Public health benefits of strategies to reduce greenhouse-gas emissions: health implications of short-lived greenhouse pollutants

Abstract

In this report we review the health effects of three short-lived greenhouse pollutants-black carbon, ozone, and sulphates. We undertook new meta-analyses of existing time-series studies and an analysis of a cohort of 352,000 people in 66 US cities during 18 years of follow-up. This cohort study provides estimates of mortality effects from long-term exposure to elemental carbon, an indicator of black carbon mass, and evidence that ozone exerts an independent risk of mortality. Associations among these pollutants make drawing conclusions about their individual health effects difficult at present, but sulphate seems to have the most robust effects in multiple-pollutant models. Generally, the toxicology of the pure compounds and their epidemiology diverge because atmospheric black carbon, ozone, and sulphate are associated and could interact with related toxic species. Although sulphate is a cooling agent, black carbon and ozone could together exert nearly half as much global warming as carbon dioxide. The complexity of these health and climate effects needs to be recognised in mitigation policies.

Figure. Relative contributions of human sources to sulphur dioxide (A) and black carbon (B) emissions

In each case, the distribution of climate effects differs from the distribution of the primary pollutant. For sulphur dioxide, the extent of transformation to the climate-active species, sulphate, will vary by location. For black carbon, the different sources produce cooling organic-carbon aerosols in varying amounts. The climate implication is due to the net radiative forcing of the two linked emissions. Only half the total forest and grassland emissions are counted here as an estimate of the proportion that is due to human activities. 2005 sulphur dioxide estimates are interpolated between the 1995 and 2030 estimates calculated by Unger and colleagues. Since there are no official inventories and because methods vary across investigators, these estimates should be regarded as approximate. Estimates of black carbon emissions are from the black carbon emissions inventory.