Inside EPA’s
Risk Policy Report
April 18, 1997 * Volume 4, Number 4
Commentary
A Technically Supportable Middle Ground in the Particulate Matter Debate
By Robert A. Michaels, PhD, CEP
The controversy about a new National Ambient Air Quality Standard (NAAQS) for airborne particulate matter (PM) proposed by the U. S. Environmental Protection Agency (EPA) has polarized industry and environmental interests. However — with interest from industry, federal and state regulators, and congressional staff — a potential middle ground is garnering bilateral support, specifically, refocusing the EPA proposal on a one-hour particle mass limit to augment the one-day PM-10 limit already in effect. This may seem like mere bookkeeping, but it might save many of the 60,000 PM-associated deaths annually which the EPA and environmental advocates attribute to PM, and many of the billions of dollars which industry representatives claim must be expended to comply with EPA’s current NAAQS proposal. This Commentary addresses the one-hour alternative to tightening the 24-hour PM standard.
EPA proposes to tighten the existing NAAQS by controlling 24-hour PM-2.5 to a more stringent degree than is accomplished by the current 24-hour PM-10 NAAQS of 150 micrograms/cu M. The basis for this proposal is EPA’s belief that reported associations of death and other adverse health effects with marginal elevations of daily (24-hour average) PM are causal and attributable to those elevated daily PM levels. However, two observations cast doubt upon this belief. First, epidemiological data suggest the absence of a threshold for the non-cancer effects of PM, whereas non-cancer effects typically do have thresholds. Second, animal bioassays fail to confirm that marginal elevations of PM above background can cause adverse effects, let alone deaths. Accordingly, EPA concluded in its Criteria Document, which elucidates technical issues which may form the basis for PM risk management policy options, that the causality of the associations, and therefore the technical basis for the proposed NAAQS, both are doubtful. EPA’s Staff Paper, setting forth PM risk management policy options for selection by the EPA Administrator, fails to adequately reflect this important Criteria Document conclusion (Risk Policy Report, July 19, 1996, p34).
A detailed technical explication justifying the one-hour PM risk management alternative is beyond the scope of a Risk Policy Report Commentary. However, in brief, the effects associated with daily average PM elevations are also consistent with a model involving causation by higher concentrations of PM acting over shorter durations. Higher PM concentrations occurring briefly (‘excursions’) would be nearly invisible to 24hour average PM measurements. Four criteria would strengthen the short-term causation model: 1) if real-time measurements of PM concentrations include brief excursions well above the 24-hour PM average; 2) if exposing animals or humans briefly (up to a few hours) to the higher PM concentrations attained in such excursions can cause adverse health effects; 3) if the severity of PM effects is greater when a given time-weighted average particle concentration includes excursions than when exposure to the same time-weighted PM average is constant over the full exposure duration; and 4) if adverse effects, though more severe with longer exposure, are more strongly associated with short-term exposures when exposure duration is normalized, for example, quanti-
Dr. Michaels is a toxicologist and health risk assessor with the Schenectady, New York-based RAM TRAC Corporation. His work was supported in part by the Albany, New York-based Rupprecht & Patashnick Co., Inc.
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Commentary
fied as the degree of effect per unit of exposure time (such as per hour).
Distinguishing between the 24-hour vs. the short-term causation model is essential for selecting a PM risk management strategy because tightening the 24-hour PM-10 or PM-2.5 limit will fail to protect public health if the short-term model is correct. Its correctness, however, would be good news for all parties, because the short-term model suggests a dramatically more economical PM risk management strategy. Specifically, if brief PM excursions are causing the adverse effects, the effects can be averted by controlling the excursions (to a one-hour maximum of perhaps 300 micrograms/cu M for PM-10 or 150 micrograms/cu M for PM-2.5) and leaving the 24-hour limit unchanged. In short, a one-hour NAAQS would prevent deaths caused by PM excursions lasting from minutes to an hour, caused by PM reaching levels known to have killed people, such as during the famous two-week London fog in 1952, in which approximately 4,000 people died.
If PM-associated deaths are indeed caused by brief PM excursions rather than sustained but smaller PM elevations, then tightening control over PM in the 2.5-micron or 10-micron range as proposed will save few if any lives because it fails to control the excursions. In many if not most industries, controlling PM excursions can be achieved more economically than controlling daily averages, because industrial processes often include brief periods of reduced PM control, when excursions occur. For example, plastics factories may produce several types of plastics using the same (extrusion) equipment operated with resins at different temperatures. Shifting to a new plastic type requires purging the
Refocusing the EPA proposal on a one-hour
particle mass limit... previous resin, changing the temperature, and running the equipment with the might save many of the new resin. PM emissions during these transitional phases of operation can be 60,000 PM-associated controlled more economically because they require improvements to only a
’ small part of overall operations. For example, plastics factories can run resins deaths annually . . . and requiring higher temperatures after first running lower-temperature resins, or many of the billions of they can assure adequate temperature reduction prior to introducing lowerdollars which industry temperature resins.
representatives claim The EPA has so far Tejected the one-hour PM risk management option because findings supporting it had not been reported in a peer-reviewed must be expended. journal by the April finalization date of the PM Criteria Document. However, in November the peer reviewed journal Aerosol Science and Technology published my report documenting brief excursions reaching up to twice the PM levels of the 1952 London fog (criterion 1; see above). Several reports (some postdating the Criteria Document) now clearly indicate that brief exposures to PM levels reached in excursions have caused adverse effects in animals and humans, and death in animals (criterion 2). One report (by Michaels and Kleinman, in press) now reveals greater effect severity (in rats) when a given time-weighted average PM concentration includes excursions compared to when exposure is constant (criterion 3). Finally, one report (by Delfino, et al.; in press) reveals that symptom severity in asthmatics is associated with PM elevations, but that the strength of the associations depends upon the PM averaging time (criterion 4). Significantly, the strength of the PM association with symptom severity observed by Delfino, et al. increased as the PM averaging time changed from 24-hour to eight-hour to one-hour.
Even though PM exposure normalized per unit of time may be more strongly associated with health effects when PM averaging times are shorter, as described above, continued exposures do increase effect severity, raising the issue of whether short-term effects meet established criteria of effect severity to justify standard setting under the Clean Air Act. Accordingly, I examined regulatory criteria recommended by the American Thoracic Society (ATS) and adopted by the EPA in setting forth evidentiary criteria for regulation under the Clean Air Act. This analysis revealed that currently available evidence should be sufficient to justify EPA action under the short-term causation model. Indeed, the evidence for the short-term model now appears to be stronger than that for the longer-term causation model, which attributes serious effects to unreasonably low PM levels. This issue will be further addressed at a session of the annual Air and Waste Management Association meeting (in Toronto this June), which I will co-chair with Michael Kleinman of UC/Irvine, dedicated to the ‘dose-rate’ issue.
The EPA requested ATS to define toxicological effects, and degrees of effect, which might form the basis for limiting air pollutants, and both ATS (in 1985) and EPA (in 1989) have published these guidelines. Their implementation leaves much to the discretion of the implementor, but the guidelines are prescriptive and valuable nonetheless. According to EPA policy, air pollution limits must be set based upon a clinically significant adverse effect, not any
effect, as defined by ATS: “We define ‘adverse respiratory health effects’ as medically significant physiologic or pathologic changes . . .” (source document, p666).
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Commentary
Controlled human exposure studies focusing upon asthmatics during exercise and/or at rest reveal symptoms which are milder than (for example) death and asthma attacks, such as bronchoconstriction, increased airway resistance, reduced FEV1 (Forced Expiratory Volume at 1 second interval) and FEV3 (at 3 second interval), reduced FVC (Force Vital Capacity), and reduced peak flow. ATS and EPA resolve the issue of whether these less severe effects can form the basis for air pollutant standard setting under EPA’s Clean Air Act implementation policies in a manner consistent with ethical constraints which preclude allowing controlled clinical exposures to progress far enough to include triggering an asthmatic attack. According to EPA and ATS guidelines, this clinical endpoint need not be experimentally induced to be used in standard setting. Rather, the guidelines justify inferring greater severity of response with greater exposure. According to ATS: “[The] spectrum of biologic responses would be evoked more strongly as the concentrations of pollutants are increased” (ATS, 1985, p666).
The Clean Air Act requires EPA to protect public health with a margin of safety to anticipate future research findings, that is, based upon both published and emerging research. Although more research will be necessary to definitively quantify the public health significance of PM excursions, many now believe that the threshold for cautionary regulation already has been crossed. Given the magnitude of public health and economic implications of the new NAAQS, EPA should rise to the procedural challenge of considering evidence post-dating the Criteria Document, and thereby tailor the new PM NAAQS to pro-actively meet the needs of public health and economics simultaneously.
Literature Cited
American Thoracic Society. Guidelines as to what constitutes an adverse respiratory health effect, with specific reference to epidemiologic studies of air pollution. American Reviews of Respiratory Disease, 131:666-8, 1985;
Delfino, R. J.; et al. Effects of PM-10 and ozone on symptoms and peak expiratory flow in asthmatics living at the inversion base. Abstract. Proceedings of the American Lung Association and American Thoracic Society International Conference; San Francisco, California; 17-21 May 1997;
Michaels, R. A. Airborne particle excursions contributing to daily average particle levels may be managed via a 1-hr standard, with possible public health benefits. Aerosol Science and Technology, 25:437-44, November 1996;
Michaels, R. A.; and M. T. Kleinman. Dose-rate dependent health risks vs. brief airborne particle excursions: model and evidence support one-hour control. Proceedings of the 22nd Annual Meeting of the National Association of Environmental Professionals; Orlando, Florida; 21 pp., 19-23 May 1997;
Michaels, R. A.; and M. T. Kleinman. Health Risks Support A One-Hour Airborne Particle Standard with Data Acquisition Via Rapid Automated Monitoring Instrumentation. Proceedings of the 90th Annual Meeting and Exhibition of the Air & Waste Management Association; Toronto, Ontario, Canada; 21 pp., 8-13 June 1997;
U.S. EPA. Interim Methods for Development of Inhalation Reference Doses. Washington, DC; Office of Health and Environmental Assessment, EPA/600/8-88/066F, PB90-145 723, i. p., August 1989.
RISK POLICY REPORT - April 18, 1997 37
Publication citation
Michaels, RA. A technically supportable middle ground in the particulate matter debate. Risk Policy Report, 4(4):35-7, 18 April 1997; Commentary.