A Focus on Particulate Matter and Health - Environmental Science

Any decision by the EPA administrator is likely to be controversial, as it was with ..... and given the variety and degree of observed health associat...
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Environ. Sci. Technol. 2009, 43, 4620–4625

A Focus on Particulate Matter and Health ARMISTEAD G. RUSSELL* Georgia Institute of Technology BERT BRUNEKREEF Universiteit Utrecht

SHUTTERSTOCK

Through analytical characterization, and toxicity end epidemiologic studies, the health implications of airborne particulate matter are slowly being elucidated.

exposure to elevated levels of particulate matter (PM) in cities, and a staggering 1.8 million are similarly affected by smoke from indoor solid fuel use (1). Together they rival “overweight and obesity” as causes of premature death. In Europe, it has been estimated that life expectancy is reduced by an average of 9 months as a result of exposure to anthropogenic fine particles (2). PM impacts go beyond health, including climatological concerns (3, 4). It is the recognition of the extent of PM-related health impacts that has led to this Focus issue with papers on topics tackling research challenges in areas ranging from measurement to health (denoted by bold reference numbers (5-24)). Such research is timely: the U.S. EPA is in the middle of its review of the Particulate Matter National Ambient Air Quality Standards (PM-NAAQS), with a decision on revising the standard in 2011. EPA’s supporting review (3) concludes that there are likely causal relationships between both PM2.5 (PMd means particles with diameter < d µm) and PM10 with cardiovascular and respiratory morbidity and mortality. Any decision by the EPA administrator is likely to be controversial, as it was with the last revision in 2006. While attention will be on the fine, or PM2.5 standard, other issues include the importance of ultrafine (PM0.1 [nanoparticles]), coarse (PM10-2.5), and supercoarse (d >10 µm) sizes (25) and whether particles of different composition or source have heightened health concerns. Controversy was likewise experienced in Europe when the 2008 decisions led to standards that in several ways are less stringent than they were before and certainly less tight than those in the U.S. (26).

Measurement

Particles in the air might be tiny, but they are not benign. An estimated 811,000 people worldwide die prematurely from 4620

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A major complexity in linking PM to health involves measurement. Particles come in sizes ranging over 5 orders of magnitude from about 2 nm to larger than 100 µm (for examples, see figures in (8, 22, 23)), and can contain hundreds of compounds. Health impacts are tied to both size and composition (discussed below), driving the development of new instrumentation. Measuring aerosol (or particle) size distributions commonly has been done optically (e.g., optical particle counters [OPCs]), but optical instruments have size-detection limitations larger than the smallest particles of interest. Condensation particle counters (CPCs) extend this range by growing small particles with condensing vapors (27). Equilibrating particle charge distribution and then separating them by their mobility (large particles experience more drag), as is done in electrical mobility analyzers (EMAs), allowed investigators to quantify even smaller particles. Currently, in situ devices can quantify the number of particles down to about 2 nm, which contain just a few molecules. If both the size and composition is desiredsas is usually the casesthe most common approach for characterizing the size-resolved chemical composition of an aerosol has been to use an impactor, which relies on the inertia of particles to separate them from air (e.g., a multiorifice uniform deposit impactor [MOUDI] (28)). Multiple stages allow for collecting particles of increasingly small sizes, though this approach is 10.1021/es9005459

 2009 American Chemical Society

Published on Web 06/01/2009

limited for particles below about 10 nm and sampling times can be long. Aerosol mass spectrometers (AMSs e.g., (29)) can characterize PM size and composition in near real time, though such instruments have limitations in terms of size range and the type of compositional information provided. As an alternative, McMurry et al. (8) present the adaptation of an EMA to collect nanoparticles for chemical analysis. To collect enough mass, the instrument operates at a low resolution and a high flow rate. Sampled aerosol is then quantified using off-line techniques. Looking forward, development of cost-effective instruments to provide continuous PM composition analysis is of most interest for providing the type of information desirable for epidemiologic studies and to support future regulation.

Emissions to Concentration: Sources, Controls, Formation, and Transformation A particle’s life can begin in a variety of ways (e.g., (30)), including direct emission or formation in the atmosphere. Characterizing emissions is difficult, being complicated by measurement device limitations discussed above, and more. No two sources are identical, and even emissions from the same source will change with time and operating conditions. Additionally, since particles undergo a variety of chemical and physical transformations in the atmosphere, what is measured at the source may not be what will be found in the atmosphere. This is particularly true for organic species (31). In this issue, three manuscripts address such complexities. Grieshop et al. (7), investigating wood combustion and diesel engine emissions, show that prior measurements of PM emissions may overlook that a fraction of the semivolatile organic compounds (SVOCs) and intermediate volatility organic compounds (IVOCs) can evaporate when being diluted with ambient air. In their presented analysis, they use the concept of volatility distributions (or basis set), a useful construct to facilitate atmospheric simulation (e.g., (17)), wherein the oxidation of organic compounds is modeled as leading to a modificationstypically loweringsof the vapor pressure. Murphy et al. (23) simultaneously measured gases and particles in the stack of a post-Panamax ship, as well as measuring the evolution of the emitted particles using an airplane. The shipboard measurements included characterizing the size-resolved chemical composition using an impactor, while the aircraft contained a suite of technologies, including an AMS, CPC, and EMA. The range of instruments allowed detailed investigation of PM properties: they found that the organic matter content increased, relative to sulfate, and was primarily a hydrocarbon-like material (having a low O to C ratio). Pakbin et al. (12) examined emissions of PM from a diesel engine using advanced control technologies. Both continuously regenerating technology (CRT) and having a selective catalytic reduction (SCRT) device following the CRT showed reductions of organic species by about two or more orders of magnitude. Under certain conditions, particles can be formed directly in the atmosphere from molecules with a very low vapor pressure (e.g., sulfuric acid) forming molecular clusters which then nucleate to form nanoparticles with diameters on the order of 2 nm and can then grow further. One of the surprises uncovered in the past decade is just how common these nucleation events may be (e.g., 32, 33). Nieminen et al. (22) push the limits of PM size measurements, using a chemical ion mass spectrometer (CIMS) to measure individual sulfuric acid molecules, and a combination of instruments to find size-resolved particle concentrations down to just 2 nm. This allowed them to follow the birth and growth of new particles. Such information is critical to understanding the nucleation process as different theories now compete to elucidate empirical measurements.

A large fraction of the observed PM mass, however, is added to preexisting particles by gas-to-particle conversion, primarily from secondary organic matter (SOA) and sulfate. While PM growth from inorganic species tends to be better understood and quantified, the amount of SOA formed is quite uncertain (e.g., (34)). Organic PM is of growing concern as there is evidence that organic material may have greater health impacts than sulfate (e.g., 35, 36). Gaseous organic compounds that were once thought to have rather negligible contribution to organic aerosol formation appear to be anything but insignificant, as they can oxidize and oligimerize to form low vapor pressure products (37, 38). Chan et al. (6) conduct laboratory experiments of the oxidation of 2-methyl3-buten-2-ol (MBO), a compound emitted in pine forests, finding it likely contributes little to SOA formation. MBO is but one of hundreds of organic gases in the atmosphere. Alternatively, Kourtchev et al. (10) approach understanding SOA formation by measuring the chemical and size characteristics of PM in a forest. They utilize a range of instruments (e.g., MOUDIs and filter-based measurements, followed by GC/MS analysis to give detailed organic speciation) to show the importance of oxidation products of biogenically emitted VOCs. The dominant mode of both secondary and primary organic compounds was in the 0.1-1 µm range. Like in other studies, only a fraction of the observed particulate organic carbon is identified. Presto et al. (20) pick up on the results of Grieshop et al. (7), showing that the compounds that may have evaporated during dilution can then oxidize and condense. Contrary to other studies, they find that aerosol formation can increase at higher NOx levels which can help explain the increased oxygenated organic levels in cities. Knowing the source of PM is of essential importance to control ambient concentrations effectively. Given how a particle is transformed in the atmosphere, identifying the source is complicated (39). Beddows et al. (14) use a statistical (or receptor) approach, somewhat different from the typical species balance approach, conducting cluster analysis on detailed particle size distribution measurements taken at four sites: roadside, a ground-level and an elevated location in London, and a more rural location. Their results show that about 15 types of clusters are consistently found, suggesting that this approach can be used for source apportionment. Murphy et al. (17) apply a chemical transport model (CTM) to study regional scale PM dynamics. Here, they implement volatility basis functions (discussed above and (7, 20)) to simulate the evaporation of a fraction of the SVOC and IVOC emissions, as well as to follow the atmospheric chemistry, transport, and transformation of other gaseous and PM emissions. Using this added flexibility appears to better capture the regional dynamics of organic carbon PM. Major uncertainties remain, including inputs such as emission rates and speciation, as well as model process descriptions such as resuspension of dust, nucleation, and the details of SOA formation. Further, the question of how well we can quantify the contribution of specific sources to ambient concentrations remains open (40).

Concentration to Exposure Being able to understand the sources of PM at the scale of an urban area does not mean a similar understanding is available for how individuals (or even populations) are exposed. Individuals move from one microenvironment to another, spending a large amount of time in buildings and vehicles with limited time outdoors, such that an individual’s exposure may not be similar to ambient measurements and the corresponding emissions (41). Exposure modelingslinking air quality model results with a model that simulates timeVOL. 43, NO. 13, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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information on differences in exposure among five transportation modes: walking, cycling, and vehicular travel by bus, car, or taxi. Not surprisingly, the highest percentage of explained variance was found for PM0.1 counts, which were lower for walking and cycling, and higher for the three invehicle modes. However, the inhaled dose, could of course still be higher in the more active modes of transport in view of the higher levels of exertion associated with walking and cycling. SHUTTERSTOCK

activity patterns of thousands of individuals and pollutant infiltration in to microenvironmentssplays a key role in NAAQS reviews to estimate the distribution of population exposures (42). However, given the lack of fundamental data and process descriptions, this approach is largely limited to the fine fraction of PM (PM2.5). Considerable work is required to address ultrafine, coarse, and supercoarse exposures. We spend most of our time indoors, notably in our own homes. It is difficult to study indoor exposures to air pollution on a large scale, so most of our information is from studies in limited numbers of homes, such as that presented by Chen and Hildemann (5), who analyzed PM size fractions and PM components. PM in the fraction larger than 2.5 µm was found to be strongly influenced by human activity. Previous work (43) indicated that the smaller particles are much harder to resuspend, which may be a reason for the findings reported herein. Such elevated indoor exposures are of interest given the emerging evidence of PM10-2.5 health effects, though we have very little direct evidence of health effects of such exposure from indoor environments. A relatively new way to look at linking sources to potential exposure is using intake fraction (44), applied herein by Ries et al. (15), to assess the importance of wood smoke emissions on exposure. While one source may emit a large amount of PM, the amount of that material that actually makes it to a human’s breathing zone might be rather small due to the source’s characteristics; e.g., the source is not near a populated area. Such knowledge can guide policies to more effectively address sources leading to the greatest potential exposure. Over the past decade or so, interest in within-city contrasts in air pollution exposure has soared. This was fuelled by observations of high exposures to specific pollutants and PM components near streets and of adverse health effects among subjects living near roadways. Such near-roadways exposures are complex as such regions can contain elevated levels of both ultrafine (PM0.1) and supercoarse (d > 10 µm) particles that are not present at such high levels further away due to their limited lifetime. As a consequence, the need for developing methods to estimate distributions of exposure on a short spatial scale has increased. As making personal measurements for long periods on large numbers of subjects is usually impossible, some form of modeling is needed. Dispersion modeling of the short spatial distribution has its limitations, and the use of land use regression (LUR) models has proliferated. Overviews of the pros and cons of such models have been published recently (45). The measurement effort to establish a database as input into the LUR models can be substantial, especially when active sampling for PM or other substances is required. Larson et al. (11) provide an elegant example of mobile monitoring, obtained from equipment mounted in a van that was driven along various intersections and streets in Vancouver, BC. The amount of variance explained in the data acquired from this program was comparable to the variance explained by models based on measurements obtained from more intensive stationary monitoring, which is promising. With the increased interest in traffic pollution, including not only exhaust emissions but also road, brake, and tire dust, the question inevitably arises as to what extent exposures to air pollution are elevated while in traffic. Early studies focused primarily on exposure to VOCs and CO, but more recently emphasis has shifted to particle metrics, as is done by Miller-Schulze et al. (24), studying taxi drivers in Shenyang, China. Measurements inside and outside taxis, as well as at the drivers’ homes, found elevated levels of some particulate nitro-polycyclic aromatic hydrocarbons (NPAHs) in taxis, but other NPAHs appear to be formed in the atmosphere, leading to more uniform, albeit still elevated, levels near roads. Nieuwenhuijsen et al. (19) provide

Exposure to Dose Let’s say that we actually could quantify the PM to which an individual is exposed, the next question is ‘What fraction is actually taken up by the body?’, that is, the dose. Lungs are quite complicated, and different parts of the respiratory system will see particles of many sizes and composition. While both laboratory and numerical modeling studies are often used to address lung deposition (e.g., 46, 47), Londahl et al. (9) used subjects exposed to PM along a busy street, sized resolved pollutant measurements, and computer modeling to identify the deposition of PM1 particles in the lung. They find, in part, that there is a difference between the deposition rates of hydrophobic and curbside particles, largely determined by their hygroscopicity. Such results can be used to evaluate recent model developments, such as recent sourceto-dose modeling that estimates the spatial- and sourcespecific intake of pollutants (48).

PM Health Effects PM is associated with a wide variety of cardiovascular- and respiratory-based health effects, with responses to exposure being both acute (e.g., increased hospital admittances for respiratory disease or premature mortality from cardiovascular disease) and chronic (reduced longevity in cities with higher PM levels); there are also indications of reproductive and developmental effects (3). Studies to better understand such responses typically take one of three forms: clinical, toxicologic, or epidemiologic. Toxicology. Specific mechanisms that lead PM to have the range of observed health impacts are still quite uncertain, and

given the variety and degree of observed health associations, it is likely that more than one is involved. Mechanistic studies, including those using human exposure to concentrated PM, have considerably increased our insights into how PM causes pulmonary and cardiovascular effects (49, 50). Pulmonary effects are associated with cellular injury and inflammation, and, increasingly researchers are finding that reactive oxygen species (ROS), either in the PM or produced by stimulated cells, can play a role (3). PM species of most interest include metals and some oxygenated organics (e.g., quinones). Hypothesized responses include up-regulating antioxidant enzymes, cell death, allergic immune response, impairing lung defense, and DNA damage (3, 51, 52). Damage can also continue to other parts of the body (e.g., the cardiovascular system). A few studies suggest that working in traffic is associated with increased levels of specific biomarkers (e.g., (53)). Wei et al. (21) used repeated urine sampling of two campus security guards exposed to heavy traffic. Postwork-shift urinary concentrations of 8-OHdG, a biomarker for oxidative DNA damage, were associated with on-site measurements of PM2.5 as well as PAH and metal content. Recent toxicologic or epidemiologic studies show that there is still much inconsistency in findings related to different size fractions. Duvall (54) in a U.S. based in vitro study of particles collected in six urban areas did not find that coarse PM (PM10-2.5) was associated with markers of lung injury or inflammation, whereas PM2.5 was. In contrast, Wegesser et al. (55) in a study of PM10-2.5 collected in a hot, rural area in the southwest U.S. found that PM10-2.5 was associated with pro-inflammatory effects in the lungs of mice after intratracheal instillation. Gerlofs-Nijland et al. (18), using PM-exposed hypersensitive rats, found that PM10-2.5 actually led to a stronger effect per mass than fine particles, and that the positive correlations were associated with particles containing PAHs and metals. Epidemiology. Given the many complexities and uncertainties in identifying the responsible biological mechanisms, the primary approach used to link PM to various health end points is through epidemiologic analyses. Whereas this research provides a very comprehensive and broad picture, the inevitable inconsistencies between at least some of the studies lead to further questions and difficulties of interpretation. One such question relates to the health effects of PM in different size fractions. The focus from a regulatory point of view in both North America and, more recently, Europe, has been on PM2.5. However, it is not so clear whether larger, but still inhalable particles up to 10 µm in aerodynamic diameter are less harmful. A 2005 review of the evidence for PM10-2.5 and PM2.5 effects on mortality (56) found very little evidence of effects of PM10-2.5 on mortality independent from those of PM2.5. Intriguing results come from a time series study in Barcelona (16) showing effects of PM10-2.5 and PM1 on cardiovascular and cerebrovascular mortality, and effects of the “in between” PM2.5-1 fraction on respiratory mortality. This being a singlecity study, the statistical power to truly separate effects of different size fractions from each other is not large, suggesting replication in larger data sets rather than changes in regulatory practice are warranted at this time. Wilson et al. (57) found PM10-2.5 had a considerable effect on cardiovascular mortality in Phoenix, AZ, but only when assessed for a population living fairly close to the monitor. However, in another large U.S. study of 3,700,000 cardiovascular and 1,700,000 respiratory hospital admissions, there were no associations between PM10-2.5 and admissions after adjustment for PM2.5 (58). The new studies, along with the EPA review, suggest that the balance is shifting toward stronger evidence for PM10-2.5 leading to premature mortality, but it needs to be recognized that these relatively small, single-city studies are reporting effect estimates for PM2.5 many times larger than those found in large multicenter studies (59). It remains to be seen what the verdict of effects of PM10-2.5 on mortality will be after more and larger

studies have been published. An example is the Multi-Ethnic Study of Atherosclerosis (MESA) AIR study (13), one of the most ambitious studies on the health effects of air pollution ever undertaken. Studying an existing cohort of subjects participating in a study on atherosclerosis development, the investigators are first conducting a very detailed exposure assessment, ranging from personal and in-home monitoring to modeling and assessment of how individuals spend their time (i.e., time activities), to be followed by epidemiologic analyses. First results of the exposure assessment show a fairly large variation of PM2.5 exposures between cities, but within at least some cities the variation is small. As this study is further extended over the next several years, its size and detail should provide more definitive results than previously available.

Looking Forward The link between PM and adverse health effects is becoming more clear, though a number of questions remain open that hinder our ability to set policies that would provide the greatest benefits. Worldwide, much of the harm, currently, comes from indoor exposure to burning solid fuels, a problem that could be mitigated relatively straightforwardly (60). Exposure to outdoor-generated PM is more complex. Research areas that appear to present the greatest challenges and hold keys to significant health benefits include understanding SOA formation both from a fundamental and operational standpoint, linking sources to exposure, and identifying what component or size of PM is most responsible for the observed health effects. While the specific mechanisms that lead to the various health effects associated with exposure to PM are still unclear and need to be elucidated, current evidence tends to suggest that the organic fraction and/or metals appear to be of most concern, and act via oxidative stress. Studies of the impacts of coarse, fine, and ultrafine particles are conflicting, and there is very little information about the potential impacts of supercoarse particles. It is apparent that multiple approaches are required. Combining mechanistic studies of concentrated real world ambient particles with observational studies of subjects exposed to PM with the same characteristics will further improve our insights into which particle exposures really matter. Such information can then identify appropriate control programs and hopefully improve everyone’s health and well-being.

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