Outdoor air pollution : assessing the environmental burden of disease at national and local levels

Overview

This guide outlines a method for estimating the disease burden associated with environmental exposure to outdoor air pollution. In a recent estimate of the global burden of disease (GBD), outdoor air pollution was estimated to account for approximately 1.4% of total mortality, 0.4% of all disability-adjusted life years (DALYs), and 2% of all cardiopulmonary disease. To obtain estimates of the impact of outdoor air pollution, population exposures are based on current concentrations of particulate matter (PM) measured as either PM10 or PM2.5 (i.e. PM less than 10 µm or 2.5 µm in diameter, respectively). PM is a mixture of liquid and solid particle sizes and chemicals that varies in composition both spatially and temporally. After multiplying the exposure concentrations by the numbers of people exposed, concentration−response functions from the epidemiological literature are applied. These functions relate ambient PM concentrations to cases of premature mortality, and enable the attributable risk to be calculated.

For the quantitative assessment of health effects, PM2.5 and PM10 are selected because these exposure metrics have been used in epidemiological studies throughout the world. In addition, over the past two decades, epidemiological studies spanning five continents have demonstrated an association between mortality and morbidity, and daily, multi-day or long-term (a period of more than a year) exposures to concentrations of pollutants, including PM. The estimated mortality impacts are likely to occur predominantly among elderly people with pre-existing cardiovascular and respiratory disease, and among infants. Morbidity outcomes include hospitalization and emergency room visits, asthma attacks, bronchitis, respiratory symptoms, and lost work and school days. However, this guide does not provide a method to quantify morbidity attributable to air pollution, since such calculations require an estimate of background disease rates in the absence of air pollution.

In most urban environments, PM is generated mainly from fuel combustion in both mobile (diesel and non-diesel cars, trucks and buses) and stationary (power plants, industrial boilers and local combustion) sources. PM can also be generated by mechanical grinding processes during industrial production, and by natural sources such as wind-blown dust. To select the most suitable interventions for reducing the disease burden associated with outdoor air pollution, an inventory of the principal local and regional sources would be useful. Typically, mobile sources contribute 50% or more of PM concentrations in urban areas. In certain cities and regions, however, other sources may predominate. In rural areas, biomass burning may be the largest source.

Estimates of the burden of disease attributed to outdoor air pollution can help set the priority for controlling air pollution, relative to other interventions that improve public health.