the standard life expectancy at the age at which death occurs. The basic formula for YLL is the following for a given cause, age and sex.
YLL = N x L
N = number of deaths
L = standard life expectancy at age of death in years
Because YLL measures the incident stream of lost years of life due to deaths, an incidence perspective is also taken for the calculation of YLD. To estimate YLD for a particular cause in a particular time period, the number of incident cases in that period is multiplied by the average duration of the disease and a weight factor is applied that reflects the severity of the disease on a scale from 0 (perfect health) to 1 (dead). The basic formula for YLD is:
YLD = I x DW x L
I = number of incident cases
DW = disability weight
L = average duration of the case until remission or death (years)
References: Murray CJL, Lopez AD (1996). The Global Burden of Disease. Cambridge: Harvard University Press
Murray CJL, Lopez AD (1996). The Global Burden of Disease. Cambridge: Harvard University Press