FEATURE
Weighing the Health Risks of Airborne Particulates Recent epidemiological studies link an increase in mortality and sickness t o particulates, but do the data justify a tighter air quality standard? TONY
REICHHARDT
O
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 hand— health 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 (i). 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 U.S. economy, because airborne particulates from many sources, including automobiles, power plants, and mines, could be subject to tighter regulation. Roger McClellan, president of the Chemical Industry Institute of Toxicology and member of EPA's Science Advisory Board, calls the particulate problem a "multibillion dollar" question. "It's a great public policy issue and it has big-stake consequences," he says. Epidemiological studies Research into the health effects of PM escalated dramatically beginning in the late 1980s. Since then many researchers—among them Douglas Dockery and Joel Schwartz of the Harvard School of Public Health, George Thurston and Kazuhiko Ito 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, including Philadelphia, 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
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respiratory conditions appear to be most vulnerable, but the data also show significantly higher 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 than 500,000 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 "increased mortality is associated with sulfate and fine particulate air pollution at levels commonly found in U.S. cities." Death rates in the most polluted cities were 15% higher than the norm when sulfates were considered as the pollutant, and 17% higher when PM 2 s 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 different picture, says Pope. "I'm getting more convinced as time goes on that it's the longterm chronic exposure that's most important in terms of real loss of health and loss of life." What's 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 factoring 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 reanalysis Consistent as the epidemiological studies have been, Pope, Dockery, and Schwartz still worry that some other factor might be causing the deaths, particularly in the case of short-term pollution episodes. They admit to two principal concerns. The effects may be attributable to artifacts of the regression method used to analyze the data, and there may be confounding factors resulting from "inadequate control of seasonal factors, epidemics, other long-wavelength trends, weather variables, or other pollutants" (i). Skeptics have seized on both these areas of uncertainty to question whether
Particulates in ambient air are produced by a variety of anthropogenic and natural sources. These particles, collected in downtown Chicago as part of the Lake Michigan Urban Air Toxics Study, are sulfate (top) and aluminum resulting from local industry. Filaments in the sulfate micrograph are Teflon filter fibers. Courtesy Robert D. Willis, ManTech Environmental Technology, Inc.
the purported associations between particulates and health risks are real. Suresh Moolgavkar at the Fred Hutchinson 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 using different statistical methods, 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" (3). 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 EPAs ongoing review of the air particulates issue (see below). Samet and Zeger will concentrate on PM-related mortality and morbidity data for five regions: PhiladelV0L. 29, NO. 8, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY • 3 6 1 A
FIGURE 1
Current levels of PM10exposure A growing number of areas in the United States have PM10 concentrations above standards set by EPA. In 1993,13 areas were added to EPA's nonattainment list, bringing the total to 83. As indicated by this selected list of nonattainment areas, long-term (annual) exposure levels can vary considerably from short-term (daily) levels. Health researchers are debating which type of particulate exposure has the greatest impact on human health.
Dallas, TX
•
Baltimore, MD
Second highest maximum 24-h concentration Weighted annual mean
Boise, ID
J
Denver, CO Phoenix, AZ New York City Chicago, IL Los AngelesLong Beach, CA
20
40
60 80 100 PM10 concentration, μg/m3
120
Source: National Air Quality and Emissions Trends Report 1993, EPA Office of Air and Radiation, Office of Air Quality Planning and Standards, October 1994, EPA 454/R-94-026.
phia, Utah Valley, St. Louis, eastern Tennessee, Bir mingham, AL, 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 in haled 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 PM10 (particles smaller than 10 micrometers). But recent mortality studies, including that using the ACS cohort, have implicated even smaller particles, PM2 5 and finer. In one Harvard study, which followed 8000 adults for 16 years, PM 2 5 showed a better associa tion with increased mortality than did PM10 (4). The question of which particles are most danger ous is further complicated by the lack of consistent monitoring data. Although PM 10 is monitored sys tematically by EPA-mandated state and local air mon itoring stations on a nationwide basis, PM2 s is not. Furthermore, different epidemiological studies have used different measures of particulate pollution. Some studies take ambient TSP data, then use a conver sion factor to determine ΡΜ ί0 levels. Others have used a measure known as the "coefficient of haze" as a sur rogate 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 McClellan. The nature of particulates PM is the only air pollutant regulated by EPA that does not specify a particular chemical composi tion. But included within that broad category are many different types of particles, from naturally oc curring windblown dust (generally the coarser par ticles) to anthropogenic combustion products (gen erally the finer ones). PM2 5, for example, includes acid 3 6 2 A • VOL. 29, NO. 8, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
aerosols such as sulfates from the burning of sulfurcontaining fuels. Some researchers believe these ac ids may be causing health effects. Once again, say researchers, basic monitoring data are lacking, as is our understanding of 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 sug gest 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 ep idemiological studies have in fact demonstrated a specific risk attributable to PM, or whether they merely show that air pollution in general causes ad verse health effects. Too many of the studies, he claims, have considered only one pollutant— particulates—and have ignored the fact that PM tends to vary with other pollutants such as carbon mon-
Health effects of particulate matter Reported health effects associated with particulate matter exposures (5). • Mortality • Increased hospital use: admissions, emer gency room visits • Increased pneumonia and exacerbation of chronic obstructive pulmonary disease: hospital admissions, emergency room visits • Exacerbations of asthma: attacks, bronchodilator use, emergency room visits, hospital admis sions • Increased respiratory symptoms: cough, up per and lower respiratory tract problems • Decreased lung function M
oxide, sulfur dioxide, and ozone. "The particulate component of air pollution appears to have be come the villain because it is a ubiquitous compo nent of air pollution, and thus serves as a proxy mea sure 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 mixtures does not dictate a par ticular pollutant or combination of pollutants that cause mortality" (6). The Hopkins team will be look ing for other potential confounding factors, includ ing temperature (mortality is already known to rise on extremely hot and cold days), humidity, influ enza 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 scientists work ing on the PM problem. What causes the effects? Even if one were to take the existing epidemiologi cal studies at face value and say that PM does in crease mortality, a great mystery would still re main: What causes the observed health effects? Animal toxicology studies conducted to date with var ious types of model particles have shown only rel atively low toxicity for PM (5). But new investiga tions, 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 "ultrafine" particles 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 inflam mation, and hyperactivity (5). In pursuing this line of inquiry, Oberdorster's group has found that even moderate levels of ultrafine Teflon particles are able to kill rats in the laboratory. Although the case is far from closed ("I'm not try ing to convince anybody that ultrafine particles are the culprits," Oberdorster said at the recent Society of Toxicology meeting), the theory has drawn con siderable interest. The notion that ultrafines cause damage because of their sheer numbers is "an at tractive biological hypothesis, and one that needs to be tested," says Moolgavkar. "I've become convinced myself that the litera ture is increasingly pointing toward the fine parti cles," agrees Pope, "but I'm not ready to jump on the ultrafine particle bandwagon yet." Part of the prob lem is that definitions in this field are still some what vague. There is no universal agreement as to what the terms "fine" and "ultrafine" mean. And be cause of the lack of systematic monitoring, "I'm not even sure we have enough data to answer the ques tion" about which particles are to blame, says Pope. EPA researchers at the Health Effects Research Laboratory in Research Triangle Park, NC, are ex ploring 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 moni whether the tors need to be in epidemiological creased, they say, and the monitors studies demonstrate themselves need to be improved to ac a specific risk f r o m count for different kinds of particles. particulates or EPA currently is spending just over simply adverse $7 million a year on PM research, which health effects f r o m most scientists in the field say is not air pollution in enough to answer all the unresolved general. questions. Testify ing before Congress last February, McClellan called for a dramatic in crease 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 astound ed" if Congress approved that funding level. But EPA is paying closer attention to the PM prob lem these days, largely because of a court order that
FIGURE 2
Particulates and mortality A study published this year by Arden Pope and colleagues found that fine particles were associated with higher mortality rates (2). The study, which controlled for risk factors such as smoking, education, and age, concluded that "the increase in risk is not attributable to tobacco smoking, although other unmeasured correlates of pollution cannot be excluded with certainty."
1000
900
Λ
•.·.·.
«_ · ST·
V.1
S 800 "Ο
I 700
·-*!.
600 -
10
15 20 25 Fine particles, μg/m3
30
35
Source: American Journal of Respiratory and Critical Care Medicine. 1995, vol. 151
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A
Fine particles profile Formation processes: chemical reaction, nucleation, condensation, coagulation, evaporation of fog and cloud droplets in which gases have dis solved and reacted Composition: sulfate, nitrate, ammonium, hydrogen ion, elemental carbon, organic compounds, PNA, Pb, Cd, V, Ni, Cu, Zn, particle-bound water, bio genic organics Solubility: largely soluble, hygroscopic, and deli quescent Sources: combustion of coal, oil, gasoline, diesel, wood; atmospheric transformation products of NO, S02, and organics including biogenic organics such as terpenes; high-temperature processes, smelters, steel mills Lifetime: days Travel distance: hundreds of kilometers Source: Reference 7.
forces the agency to review its regulations on par ticulates. 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 re viewed was in 1987. After EPA missed the five-year deadline in 1992, the American Lung Association sued successfully to force the agency to revisit the stan dard. The new standard will be published in mid1996 and will take effect in 1997. Between now and then, the scientific rationale for the proposed PM standard will be subjected to in ternal and external review. Last April, EPA p u b lished an encyclopedic Criteria Document summa rizing known information about air particulates and their health effects. Although the document is meant to be a neutral compendium of technical informa tion, including evidence for all sides of the debate, it does occasionally venture into a cautious conclu sion: "There appears to exist credible evidence for a likely very small, but real PM effect on h u m a n health in some susceptible subpopulations (includ ing contributing along with other risk factors to pre mature deaths among the elderly with preexisting cardiopulmonary diseases) at PM 1 0 24-hour con centrations in the range of 30 to 200 μg/m 3 " (7). The Criteria Document will be followed by an agency "Staff Paper" containing recommendations for a PM standard and supporting documentation. Both docu ments will be reviewed by the agency's Clean Air Sci entific Advisory Committee this year, the Criteria Doc ument in August and the Staff Paper in November. One effect of the court order is that this peer re view process has been accelerated. A review that nor mally takes several years is being compressed into months, a situation that McClellan calls "unfortu nate, 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 10 or it could establish a new stan dard for PM 2 5, either for short-term exposure, longterm exposure, or both. Setting a new standard for PM 2 5 would require that those particles be moni 3 6 4
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tored systematically throughout the country. If die epidemiological results collected to date are confirmed, setting a new PM standard could be dif ficult. Although today's 24-hour health standard for exposure to PM 10 is 150 pg/m 3 , health effects ap pear in the studies at levels below 50 pg/m 3 . In fact, the research has yet to identify a clear threshold be low which the effects do not occur. By law, EPA is not supposed to consider eco nomic factors or technical feasibility when estab lishing standards for the six primary air pollutants. But if PM does show harmful effects at very low lev els, it could present policy makers with a dilemma. Testifying before Congress last February, McClel lan addressed this issue, which also applies to EPA's review of the ozone standard: "The [Clean Air] Act requires that standards be set to protect against ad verse effects, to protect sensitive populations, and to include a margin of safety. Increasingly, this ap pears to be unworkable as we are observing changes tiiat extend down to ambient concentrations that are routinely encountered around the country. The prob lem is likely to become even more difficult because of our extraordinary capability to identify subtle changes in the body at lower and lower levels of ex posure. . . . This leads to a quandary, in that stan dards 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 stan dard. Others have proposed that the United States work to set PM standards in cooperation with other nations and try to defuse issues related to eco nomic competitiveness should a restrictive PM stan dard impose a burden on U.S. industry (9). But even if EPA takes a cautious approach to revis ing the PM standard next year, the particulate prob lem is not likely to go away. As Mauderly told his au dience at the Society of Toxicology meeting, "It forces us to look at the issue of what is acceptable risk. It re ally gets at the nub of how we deal with health risks in a society that is, by and large, reasonably clean." References (1) Pope, C. A. Ill; Dockery, D.; Schwartz, J. Inhalation Toxi col. 1995, 7, 4-18. (2) Pope, C. A. et al. Am. J. Respir. Crit. Care Med. 1995, 151, 669-74. (3) Moolgavkar, S. et al. Epidemiology, in press. (4) Dockery, D. W. et al. Ν. Engl. J. Med. 1993, 329, 1753-59. (5) "Particulate Matter Research Program Strategy: Health and Exposure Issues" [draft]; U.S. Environmental Protection Agency: Research Trangle Park, NC, Nov. 25, 1994; Partic ulate Matter Research Program Strategy, 2AF003, Project Work Plan 2AF-004, Tip 671. (6) Samet, J.; Zeger, S. "The Association of Mortality a n d Par ticulate Air Pollution." Analytical Plan for Data Reanalysis prepared for Health Effects Institute: Cambridge, MA, 1995. (7) "Air Quality Criteria for Particulate Matter." U.S. Environ mental Protection Agency. Research Triangle Park, NC: April 1995 Draft Executive S u m m a r y 1-100; EPA/600-AP-951001A. (8) Testimony of Roger McClellan before the Subcommittee of Energy a n d Environment, House C o m m i t t e e o n Sci ence, Feb. 16, 1995. [where c a n this be accessed?] (9) Friedlander, S. K.; Lippmann, M. Environ. Sci. Technol. 1994, 28, 148A-150A. Tony Reichhardt is a freelance science journalist itor based in Washington, DC.
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