Studies Focus On Air Pollution's Effects On Health

Two studies announced by the University of Southern California on Sept. 20 cite the effects that air pollution has on human health. One study examines air pollution's role in causing early death, and another finds traffic-related pollution brings increased risk of childhood asthma.

Air Pollution Found to Pose Greater Danger to Health than Earlier Thought

Experts may be significantly underestimating air pollution's role in causing early death, according to a team of American and Canadian researchers, who studied two decades' worth of data on residents of the Los Angeles metro area.

When the epidemiologists examined links between particle pollution and mortality within more than 260 Los Angeles neighborhoods, they found that pollution's chronic health effects are two to three times greater than earlier believed. The study appears in the November issue of Epidemiology but was published early on the journal's Web site.

Among participants, for each increase of 10 micrograms per cubic meter (µg/m3) of fine particles in the neighborhood's air, the risk of death from any cause rose by 11 to 17 percent, according to Michael Jerrett, Ph.D., associate professor of preventive medicine at the Keck School of Medicine of the University of Southern California (http://www.usc.edu/schools/medicine/ksom.html) and the paper's lead author. Fine particle levels can differ by about 20 µg/m3 from the cleanest parts of Los Angeles to the most polluted.

"By looking at the effects of pollution within communities, not only did we observe pollution's influence on overall mortality, but we saw specific links between particulate matter and death from ischemic heart disease, such as heart attack, as well as lung cancers," Jerrett said. Ischemic heart disease mortality risks rose by 25 percent to 39 percent for the 10 µg/m3increase in air pollution.

Earlier studies took one or two pollution measures from several cities and compared health effects among cities. This study digs more deeply, taking pollution measures at 23 sites within Los Angeles to more accurately reflect air pollution exposure where residents live and work.

Researchers examined data from 22,906 residents of Los Angeles, Riverside, San Bernardino and Ventura counties in the American Cancer Society's Cancer Prevention Study II since 1982. They determined air pollution exposure in 267 different zip codes where participants lived. The vast number of participants allowed scientists to control for dozens of factors that influence health outcome, such as smoking, diet and education. Finally, they compiled causes of death for the 5,856 participants who died by 2000.

When considering air pollution, the epidemiologists specifically looked at levels of particulate matter, a mixture of airborne microscopic solids and liquid droplets. That includes acids (such as nitrates), organic chemicals, metals dust and allergens.

Small particles less than 2.5 micrometers in diameter pose the greatest problems to health because they can penetrate deep into the lungs and sometimes even enter the bloodstream. In this study, the researchers tracked this particulate matter, called PM2.5, across the neighborhoods of Los Angeles. It is often found in smoke, vehicle exhaust, industrial emissions and haze, driven by the burning of fossil fuels. Scientists also tracked ozone pollution, but found no link between ozone levels and mortality.

Increased deaths from heart disease jibe with the scientists' earlier research showing links between air pollution and atherosclerosis, a thickening of artery walls that may lead to heart attack and stroke. They believe particulate matter may promote inflammatory processes, including atherosclerosis, in key tissues. "We have convincing evidence that those causes of death that we might expect from inflammation, ischemic heart disease and lung disorders, are elevated in areas of higher pollution levels," he said.

Researchers also saw more than a twofold increased risk of death from diabetes, although numbers of diabetes-related deaths were smaller than those from heart disease, making findings less reliable. "People who are diabetic may be more susceptible to day-to-day fluctuations in air pollution," Jerrett said. "They may experience a state of greater inflammation-related to insulin resistance-that makes their lungs more receptive to receiving harmful particles."

Jerrett noted that findings might have been affected by participants who moved during the study or who changed their lifestyle since 1982. Another limitation is that scientists could only use participants' zip codes, rather than their home addresses, to determine their home neighborhood.

Researchers will conduct a similar study in New York City to try to duplicate findings. They hope to determine whether Los Angeles' tailpipe-emission-driven pollution poses a greater danger than that in the eastern United States, where power plants and factories contribute more heavily to pollution. They also plan to better understand pollution's effects on diabetes, and will use more specific measures to assess pollution within neighborhoods.

Because of the large number of participants in the American Cancer Society's study (more than a million people in 150 cities), policymakers in the past have relied heavily on findings from the study to set the nation's air-quality standards.

"These findings should give us some pause to think about what we need to do as a society," Jerrett said. "Restrictions on tailpipe emissions have gotten tighter, but there are more trucks and cars on the roads and people are driving farther. This study may cause us to reflect on how we use our cars, what cars we drive and whether we can do anything to make tailpipe emissions from all vehicles less harmful to health."

The Health Effects Institute and the National Institute of Environmental Health Sciences supported the research.

Michael Jerrett, Richard T. Burnett, Renjun Ma, C. Arden Pope III, Daniel Krewski, K. Bruce Newbold, George Thurston, Yuanli Shi, Norm Finkelstein, Eugenia E. Calle and Micheal J. Thun, "Spatial Analysis of Air Pollution and Mortality in Los Angeles," Epidemiology. Vol. 16, No. 6 (published early on Epi Fast-Track, http://www.epidem.com).

Researchers Find Traffic-Related Pollution Causes Childhood Asthma

Living near a freeway may mean more than the annoying rumble of cars and trucks: For children, it brings an increased risk of asthma, according to researchers at the Keck School of Medicine of the University of Southern California.

Scientists studying air pollution levels in 10 Southern California cities found that the closer children live to a freeway, the greater their chance of having been diagnosed with asthma. They report their findings in the November issue of the journal Epidemiology.

Researchers also found that children who had higher levels of nitrogen dioxide (NO2) in the air around their homes were more likely to have developed asthma. NO2 is a product of pollutants emitted from combustion engines, such as those in cars and trucks.

"These results suggest that tailpipe pollutants from freeway traffic are a significant risk factor for asthma," said lead author James Gauderman, PhD, associate professor of preventive medicine at the Keck School. "Considering the enormous costs associated with childhood asthma, today's public policy toward regulating pollutants may merit some re-evaluation."

"These results have both scientific and public health implications," said David A. Schwartz, MD, director of the National Institute of Environmental Health Sciences, the federal agency that funded the study. "They strengthen an emerging body of evidence that air pollution can cause asthma, and that exposure to outdoor levels of nitrogen dioxide and other traffic-related air pollutants may be a significant risk factor for this illness."

Researchers looked at the pollution-asthma link in 208 children who were part of the USC-led Children's Health Study, the longest investigation ever into air pollution and kids' health. The study has tracked the respiratory health of children in a group of Southern California cities since 1993.

The investigators placed air samplers outside the home of each student to measure NO2 levels. In addition, they determined the distance of each child's home from local freeways, as well as how many vehicles traveled within 150 meters (about 164 yards) of the child's home. Finally, they estimated traffic-related air pollution levels at each child's home using models that take weather conditions, vehicle counts and other important factors into account.

In all, 31 children (15 percent) had asthma. Scientists found a link between asthma prevalence in the children and NO2 levels at their homes. For each increase of 5.7 parts per billion in average NO2-which represents a typical range from low to high pollution levels among Southern California cities-the risk of asthma increased by 83 percent. Risk of wheezing and current asthma medication use also rose as NO2 levels increased.

They also found that the closer the students lived to a freeway, the higher the NO2 levels outside their homes. NO2 levels also corresponded with traffic-related pollution estimates from the group's statistical model.

It was not surprising, then, when they found that the closer the students lived to a freeway, the higher the students' asthma prevalence. For every 1.2 kilometers (about three-quarters of a mile) the students lived closer to the freeway, asthma risk increased by 89 percent. For example, students who lived 400 meters from the freeway had an 89 percent higher risk of asthma than students living 1,600 meters away from the freeway.

Interestingly, the researchers saw that air pollution from freeway traffic influenced NO2 concentrations at homes more strongly than pollution from other types of roads. Traffic counts within 150 meters of homes (which primarily comprised traffic from smaller streets) were only weakly correlated with measured NO2.

In any community, a freeway is a major source of air pollution. "Cars and trucks traveling on freeways and other large roads may be a bigger source of pollutants that matter for asthma than traffic on smaller roads," Gauderman said. Scientists also find it difficult to get good data on traffic on smaller streets, which may make it harder to find associations between asthma and local traffic.

Gauderman cautions that researchers do not yet know that NO2 is to blame for the asthma. NO2 travels together with other airborne pollutants, such as particulate matter, so it may be a marker for other asthma-causing pollutants.

Study sites included the cities of Alpine, Atascadero, Lake Elsinore, Lancaster, Long Beach, Mira Loma, Riverside, San Dimas, Santa Maria and Upland.

W.J. Gauderman, E. Avol, F. Lurmann, N. Kuenzli, F. Gilliland, J. Peters and R. McConnell, "Childhood Asthma and Exposure to Traffic and Nitrogen Dioxide," Epidemiology. Vol. 16, No. 6, November 2005.

This article originally appeared in the 09/01/2005 issue of Environmental Protection.

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