Air Pollution as a Climate Forcing: A Workshop
Summary E. Health Effects
Background — Much known about the health effects of air pollution. Numerous epidemiologic, human clinical, and laboratory studies in North America and Europe have identified a number of health effects associated with ambient air pollution. In addition, studies in the developing world have identified health effects for people exposed to intense indoor pollution, especially from low efficiency cooking stoves using a variety of biomass fuels. Although there continue to be uncertainties concerning each of these types of effects, there is consensus that air pollution is linked to a number of effects, e.g.:
- exacerbation of respiratory and cardiovascular disease, and premature mortality associated with exposure to ambient pollution,
- enhanced infectious disease, blindness, and respiratory effects from exposure to indoor air pollution, and
- emerging evidence of immune system and developmental effects.
In general, the effects are smaller than those attributed to several other causes (e.g. smoking). However, recent epidemiologic evidence in the United States (1) suggests that the widespread exposures, and the likely magnitude of effects, would result in these effects being measurable on a global scale.
Effects of Different Ambient Pollutants. Although there is general consensus that pollution is linked to health effects, the studies to date have found different effects attributable to different ambient air pollutants, and with differing degrees of magnitude for those effects (2,3). There are also differences in the degree to which ambient air pollutants might directly affect climate change: some pollutants for which there are well-understood health effects (e.g. sulfur dioxide and lead) have not been linked directly to climate forcing, while others (e.g. aerosols and ozone) have been more directly linked.
Figure 1 summarizes in general form the effects that have been attributed to ambient air pollutants that have also been associated with climate forcing. Among those effects ascribed to these pollutants are:
- Black Carbon aerosol — premature mortality, respiratory and cardiovascular hospitalization, lung cancer (diesel exhaust), allergy exacerbation;
- Sulfate aerosol — premature mortality, respiratory hospitalization, lung cancer, allergy exacerbation;
- Ozone — respiratory and cardiovascular hospitalization; allergy exacerbation;
- NOx — respiratory hospitalization and allergy exacerbation; (also a precursor);
- CO — premature mortality and cardiovascular hospitalization
Effects of Indoor Pollutants. In addition to exposures to ambient pollution, populations are exposed to a variety of indoor air pollutants as a result of cooking, smoking, and other activities. In most instances, these exposures, which are a combination of ambient and internally-generated emissions, are significantly larger than those experienced outside the home. This is especially true in the developing world, where the primary sources of cooking energy and heat is the inefficient combustion of wood, poor quality coal, and biomass such as animal dung. In these interior settings, there is evidence of significant health effects due to the exposure, especially among women, who are the operators and users of the cooking combustion sources, and their children. Among other potential health effects, exposure to these largely carbonaceous aerosol and gaseous pollutants has been found most strongly to be associated with acute respiratory infections in young children and chronic obstructive pulmonary disease and lung cancer in adult women. There is also moderate evidence of enhancement of tuberculosis, cataracts and increased blindness, and asthma attacks, and emerging evidence of enhanced promotion of infectious disease (4).
-- FIGURE 1 -- Figure 1. Relative Strength of Evidence of Health Effects from Different Ambient Air Pollutants
Estimating the Public Health Impact. Two approaches for estimating the public health impact were described during the meeting.
The first is efforts now underway under the auspices of the World Health Organization to estimate the Global Burden of Disease (GBD) for a variety of preventable exposures, including indoor and outdoor pollution (5). Based on synthesis of currently available epidemiology and economics-based estimates of ambient air pollution levels worldwide, that analysis has yielded a preliminary estimate, made using PM as an indicator of air pollution, that 1%-4% of mortality is attributable to exposure to particulate matter air pollution (PM). Although data is available on a variety of other health effects (e.g. respiratory hospitalization and ozone exposure) the GBD effort has not yet made comparable global estimates of impact, nor of other measures of impact (e.g. quality adjusted life years or QALYs). Although these estimates of effect have been capable of linking these effects to specific components of the complex mixture that makes up PM. Comparable efforts are underway to estimate the Global Burden of disease from indoor air pollution.
The second approach has been to estimate the benefits of specific interventions, for example in China and Chile. In these analyses, substantial benefits have been estimated, again primarily based on estimates of avoided premature mortality associated with reductions in exposure to PM. To date, these estimates have been based on extrapolation from effects documented in two studies of long term exposure and mortality in the United States (6,7) and to some extent using local health studies. Although these estimates use care in estimating effects in developing countries based on United States data, they would undoubtedly be improved with enhanced availability of epidemiologic studies in the developing countries themselves.
What Do We Need To Learn? Based on the presentations and discussions at the workshop, several key needs emerged for future research:
- Better Understanding of Effects of Different PM Components and Sources. It is clear that one of the principal needs to better examine the relative co-benefits and disbenefits of reducing ambient pollutants from both a climate and health basis will be to better determine the relative toxicity of the different air pollution components. While there are today relatively well documented effects of several components of the mixture — e.g., traffic emissions and diesel particulate matter, sulfates, and ozone — it is difficult in the current literature to ascribe stronger effects to one or another of these pollutants. This will be a significant challenge, in part because of the significant collinearity of exposure, and in part due to the diversity of effects and of potentially susceptible populations. Even as initial actions to reduce exposure move forward (e.g. tougher standards in the United States, Europe, and Japan for heavy-duty diesel engines), this set of constituents will need systematic epidemiology and toxicology studies over the next decade to identify relative toxicity and inform future control strategies (8).
- Improved Concentration-Response Data in Developing World. Eighty percent of current time series studies of air pollution effects, and 100% of cohort studies, have been conducted in Europe and North America. Although there are today reasonable methods of extrapolating from these studies to the developing world, the accuracy of estimated effects, and the willingness of local political leaders to accept the results, will be enhanced with increased use of local studies.
- Estimates of Morbidity Impact for PM and Ozone. To date, most efforts to estimate the health benefits of reducing air pollution have been estimates of the reductions of premature mortality likely to result from pollution reductions. Although these effects are significant, and likely carry the largest estimates of economic benefits, it is also important to estimate the likely benefits from reduced hospitalizations and reduced loss of work days as a result of pollution reductions. This is feasible given existing health and exposure data, but requires time and resources in order to be implemented. It is important to note that these impacts, once estimated, are likely to be large in number, but not in economic value (9).
- Better understanding of exposure to, and effects of, biomass source ambient BC, e.g., from residential fuel and wood smoke.
- Better understanding of synergistic effects. Populations are subjected simultaneously to multiple pollutant exposures. There is the potential of one pollutant to activate and/or facilitate the effects of others (e.g., PM as the "escalator" for the introduction of metals, organics, and gases into the deep lung).
How Do the Atmospheric and Health Communities Need To Work Together? The successful addressing of these questions will require enhanced collaboration among the health and atmospheric communities. This will require (a) developing common understandings of key facts, e.g., what is black carbon and how best to represent it in health studies, (b) identifying atmospheric data sources that could sharpen future health analyses, e.g., atmospheric sulfate, BC data sets that might be applied in health studies to help distinguish effects, (c) the bringing together of source-specific climate forcing data with source-specific health analysis, and (d) developing better means to communicate results — in an understandable fashion — to decision makers in the developed and developing worlds.
Summary and Conclusions. There is evidence of health effects from both aerosol and gaseous pollutants to which populations are exposed both outdoors and indoors. To date there is evidence of effects from most components (constituents) of the aerosol mixture, and it has not been possible to disentangle the effects of one component from the others. Estimates of mortality effects due to worldwide and local PM pollution have been made, suggesting substantial effects on premature mortality (1%-4% of all mortality depending on region). Estimates of morbidity effects from PM and ozone have yet to be made. Looking forward, there is a need for improved efforts to distinguish the effects of different components of the PM aerosol, increased number of studies of effects in developing countries, and better understanding of the effects of aerosol from biomass combustion. Enhanced cooperation among atmospheric and health scientists will facilitate the timely addressing of these remaining questions.
- Pope CA, Burnett RT, Thun MJ, Calle EE, Krewski D, Ito K, Thurston G. Lung Cancer, Cardiopulmonary Mortality, and Long-term Exposure to Fine Particulate Air Pollution. JAMA 2002; 287(9):1132-1141
- National Research Council, Committee on Research Priorities for Airborne Particulate Matter. Research priorities for airborne particulate matter: I. Immediate priorities and a long-range research portfolio. Washington, DC: National Academy Press, 1998.
- Holgate ST, Samet JM, Koren HS, Maynard RL, eds., Air Pollution and Health. Academic Press, San Diego CA. 1999
- Smith K, Estimating the Global Burden of Disease from Indoor Air Pollution, Abstract, Air Pollution as a Climate Forcing — A Workshop; Honolulu, Hawaii: May 2002
- Dockery DW, Pope CA, III, Xu X, et al. An association between air pollution and mortality in six U.S. cities. N. Engl. J. Med. 1993; 329:1753-9.
- Krewski D, Burnett RT, Goldberg MS, et al. Reanalysis of the Harvard Six Cities Study and the American Cancer Society Study of particulate air pollution and mortality. Special Report. Boston, MA: Health Effects Institute, 2000. (http://www.healtheffects.org/Pubs/Rean-ExecSumm.pdf).
- Health Effects Institute, Understanding the Toxicity of Components of the Particle Mix: Recent Progress and Next Steps, Perspectives Series No. 2, Boston, MA: Health Effects Institute, 2002
- United States Environmental Protection Agency, Final Report to Congress on Benefits and Costs of the Clean Air Act, 1970 to 1990; 1997: EPA 410-R-97-002; http://www.epa.gov/oar/sect812/