Air Pollution and Cardiovascular Disease: The Saga Continues
Pub Date: Monday, May 10, 2010
Author: Jonathan M. Samet, MD, MS
Brook RD, Rajagopalan S, Pope CA 3rd, et al. Particulate matter air pollution and cardiovascular disease: an update to the scientific statement from the American Heart Association. Circulation 2010. Epub May 10, 2010. 10.1161/CIR.0b013e3181dbece1.
This year, 2010, marks the 40th anniversary of the passage of the 1970 Clean Air Act Extension and the founding of the U.S. Environmental Protection Agency. Over that span, the standards covering the major air pollutants, the National Ambient Air Quality Standards (NAAQS), have become ever more stringent, driving improvements in air quality that date to the 1950s and 1960s. Yet, while air quality has greatly improved in the United States and in much of Europe, epidemiologic studies continue to link outdoor air pollution at current levels to a wide range of adverse health effects. Much of the recent evidence relates to effects of air pollution on the cardiovascular system, a lesser focus of research in the past.
This commentary accompanies the publication of the second American Heart Association (AHA) Scientific Statement on Particulate Matter Air Pollution and Cardiovascular Disease. The first, published in 2004, was written in response to the emerging literature on airborne particulate matter and cardiovascular disease. Its authors concluded that short-term exposure to airborne particles increases acute cardiovascular mortality as well as hospital admissions, and that long-term exposure reduces life expectancy. The implications of this evidence were noted: the NAAQS need to be met and to be sufficiently stringent; clinicians and patients with cardiovascular disease need to understand the implications of exposure to particulate matter air pollution; and research is needed to meet persistent gaps in scientific understanding of airborne particles and health. The statement has been widely and increasingly cited (Figure 1).
Figure 1. Citation analysis for Brook et al. Air pollution and cardiovascular disease: a statement for healthcare professionals from the Expert Panel on Population and Prevention Science of the American Heart Association, 2004.
The second statement, published 6 years later, provides a comprehensive, though not exhaustive, review of the new evidence. Since 2004, the literature has expanded rapidly. A PubMed search on "particulate matter air pollution and cardiovascular disease" identifies 534 citations for the period 2004 to 2010. The new statement cites 426 references, compared with 194 in the first. It offers more certain conclusions and expands the list of adverse cardiovascular outcomes associated with particulate matter air pollution. The authors also comment on the advances in understanding of mechanisms by which inhaled particles cause adverse cardiovascular effects. As a major, overarching finding, the statement concludes "...that the overall evidence is consistent with a causal relationship between PM2.5 exposure and cardiovascular morbidity and mortality."
This same evidence is now under consideration by the Environmental Protection Agency, as it moves through the process by which it assesses the adequacy of the public health protection provided by the NAAQS for particulate matter. The process begins with a systematic review of the evidence since the last assessment, completed in 2004. In a new agency process, the first document is the Integrated Science Assessment (ISA), which reviews and synthesizes the evidence and reaches conclusions on the strength of evidence for causation of adverse health effects. The process is comparable to that long incorporated in the U.S. Surgeon General's reports on smoking and health. In the recently finalized ISA, the Environmental Protection Agency provided conclusions on the strength of evidence for causation of adverse health effects by PM2.5 (particulate matter less than 2.5 microns in aerodynamic diameter-"fine" particles), PM10-2.5 (particulate matter in the size range between 2.5 and 10 microns in aerodynamic diameter-"coarse" particles), and ultrafine particles (particulate matter less than 0.1 microns in aerodynamic diameter) (Table 1). The conclusions for PM2.5 are comparable to those reached in the updated AHA statement.
The next step in the process for considering revision of the NAAQS is the Quantitative Health Risk Assessment. The agency uses risk analysis to estimate the burden of morbidity and premature mortality associated with airborne particulate matter and to explore the extent to which this burden can be reduced under various alternative NAAQS. The key outcome considered in this analysis is ischemic heart disease mortality, using the estimate of PM2.5 associated risk from the American Cancer Society cohort study.[8,9] This most recent analysis of data from this key study takes other factors into account to the extent possible and also makes refined adjustments for statistical complications of the data.
For airborne particles, the agency finds that the evidence supports the assumption of a linear, no-threshold relationship between concentration and risk for adverse health effects. This assumption implies that any increment of exposure beyond background conveys some risk to health and that a threshold concentration below which health effects do not occur cannot be identified. Under the Clean Air Act, the administrator is called on to set NAAQS that protect health with "an adequate margin of safety." With the assumption of a no-threshold model for risk, the administrator cannot directly meet this mandate and the Quantitative Health Risk Assessment provides a basis for promulgating NAAQS that best protect the public's health.
In the final translational step, the Policy Analysis considers the current and alternative NAAQS for particulate matter. The Policy Analysis draws on the Quantitative Health Risk Assessment in considering potential changes to the four elements of the NAAQS: the indicator used (e.g., PM2.5), the statistical form (e.g., 99th percentile value), the averaging time, and the concentration. The Policy Analysis is the foundation for the administrator's policy judgment in promulgating NAAQS that meet the public health mandate of the Clean Air Act. While the Policy Analysis for particulate matter is still undergoing revision, its analyses suggest a need for lowering the concentrations for the 24-hour and annual standards.
This most recent consideration of the NAAQS for particulate matter comes nearly two decades after concerns were raised about the health effects of airborne particles by the findings of time-series studies of daily mortality and air pollution (see Pope and Dockery 2006  for a review). Wide-ranging research has followed; the Environmental Protection Agency has funded centers for research on particulate matter; and the National Research Council set out a 13-year agenda for research to reduce key uncertainties. Nonetheless, uncertainties persist and the AHA statement offers recommendations for further research. Research on airborne particles is complicated by the diversity of their sources, their complex chemical and physical characteristics, and their presence in heterogeneous mixtures with other pollutants. Identifying those characteristics that determine the toxicity of particles has been a high priority, so that the sources contributing to risk can be targeted for control.
While the AHA statement addresses particulate matter in outdoor air, the population is exposed to other types of particles that also have adverse health effects. Some are regulated: workplace exposures to coal dust, asbestos, and silica and the exposure of the public to secondhand smoke in workplaces and public places. In homes, cooking and hobby activities generate particles and exposure to secondhand smoke persists around the world for millions of people, particularly women and children.[13,14] Worldwide, exposure to smoke from biomass fuel combustion remains a major contributor to acute respiratory illness in infants and children and to risk for chronic obstructive pulmonary disease and lung cancer in adults.[15-17] The cardiovascular consequences of active smoking and of exposure to secondhand smoke are well documented.[5,18] The findings on particulate matter pollution of ambient air suggest that attention should also be directed at potential cardiovascular disease risk associated with these other airborne particles.
In closing, Brook and colleagues offer another landmark review in this updated AHA statement. Like its predecessor, the new statement will be an invaluable resource for researchers and clinicians and for informing policymakers, patients, and the public at large about the cardiovascular consequences of airborne particles. Like its predecessor, it will be widely cited. Perhaps, by the third or fourth revision, the remaining questions about particulate matter air pollution and health will be answered or made irrelevant by further gains in air quality.
- U.S. Environmental Protection Agency. National Ambient Air Quality Standards (NAAQS). Available at: http://www.epa.gov/air/criteria.html.
- Bachmann J. Will the circle be unbroken: a history of the U.S. National Ambient Air Quality Standards. J Air Waste Manag Assoc 2007;57(6):652-697.
- Brook RD, Rajagopalan S, Pope CA III, et al. Particulate matter air pollution and cardiovascular disease: an update to the scientific statement from the American Heart Association. Circulation 2010; this issue.
- Brook RD, Franklin B, Cascio W, et al. Air pollution and cardiovascular disease: a statement for healthcare professionals from the Expert Panel on Population and Prevention Science of the American Heart Association. Circulation 2004;109(21):2655-2671.
- U.S. Department of Health and Human Services. The health consequences of smoking. A report of the Surgeon General. Atlanta, GA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, Office on Smoking and Health; 2004.
- U.S. Environmental Protection Agency. Integrated science assessment for particulate matter (final report). Washington: U.S. Environmental Protection Agency; 2009. EPA/600/R-08/139F.
- U.S. Environmental Protection Agency. Quantitative health risk assessment for particulate matter (second external review draft). Washington: U.S. Environmental Protection Agency; 2010. EPA-452/P-10-001.
- Pope CA III, Burnett RT, Thun MJ, et al. Lung cancer, cardiopulmonary mortality, and long-term exposure to fine particulate air pollution. JAMA 2002;287(9):1132-1141.
- Krewski D, Jerrett M, Burnett RT, et al. Extended follow-up and spatial analysis of the American Cancer Society study linking particulate air pollution and mortality. Res Rep Health Eff Inst 2009(140):5-114; discussion 115-136.
- 10. U.S. Environmental Protection Agency. Policy assessment for the review of the particulate matter national ambient air quality standards (first external review draft). Washington: U.S. Environmental Protection Agency; 2010. EPA 452/P-10-003.
- Pope CA III, Dockery DW. Health effects of fine particulate air pollution: lines that connect. J Air Waste Manag Assoc 2006;56(6):709-742.
- National Research Council. Research priorities for airborne particulate matter: IV. Continuing research progress. Washington: National Academies Press, 2004.
- World Health Organization. WHO report on the global tobacco epidemic, 2009: implementing smoke-free environments. Geneva: World Health Organization, 2009.
- Wipfli H, Avila-Tang E, Navas-Acien A, et al. Secondhand smoke exposure among women and children: evidence from 31 countries. Am J Public Health 2008;98(4):672-679.
- World Health Organization. The global burden of disease: 2004 update. Geneva: World Health Organization, 2008.
- Dherani M, Pope D, Mascarenhas M, et al. Indoor air pollution from unprocessed solid fuel use and pneumonia risk in children aged under five years: a systematic review and meta-analysis. Bull World Health Organ 2008;86(5):390-398C.
- Zhang JJ, Smith KR. Household air pollution from coal and biomass fuels in China: measurements, health impacts, and interventions. Environ Health Perspect 2007;115(6):848-855.
- U.S. Department of Health and Human Services. The health consequences of involuntary exposure to tobacco smoke. A report of the Surgeon General. Atlanta, GA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, Coordinating Center for Health Promotion, National Center for Chronic Disease Prevention and Health Promotion, Office on Smoking and Health; 2006.
-- The opinions expressed in this commentary are not necessarily those of the editors or of the American Heart Association --