Weighing the Health Risks of Airborne Particulates Recent epidemiological studies link an increase in mortality and sickness to particulates, but do the data justify a tighter air quality standard? T n N Y REICHHARDT
pening a panel discussion at last spring’s meeting of the Society of Toxicology in Baltimore, moderator Joe Mauderly told his audience that the subject at handhealth risks from particulate air pollution-was “one of the most absolutely fascinating dilemmas that I’ve seen in my professional career.” Mauderly, director of the Inhalation Toxicology Research Institute in Albuquerque, NM, isn’t alone in that opinion. Increasing numbers of epidemiologists, toxicologists, atmospheric chemists, and public health officials are delving into a scientific puzzle whose answer could have profound consequences for American society: Are small particles in the air killing people? Dozens of studies conducted in the past five years suggest that they are (11. Even more disturbing, the studies show statistical associations between airborne particulate matter (PM) and increased mortality and sickness, even at levels well within current national air quality standards. Although the implied risks to individuals are small compared with health factors such as smoking, they are large compared with typical environmental risks from toxic compounds and carcinogens in the air and water. If these results stand up to scrutiny, they could have wide-ranging implications for the US. economy, because airborne particulates from many sources, including automobiles, power plants, and mines, could be subject to tighter regulation. Roger McCleUan, president of the Chemical Industry Institute ofToxicology and member of EPAs ScienceAdvisory Board, calls the particulate problem a “multibillion dollar” question. “It‘sa great public policy issue and it has hig-stake consequences,” he says.
Epidemiological studies Research into the health effectsof PM escalated dramatically beginning in the late 1980s. Since then many researchersarnong them Douglas Dockery and Joel Schwartz of the Harvard School of Public Health. George Thurston and Kazuhiko It0 of New York University, and Arden Pope of Brigham Young University-have published epidemiological studies that have compared air quality data with health statistics for dozens of American cities, includingPhiladelphia, Detroit, Cincinnati, Minneapolis, Seattle, St. Louis, and Steubenville, OH. The U.S. studies, along with similar research in locations ranging from Brazil to Germany, consistently link higher levels of particulates to increased risks of respiratory-, cardiovascular-, and cancerrelated deaths, as well as pneumonia, lung function loss, hospital admissions, asthma, and other respiratory problems. Individuals with existing 360 A . VOL. 29. NO. 8. 1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY
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respiratory conditions appear to be most wlnerable, but the data also show sigtuficantlyhigher death rates for the general population in areas with high levels of particulate matter in the air. In one recently published study, Pope, Dockery, and colleagues examined the relationship between ambient particulate levels in 151 U.S. metropolitan areas and mortality data for more than500,000 adults enrolled in a prospective study for the American Cancer Society (ACS) (2).After controlling for smoking, education, and other risk factors, the authors concluded that “particulate air pollution was associated with cardiopulmonary and lung cancer mortality,”and that “&creasedmortalityis &sociatedwith sulfate and fine Dartidate air Dollution at levels cammanly found in U.S. cities.” Death rates in the most polluted citieswere 15%higher than the norm when sulfates were considered as the pollutant, and 17% higher when PM,, particulates (smaller than 2.5 micrometers in aerodynamic size) were considered in the statistical analyses. Along with these long-term effects, other studies show clear increases in mortality associated with short-term pollution episodes. Although this may be a case of pushing people who are already sick “over the edge,” the ACS cohort study and other similar research paint a differentpicture, says Pope. “I‘m getting more convinced as time goes on that it’s the longtern important in terns exposurethat’s of real loss of heal& and loss oflife,,, more, he says, “this really is a substantial loss of life.We’re talking an average loss of life of a year or more” in an average American city-more in areas with high levels of particulates. Pope and his colleagues have zeroed in on particulates as the culprits only after statistically factaring out the effects of other air pollutants. They point to two areas in particular-the Utah Valley region and Santa Clara, CA-where increased mortality is associated with particulates, even in the relative absence of other pollutants such as ozone and sulfur dioxide.
Skepticism and reanalpis Consistent as the epidemiological studies have heen, Pope, D ,and Schwa& still w o v that some other factormight be causing the deaths, particularIy in the case of short-term pollution episodes. They admit to two principal concerns. %e effectsmay be andbutable to artifacts of the regression method used to analyze the data, and there may be confounding factors resulting from “inadequate control of seasonal factom epidemics, other long-wavelength trends, weather variables, or other pollutants” (1).Skepticshave seized on both these areasof uncertainty to question whether
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I Palticulates in ambient sir are pmducad by a variety of anthropogenic and natural sources. nesa particlar. collected in downtown Chicago as par( d the L a bMichigan Urban Air Toxic0 Study. are sulfate (top) and aluminum resulting horn local indnshy. Filamenk in the sulfate micrograph are Teflon tibr D, Willis, Technology, lnC.
the purported associations between particulates and health risks are real. Suresh Moolgavkar at the Fred Hutcbinson Cancer Research Center in Seattle is one of those skeptics.When he and his colleagues reanalyzed Schwartz and Dockery‘s data from Steubenville and Philadelphia usingdifferent statisticalmethods, they found that “although there was an association between air pollution and mortality, it was not possible to isolate one component of air pollution as being responsible” (31. Partly to reconcile these and other conflicting results, a team led by Jonathan Samet and Scott Zeger at Johns Hopkins University has begun yet another reanalysis of the epidemiological record for PM using data provided by the original researchers. Phase one of their study, which is being conducted for the Health Effects Institute, will feed into EPKs ongoing review of the air particulates issue (see belav). Wet and Zeger will concentrate on PM-relatedmortality and morbidity data for five regions: PhiladelVOL. 29. NO. 8. 1995 /ENVIRONMENTAL
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considerablyfrom short-term ldail est impact on human health.
phia. UtahValley, St. Louis, eastern Tennessee, BUmingham, M, and Santa Clara. Like other researchers who have examined the health effects of PM, the Hopkins team faces a host of unresolved issues. One key question has to do with particle size. Does all PM lead to sickness and death? Particles bigger than 10 micrometers are not inhaled deeply into the lungs, which is why EPA switched in 1987 from an air quality standard based on total suspended particulates (TSP) to one based on PM,, (particlessmaller than 10 micrometers). But recent mortality studies, including that using the ACS cohort, have implicated even smaller particles, PM,, and her. In one H m d study, which followed BO00 adults for 16 years, PM,,5 showed a better association with increased mortality than did PM,, (4). The question ofwhich particles are most dangerous is further complicated by the lack of consistent monitoring data. Although PM,, is monitored systematically by EPA-mandated state and local air monitoring stations on a nationwide basis, PM,, is not. Furthermore, different epidemiologicalstudies have used Merent measures of particulate pollution. Some studies take ambient TSP data, then use a conversion factor to determine PM,, levels Others have used a measure known as the “coefficientof haze” as a SUTrogate for particulates As a result, some scientists are wary of drawing hard conclusions about different health effects based on particle size. “I don’t think we’ve got the data,” says McCleUan.
The nature of particulates
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PM is the only air pollutant regulated by EPA that does not specify a particular chemical composition. But included within that broad category are many diflerent types of particles, from naturally occumng windblown dust (generally the coarser particles) to anthropogeniccombustion products @ene d y the liner ones). PM,,, for example,indudes acid
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aerosols such as sulfates from the burning of sulfurcontaining fuels. Some researchers believe these acids may be causing health effects. Once again, say researchers, basic monitoringdata are lacking, as is our understandingof how different acid aerosols are transported and changed in the atmosphere. Information on exposures to PM is similarly sparse. Although few personal monitoring studies have been conducted to date, the data that exist suggest that personal exposures to PM, both indoors and outdoors, are higher than ambient levels measured at the same time (5). Critics like Moolgavkar question whether the epidemiological studies have in fact demonstrated a specific risk attributable to PM, or whether they merely show that air pollution in general causes adverse health effects. Too many of the studies, he claims, have considered only one pollutantparticulates-and have ignored the fact that PM tends to MD/ with other pollutants such as carbon mon-
rted health effects associated with particulatf Increased hospital use: admissions, emerIncreased pneumonia and exacerbation of chronic obslructive pulmonary disease: hospital admissions, emergency room visits Exacerbations of asthma: attacks, bronchodilator use, emergency room visits, hospital admissions Increased respiratory symptom: cough^ tinper and lower respiratoly tract problems
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oxide, sulfur dioxide, and ozone. “The particulate component of air pollution appears to have become the villain because it is a ubiquitous component of air pollution, and thus serves as a proxy measure of pollution,” he writes (3). With this in mind, the Hopkins team went into its review of PM with the following neutral premise: “Our current understanding of the health effects of complex air pollutant mixhues does not dictate a particular pollutant or combination of pollutants that cause mortality” (6‘).The Hopkins team will be looking for other potential confoundingfactors, including temperature (mortality is already known to rise on extremely hot and cold days), humidity, influenza epidemics, and age. Pope, Dockery, and Schwartz have controlled for some of these factors in past studies, but more work needs to be done in the area of potential confounders, say scientistsworking on the PM problem.
What causes the effects? Even if one were to take the existing epidemiological studies at face value and say that PM does increase mortality, a great mystery would still remain: What causes the observed health effects? Animal toxicology studies conducted to date with MIious types of model particles have shown only relatively low toxicity for PM (5). But new investigations, using previously unstudied types of particles, are beginning to provide some tentative insights into the “how” question. Gunther Oberdorster and his colleagues at the University of Rochester in New York have focused their attention on “ultratine”panicles as small as one hundredth of a micrometer in diameter. Although these particles have very low mass, there are many of them, and their sheer numbers mean that they present a large total surface area when they reach the alveoli in the lungs. Once bound to the alveoli, they may induce oxidant production, lung d a m mation, and hyperactivity (5).In pursuing this line of inquiry, Oberdorster’s group has found that even moderate levels of ultratine Teflon partides are able to kill rats in the laboratory. Although the case is far from closed (“I’m not trying to convince anybody that ultraline particles are the culprits,” Oberdorster said at the recent Society of Toxicology meeting), the theory has drawn considerable interest. The notion that ultraiimes cause damage because of their sheer numbers is “an attractive biological hypothesis, and one that needs to be tested,” says Moolgavkar. “I’vebecome convinced myself that the literature is increasingly pointing toward the fine particles,” agrees Pope, “but I’m not ready to jump on the ultrafine particle bandwagon yet.” Part of the problem is that definitions in this field are still somewhat vague. There is no universal agreement as to what the terms “fine”and “ultrafine”mean. And because of the lack of systematic monitoring, “I’mnot wen sure we have enough data to answer the question” about which partides are to blame, says Pope. EPA researchers at the Health Effects Research Laboratory in Research Triangle Park, NC, are exploring other possible mechanisms for PM-related biological damage, including the effects of organic
matter and transition metals such as iron attached to the particle surface.These entities could react with cell membranes. proteins, and cell receptors. Not surprisingly, scientists working on the PM problem agree that more research and more monitoring Critics question are necessary. The numbers of moniwhether the tors need to be increased, they say, epidemiological and the monitors studies demonstrate themselves need to be improved to account for different a specific risk from kinds of particles. particulates or EPA currently is spending just over simply adverse $7 million a year on PM research, which health effects from most scientists in the field say is not air pollution in enough to answer all the unresolved general. questions. Testifying before Congress last February, McClellan called for a dramatic increase in PM research funding, to $30 million a year. That is unlikely to happen, says one EPA scientist working on PM research, who “would be astounded” if Congress approved that funding level. But EPA is paying closer attention to the PM problem these days, largely because of a court order that
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ine particles profile i d o n processes: chemical reaction, nucleion, condensation, coagulation, evaporation of g and cloud droplets in which gases have dished and reacted omposition: sulfate, nitrate. ammonium, hydro n. elemental carbon.. oroanic comoounds. PN Pb;Cd, V, Ni, Cu, Zn, particle-bound water,.biogenic organics Solubilii: largely soluble, hygroscopic, and deliquescent Sources: combustion of coal, oil, gasoline, diesel, wood: atmospheric transformation products of NO, SO, and organics including biogenic organics such as terpenes; high-temperature processes, smelters,steel mills Lifetime: davs Travel distance: hundreds of kilometers Source: Reference 7.
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forces the agency to review its regulations on particulates. PM is one of six air pollutants regulated by the National Ambient Air Quality Standards. By law, EPA is required to review these standards every five years, but the last time the PM standard was reviewed was in 1987. After EPA missed the five-year deadline in 1992,the American Lung Association sued successfully to force the agency to rexisit the standard. The new standard will be published in mid1996 and will take effect in 1997. Between now and then, the scientXc rationale for the proposed PM standard will be subjected to internal and external review. Last April, EPA published an encyclopedic Criteria Document summarizing known information about air particulates and their health effectsAlthough the document is meant to be a neutml compendium of technical information, including evidence for all sides of the debate, it does occasionallyventure into a cautious conclusion: "There appears to exist credible evidence for a likely very small, but real PM effect on human health in some susceptible subpopulations (including contributing alongwith other risk factors to premature deaths among the elderly with preexisting cardiopulmonary diseases) at PM,, 24-hour concentrations in the range of 30 to 200 pg/m3" (7). TheCrireriaDocummtdbefoUdbyanagency "Staff Paper" containing recommendations for a PM standard and supporting documentation. Both documents will be reviewed by the agencys Clean Air Scienli6c Advisory Committeethisyear, the Criteria Document in August and the Staff Paper in November. One effect of the court order is that this peer review process has been accelerated.A review that normally takes several years is being compressed into months, a situation that McCleUan calls "unfortunate, due to the gravity of the decisions to be made." EPA has a wide range of options in setting a new PM standard. It could merely reaffirm the existing standard for PM,, or it could establish a new standard for PM,,, either for short-term exposure, longterm exposure, or both. Setting a new standard for PM, would require that those particles be moni364 A
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tored systematically throughout the country If the epidemiologjd results collected to date are confirmed, setting a new PM standard could be difficult. Although today's 24-hour health standard for exposure to PM,, is 150 pg/m3, health effects appear in the studies at levels below 50 pg/m3. In fact, the research has yet to identify a clear threshold below which the effects do not occur. By law, EPA is not supposed to consider economic factors or technical feasibility when establishing standards for the six primary air pollutants. But if PM does show harmful effects at very low levels, it could present policy makers with a dilemma. Testifying before Congress last February, McClelIan addressed this issue, which also applies to EPAS review of the ozone standard: "The [Clean Air] Act requires that standards be set to protect against adverse effects,to protect sensitive populations, and to include a margin of safety. Increasingly, this appears to be unworkable as we are observing changes that extend down to ambient concentrations that are routinely encountered around the country.The problem is liely to become even more difficult because of our extraordinary capability to identify subtle changes in the body at lower and lower levels of exposure. . . . This leads to a quandary, in that standards are being set at levels which realistically may not be attainable for much of the United States" (8). This quandary has led some, like McClellan, to propose that EPA go slowly in revising the PM standard. Others have proposed that the United States work to set PM standards in cooperationwith other nations and try to defuse issues related to economic competitiveness should a restrictive PM standard impose a burden on US. industry (9). But even if EPA takes a cautious approach to m i s ing the PM standard next year, the particulate problem is not likely to go away As Mauderly told his audience at the Society of Toxicology meeting, "It forces us to look at the issue of what is acceptable risk. It really gets at the nub of how we deal with health risks in a society that is, by and large, reasonably clean."
References (11 Pope. C.A. III: Dockery, D.; Schwartz, 1. Inhalation Toxicol. 1995,7,4-18. (21 Pope, C. A. et al. Am. 1.Respir Crif.Care Med. 1995,151, EfcL7A 1 " "
(31 Moolgavkar, S. et al. Epidemiology, in press.
(4) Dockery, D. W. et al. N. Engl. 1.Med. 1993,329.1753-59.
(51 '"particulateMatter ResearchProgram Strategy: Health and exposure Issues" Idraftl; U.S. Environmental Protection Agency: Research 'Bangle Park, NC, Nw.25,1994; particuiate Matter Research Program Strategy. 2Am03, hoject Work Plan 2AF-004,Tip 671. (61 Samet, 1.; Zeper, S. "TheAssociation of Mortality and Particulate Air Pollution."Analytical Plan for Data Reanalysis prepared for Health ERects Institute:Cambridge, MA, 1995. (7)"Air Quality Criteria for Particulate Matter." US.Environmental ProtectionAgency. Rerearch'ItiangJeM , N C April 1995 Draft Executive Summary 1-100; EPA1600-AP-95IOOlA. (81 Testimony of Roger Mcclellan before the Subcommittee of Energy and Environment, House Committee on Science, Feb. 16,1995.fwherecan lhls be aeeessedll (9)Friedlander, S.K Uppmann, M. hvimn Sci. TechnoL 1994, 28,148A-150k
Tbny Reichhardr is a freelancesciencejoumalist and editor based in Washington, DC.
