The burden of disease attributable to ambient air pollution can be expressed as : Number of deaths, Death rate (both crude and age-standardized), Number of disability-adjusted life years (DALYs), DALYs rate (both crude and age-standardized), Number of years life lost (YLLs), YLLs rate (both crude and age-standardized). Death (DALYs, or YLLs) rates are calculated by dividing the number of deaths by the total population (or indicated if a different population group is used, e.g. children under 5 years). Age-standardized rates adjust for differences in population age distributionby applying the observed age-specific mortality (or other health outcomes) rates for each population to a standard population. The age-standardized rates can therefore be used to compare the rates of countries without being affected by the differences in age distribution from country to country. Evidence from epidemiological studies have shown that exposure to ambient air pollution is linked, among others, to the important diseases taken into account in this estimate: Acute respiratory infectionsin young children (estimated under 5 years of age); Cerebrovascular diseases in adults (estimated above 25 years); Ischaemic heart diseases in adults (estimated above 25 years); Chronic obstructive pulmonary disease in adults (estimated above 25 years); and Lung cancer in adults (estimated above 25 years).