ES&T VIEWs
Revising the Particulate AmbientAirQuality Standard SCIENTIFIC AND ECONOMIC DILEMMAS
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BY S. K. F R I E D L A N D E R AND M O R T O N L I P P M A N N
ecent epidemiological studies indicate that increases in human mortality [1-9) and morbidity (10-23) have been associated with levels of particulate pollution significantly lower than those previously thought to affect h u m a n health. If the evidence withstands critical evaluation, we will probably need to revise downward the National Ambient Air Quality Standard (NAAQS) for particulate matter (PM), even though the scientific basis for making the decision is weak. Tightening the standard will have significant economic conse148 A
quences for U.S. industry and will affect air pollution standards internationally. Scientific and economic dilemmas may turn out to be minefields, unless we think the problem through carefully. Over the periods 1978-82 and 1983-87, we chaired the Clean Air Scientific Advisory Committee (CASAC) set up by Congress to advise the EPA administrator on the ambient air quality standards. Our committee met regularly to review criteria documents prepared by the EPA Office of Environmental Criteria and Assessments and staff papers prepared by the EPA Office of Air Quality Planning and Stan-
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dards. The criteria documents summarize the effects of certain pollutants on human health and welfare. ("Welfare" refers to effects other than human health, including visibility and plant damage.) The staff papers summarize the literature most directly related to the setting of the standard. The pollutants currently covered by the NAAQS are ozone, nitrogen oxides, carbon Views are insightful commentaries on timely environmental topics, represent an author's opinion, and do not necessarily represent a position of the society or editors. Contrasting views are invited.
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monoxide, lead, sulfur oxides, and particulate matter. The criteria documents The criteria documents, multivol ume works on each pollutant, and the staff papers were reviewed in public sessions at which environ mental organizations, i n d u s t r y trade associations, and individual citizens made presentations. CASAC then prepared reports to the administrator, commenting on the scientific databases in the docu ments and papers and including the recommended concentration ranges over which the standards could rea sonably be set. It was the responsi bility of the administrator to make the final selection of the standard. The Clean Air Act called for peri odic reviews of the standards on a five-year cycle, but this provision has not been strictly observed (24). The standard-setting process that developed was lengthy and some times tedious, but the CASAC par ticipation succeeded in defusing what had often been acrimonious disputes involving industry, the government, and environmental groups. These disputes had spilled over into the law courts, further de laying decisions on standards. The development of air quality standards for regulating particulate air pollution is a daunting task. The atmospheric aerosol is composed of particles ranging in size from a few nanometers to 50 μπι, and each par ticle may consist of a mixture of chemical species. The material composing the aerosol comes from many different sources that can be identified, in part, by source attri bution methods based on chemical signatures (25, 26). Much of the mass of the atmospheric aerosol, es pecially the submicrometer portion, is the result of gas-to-particle con version. Even if all particulate mat ter were removed from emissions to the atmosphere by gas-cleaning de vices, a substantial portion of the submicrometer mass would remain because of the conversion of sulfur and nitrogen oxides and organic va pors to particulate matter [27, 28). Nonspecificity of particulates The particulate standard is the only ambient air quality standard that is not chemically specific; one of its components, lead, is itself reg ulated independently of the rest of the aerosol. The concentrations of other components, such as sulfates, nitrates, and organics resulting from gas-to-particle conversion, are af
fected by controls designed to ad dress the NAAQS for sulfur and ni trogen oxides and ozone. Although there have been significant ad vances in our understanding of the dynamics of the atmospheric aero sol, the chemical and biochemical processes that underlie its effects on human health are very poorly un derstood; the standard is based on the total mass of the size fraction that penetrates into human lungs (< 10 μπι in aerodynamic diameter). Indeed it may be that the health ef fects of the atmospheric aerosol do
cals and other metastable compo nents, may be much more biochem ically active t h a n the aerosol components measured many hours, days, or weeks later. The deposited aerosol is composed of the end products of the atmospheric chemi cal and physical processes that gen erated the short-lived components. Biochemically active components may have been present at the parti cle surface or inside the particles, or may originally have been present in the gas before being converted to particulate matter. Such uncertain ties have long hampered efforts to develop scientifically based stan dards for particulate air pollution.
The paniculate standard is the only ambient air quality standard that is not chemically specific. not depend on the specific chemical components. There have been con tinuing efforts to identify the role of biologically active chemical species (such as sulfuric acid) that may cause a disproportionate fraction of the health effects. In evaluating the health effects of aerosol chemical components, we should not rely entirely on the chemical analysis of particulate matter collected routinely from the atmosphere. The chemical analysis is usually conducted long after the aerosol is sampled on filters or other sampling devices. Short-lived chemical species in the gas and/or aerosol phase, including free radi
Health effects The recent epidemiological stud ies (1-23) indicating health effects at mass concentrations of particu late matter below the existing stan dards are of two types: those that fo cused on the h e a l t h effects of particles emitted by a steel mill op erating in a valley in Utah and those that focused on mortality and mor bidity associated with particles in urban atmosphere. It is unlikely that the new evidence will be the "smoking g u n " that points to a "killer aerosol." We can only specu late on what special aerosol proper ties, if any, caused the observed health effects. Perhaps changes in steel mill operating conditions have led to increased emissions of submicron aerosols, which are likely to pose a greater health risk than the same mass of coarse particles. Per haps there have been changes in the composition of urban aerosols re sulting from changing automotive and power plant fuels. Despite these uncertainties, the epidemiological data are suffi ciently coherent to compel us to re view the existing standard for par ticulate matter (29). A significant downward revision in the standard will probably cost industry (and the public) billions of dollars. Height ened concern about the effects of air pollution standards on the competi tiveness of U.S. manufacturers has been reflected in the bitter debate over the environmental conse quences of the North American Free Trade Agreement. Similar concerns appear in the 1990 Clean Air Act (30), which calls for the "harmoni zation" of air pollution standards among the United States and its trading partners. There are many precedents for international cooper ation on air quality. An example is the Montreal Protocol, in which the
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advanced industrial nations agreed to limit atmospheric emissions of fluorocarbons. The scale of the ef fects of particulate pollution ex tends from local to global, so inter national cooperation is justified for scientific and economic reasons. The harmonization of standards among nations called for in the Clean Air Act does not necessarily mean equalization. Harmonization will require some rough parity among the standards set by the par ticipating nations. Harmonization will also stimulate development of internationally compatible monitor ing and measurement networks and the regular exchange of information on pollutant concentrations. Such information will be required in tracking transfrontier air pollution, an important issue for both North America and Europe (31). In the case of particulate pollu tion, the experience of other nations has always influenced the U.S. cri teria documents and standards. Of special importance was the London smog episode of 1952 in which 4000 excess deaths were reported. Because the biological mechanisms for the effects of the pollutants were not understood, interpreting the fragmentary data on relationships between health statistics and pol lutant concentrations was very con troversial. British scientists pre pared a lengthy critique of the draft U.S. criteria document for particu late matter and sulfur oxides (32) in which they argued that the London data were used to attribute human health effects at unrealistically low particulate concentrations (33). Steps for revisions It will be in the general interest to broaden our next round of particu late standards setting to include wider international participation. This could be accomplished in a three-stage process. The first stage would involve joint workshops sponsored by the United States, the European Community, and other in terested parties with the aim of reaching consensus on a common set of data on which to base the standards. Research needs for the characterization of the atmospheric aerosol and its biomedical effects should be identified. This program should be initiated in a timely fash ion to help guide the standardsetting process and continued even after the standard is set until the sci entific issues are resolved. In the second stage, each country or group of countries would set its
own standard and the associated air-monitoring and measurement systems. Although this stage should involve consultation among the par ticipating nations, each country would set standards in its own way, following its own timetable. In the final stage, there should be fol low-up discussions on methods of implementation and comparison of results for air quality and health and welfare effects. Broadening the standard-setting procedure to include more interna tional participation will help us deal with an issue of enormous sci entific complexity and major inter national economic and policy im plications. It will help achieve the harmonization of standards called for by the 1990 Clean Air Act Amendments. Finally, it will help defuse contentious international is sues associated with trade and com petitiveness in much the same way that disputes about standards in the United States were reduced through the CASAC review processes.
Health 1991, 46, 135. (21) Pope, C. A. ; Kanner, R. E. Am. Rev. Respir. Dis. 1993, 147, 1336. References 22, 23 concern chronic respi ratory disease (22) Schwartz. J. Environ. Res. 1993, 62, 7. (23) Abbey, D. E. et al. /. Expos. Anal. En viron. Epidemiol. 1993, 3, 99. (24) Lippmann, M. Aerosol Sci. Technol. 1987, 6, 93. (25) Friedlander, S. K. Environ. Sci. Tech nol. 1973, 7, 235. (26) Gordon, G. E. Environ. Sci. Technol. 1988,22, 1132. (27) Hering, S. V.; Friedlander, S. K. Atmos. Environ. 1982, 16, 2647. (28) Seinfeld, J. H. Atmospheric Chemistry and Physics of Air Pollution; Wiley: New York, 1986. (29) Bates, D. V. Environ. Res. 1992, 59, 336. (30) Clean Air Act, Public Law 101-549, Nov. 15, 1990, Section 811. (31) R a n d C o r p o r a t i o n W o r k s h o p on Transfrontier Air Pollution and the Law of the Atmosphere, May 1993, Delft, The Netherlands. (32) "U.S. EPA Air Quality Criteria for Particulate Matter and Sulfur Diox ides"; U.S. Environmental Protection Agency: Washington, DC, Dec. 1982; EPA-600/8-82-029a,b,C (33) Holland, W. W. et al. Am. J. Epide miol. 1979, 110.
References (1)
Ozkaynak, H.; Thurston, G. D. Risk Anal. 1987, 7, 449. (2) Archer, V. E. Arch. Environ. 1990, 45, 325. (3) Fairley, D. Environ. Health Perspect. 1990, 89, 159. (4) Kinney, P. L.; Ozkaynak, H. Environ. Res. 1991, 54, 99. (5) Dockery, D. W.; Schwartz, J.; Spengler, J. D. Environ. Res. 1992, 59, 326. (6) Schwartz, J.; Dockery, D. W. Am. Rev. Respir. Dis. 1992, 145, 600. (7) Schwartz, J.; Dockery, D. W. Am. J. Epidemiol. 1992, 235, 12. (8) Ito, K.; Thurston, G. D.; Lippmann, M. Arch. Environ. Health, 1993, 48, 213. (9) Dockery, D. W. et al. Ν. Engl. J. Med. 1993, 329, 1753. References 10-14 concern hospital admis sions and emergency room visits (10) Bates, C. V.; Sizto, R. Environ. Res. 1987, 43, 317. (11) Pope, C. A. Arch. Environ. Health 1991, 46, 90. (12) Thurston, G. D. et al. /. Expos. Anal. Environ. Epidemiol. 1992, 2, 429. (13) Sunyer, J. et al. Am. J. Epidemiol. 1993, 137, 701. (14) Schwartz, J. et al. Am. Rev. Respir. Dis. 1993, 147, 826. References 15-18 concern symptoms and restricted activities (15) Ostro, B. D. Risk Anal. 1990, 10, 421. (16) Pope, C. A. et al. Am. Rev. Respir. Dis. 1991, 144, 668. (17) Ransom, M. R..; Pope, C. A. Environ. Res. 1992, 58, 204. (18) Ostro, B. D. et al. Am. J. Epidemiol., 1993, 137, 691. References 19-21 concern pulmonary function (19) Schwartz, J. Environ. Res. 1989, 50, 309. (20) Chestnut, L. G. et al. Arch. Environ.
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S. K. Friedlander is Parsons Professor of Chemical Engineer ing and director of the Aerosol Tech nology Laboratory at the University of California-Los An ge 1 es. He has worked on aerosol size distribution dynamics, discovering that very small particles in gases tend to approach asymptotic size distributions independent of their initial properties. He also introduced the widely used method of receptor modeling for relat ing air quality to emissions sources. He is currently working on the processes controlling the formation of nanophase particles and their assembly into ag glomerate structures.
Morton Lippmann is a professor at the Nelson Institute of Environmental Medicine of the New York Univer sity Medical Center. His research has been focused on particle deposition and clearance in the lungs, on the use of inert tracer aerosols as probes in in vivo airway dimensions and lung function, and on identifying and characterizing the responses of human populations in natural setting to exposures to particu late and vapor-phase pollutants.
