can damage animals’ respiratory tracts and cardiovascular and central nervous systems. Once inside the body, ultrafine particles On a cold January morning in 2008, Background levels might also be able to travel to Like some manufactured and a white van patrolled the streets of other organs in unexpected ways. naturally occurring nanopartia neighborhood that sits just outOberdo¨rster points to one forgotcles, ultrafine particles can be side of Boston, several hundred ten pathway: through the nose to difficult to measure, impossible meters (m) from two major freethe olfactory system, then to the to detect in biological tissues in ways. Inside the van, researchers central nervous system, and fireal time unless they are labeled, from Tufts University and their colnally to the brain. (This route was and tricky to track behaviorally leagues took a first spin found in the 1940s in around the neighborhood studies of viruses in monin the mobile laboratory keys.) built by Aerodyne ReEngineered nanopartisearch, Inc. That day, theycles get more attention collected gigabytes of data than ultrafine particles, on the levels of a suite of Oberdo¨rster comments, pollutants that came from particularly for industrial the traffic emissions raining and medical purposes down around them: CO, such as drug delivery. This NO, NO2, black carbon, focus, taken together with PAHs, and three kinds of the difficulty of measuring particulate matter, from these particles in real encoarse particles to fine parvironments, means that ticles larger than 2.5 mithe bulk of the data on crometers (PM2.5) down to Drivers stuck in traffic have more to worry about than accidents ahead. Ultrafine particles, among other components both manufactured nanoultrafine particles at of vehicle emissions, could be damaging their health. particles and ultrafines 10-100 nanometers in comes from in vitro studdiameter. ies or from animals exposed to because of their tendency to That morning, the researchers well-characterized nanoparticles clump or change in other ways found that one cubic centimeter in known quantities. once they are released into the of Somerville’s air held about “It’s easy enough to put lots of atmosphere. Such tiny particles 120,000 ultrafine particles, as material on those cells, [and] even also have large surface-to-volreported by one of the lead inthe most benign material causes ume ratios, which gives them vestigators, Doug Brugge of harm if you put in enough,” extra area for surface chemical Tufts. The team wants to explore Oberdo¨rster says. The question is reactions, as well as the ability the connection between expohow to extrapolate those lab reto act as vehicles for transportsure to these ultrafine particles sults “to real-world implications.” ing other contaminants. They and human health. also change quite rapidly, from Ultrafine particles’ contributions Traffic ahead the first few seconds after they to human-health problems have The best place to go for realexit a tailpipe, for example, to attracted increasing attention from world data seems to be the open minutes and hours later as they environmental scientists, as indiroad. Any driver heading down undergo chemical transformacated by some of the research pubthe freeway breathes in a range of tions in their airborne travels. lished in this issue of ES&T and particles from the vehicles ahead. First examined for their poselsewhere. Research communities The mix includes metals, PAHs sible health impacts in the 1990s that are working to answer these attached to particles, unburned by Gu ¨ nter Oberdo¨rster of the Uniquestionssin public health, envioil droplets, and other compoversity of Rochester and others, ronmental monitoring, medicine, nents that have as-yet-unknown nanosized ultrafine particles did and moresare moving toward the impacts when taken in altogether. not set off alarm bells until reidea that such tiny particulate matBut the key constituents that imsearchers and policy makers ter could be as important as the pact human health have been turned their attention to the larger particles that are already thought to be particles at around growing market for manufactured regulated. But much remains to be PM2.5. nanoparticles. “Since then, this discovered about these ulftrafine has become a very important Regulations in the U.S. and Euparticles and exactly how they field,” Oberdo¨rster says, with indirope started with coarse particumight cause damage. cations that nanosized particles late matter, PM10, and expanded JUPITERIMAGES
Breathing less easily with ultrafine particles
10.1021/es901328h
2009 American Chemical Society
Published on Web 05/20/2009
July 1, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 4615
DOUG BRUGGE
completed later this year, and example, traffic changes accordto PM2.5 as epidemiological studthey hope to see health outing to time of day, which leads to ies connected those fine parcomes correlated with specific different emissions levels in the ticles to mortality. Coarse PM10 particle exposures. morning than at night. Meteorosticks in the upper reaches of “It’s important to do,” Brugge logical conditions on a nice, the lung, as does PM2.5, where says. “Putting the two together is windy day may carry air pollution their role in asthma and impact really at the center of what makes away quickly, but quiet windless on the cardiovascular system this a potentially valuable mornings mean that emissions have been well-documented. But project.” stick around. Temperature, relaPM2.5 and smaller particles can tive humidity, wind speed: all of get deeper into the lungs and these factors “are highly into the alveolisand correlated, and it is hard eventually across the to analyze the indepenthinner membranes into dent effects” in some the bloodstream, where studies, Nieuwenhuijsen they might cause clots or says. other damage. Those variables also About 150 epidemiologimay differ depending on cal studies made the conthe urban areas studied, nection between PM2.5 Nieuwenhuijsen and othparticles and death, says ers suggest, and that Robert Devlin, a remight explain some of the searcher at the U.S. EPA differences reported in the Research Triangle Park. literature for Los Angeles, “Almost one million for example, versus people a year die because smaller urban areas on the of particulate matter expo- Researchers at Tufts will be driving a mobile lab, like the one in this van built by Aerodyne Research, into neighborEast Coast. sure,” according to the hoods outside Boston to track emissions from trafficAndre Nel of the UniWorld Health Organizaheavy roads. The team will conduct surveys to see how versity of California Los tion, he adds. “I don’t people might be exposed in their daily lives to different veAngeles says that particuthink anyone disputes that hicles’ emissions and will look for correlations with health effects. late matter “gets really fine particles are killing complex on a physicalpeople. I don’t think anychemistry level. Once particles are one doubts ultrafine particles Location, location, location emitted, they change size; the have effects. The question is, how “Ultrafine particles are more renumbers change with proximity do they compare with other parlated to being in a specific transto [the] freeway; the numbers ticles?” For example, although port environment,” comments change during the day; and coarse particles may exacerbate Mark Nieuwenhuijsen of the chemical composition changes asthma, they may not be as hazCentre de Recerca en Epidemio[during] travel through the atmoardous as fine particles; but, all logia Ambiental (Spain). He and sphere,” as the particles move three size classes could cause carhis coauthors tracked pedestrifrom, say, Los Angeles to Riverdiovascular damage or lead to ans on London streets; in their side, which is about 50 miles heart attacks. Devlin says new paper in this ES&T issue, they away. studies are needed to take into report that people are exposed But a more important consideraccount both large-scale exposure to high levels of CO and ulation may be where the particles and epidemiology in real-world trafines on busier roads, with end up in an animal or cell, or situations. exposure dropping off on the which animal or person they end That’s the goal of the Tufts quieter streets nearby. up in. “The way you are breathing study currently in progress. The Previous studies have shown and the shape of your lungs” team will measure concentrathat the concentration of ulcould make a difference to expotions from the highway itself to trafines drops off dramatically sure levels, says Jakob Lo¨ndahl of the neighborhoods in Somerville near the 200-m mark (though Lund University (Sweden), who 400 m away (with control sites PM2.5 might travel farther). “In a led a team of researchers report1000 m away) and try to identify car, [a driver] gets quite a bit of ing in this ES&T issue on the upwhen and where people are exposure to this. On the pavetake in the lungs of emissions most exposed to different parment, exposure is going to be from traffic and wood combusticles. The researchers expect to quite a bit less,” Nieuwenhuijsen tion. “Some people get a much have their first whole-neighborsays. Exposure studies must achigher dose than others; it could hood human-health surveys count for a variety of factors: for 4616 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / July 1, 2009
be a factor of two in difference, which could partly explain why some people are more vulnerable than others.” Although Lo¨ndahl is among the researchers who suspect that ultrafines stay in the lungs, he says that some recent studies indicate transport to other organs.
Measuring up to expectations Scientists have yet to come to consensus on how ultrafine particles create health problems. They still need to determine which characteristic to monitor: is it the number (or concentration) of particles, their surface area, or their mass? Researchers seem to tilt toward numbers and surface area, because these give a better idea of the chemical reactions that could occur or the metals or other constituents that might be present on a calculated surface area. In general, the particles are so small that their mass is almost impossible to measure. “It really depends on which endpoint we’re looking at, if number, concentration, or surface area is the best predictor” of human-health outcomes, says Junfeng (Jim) Zhang of Rutgers
University. “It’s too early to say what’s the best thing to do.” Zhang, who coauthored research in this issue examining PM2.5 and oxidative DNA damage in people, also was on a team reporting to the Health Effects Institute on real-world diesel exposures and people with asthma. In that study, more than 80% of the emissions samples were composed of ultrafines. Zhang suggests that differences in damaging activity may exist even among different size classes of ultrafine particles: a few nanometers versus a few tens of nanometers might make the difference. One likely metric for harm seems to be oxidative stress. Nel’s team has been developing a “testin-a-tube” for oxidative properties of particulate matter. They want to find a fast way to put an air sample in a test vessel and find out which metals, particles, and other constituents there could damage a cell’s activity, and how that might translate to whole organisms.
New endpoints Taking these considerations from observation to regulation
could lead to solutions that range from local to global in their scope. Nieuwenhuijsen suggests cities might plan separate biking and walking paths or create canyon-like traffic corridors that generate wind to carry away pollution. Devlin says that EPA and others are working to evaluate a new generation of engines and filters, which are expected to be on the market by 2010, to see how clean they will be with respect to particle emissions. Devlin adds that other sources, such as power-plant and ship emissions, also will have to be considered under any regulatory scheme. Nel suggests that using a CO2 standard to regulate fossil fuel combustion could control much of the source of ultrafine particles. In the meantime, getting to the answers on how ultrafines impact human health could be as complex as the particulate emissions themselves. The behavior of these nanoparticles in cells, animals, and the atmosphere, and the chemical reactions on their surfaces, remain to be parsed. —NAOMI LUBICK
July 1, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 4617
