January 25, 2008 — The tiniest particles of air pollution from vehicle emissions may be the deadliest to people at risk for heart disease, according to a new study by researchers at UCLA, the University of Southern California, UC Irvine and Michigan State University.
The multi-campus team used new technologies and exposure designs developed by Constantinos Sioutas, a professor in the Sonny Astani Department of Civil and Environmental Engineering at USC, professors Andre Nel and Jesus Araujo of UCLA, and Michael Kleinman, professor of community and environmental medicine at UC Irvine, to
Constantinos Sioutas in his pollution lab prepares to take some measurements.
track nano-sized particles. Reporting their findings in the January 17 online issue of Circulation Research
, the researchers said these microscopic specks of pollution are not only more abundant in urban areas than larger, coarser particles, but also far more toxic.
“The size of nanoparticles has posed a great challenge to scientists in the past in studying their chemical and toxic effects,” said Sioutas, the Fred Champion Professor in Civil and Environmental Engineering and co-director of the Southern California Particle Center and Supersite (SCPCS). “Now that we have found ways to sample them in the form that they exist in the atmosphere, expose humans or animals to high concentrations of them, and track them once they are inhaled, we are finding that they are perhaps the most toxic of all the pollutants in city air.”
Nanoparticles measure less than 0.1-0.2 micrometers in diameter, or about one-thousandth the size of a human hair. The Environmental Protection Agency (EPA) currently does not regulate particle pollutants that small, partly because those particles were too difficult to separate from the larger particulate mass in the atmosphere. The study, which was led by Nel, UCLA’s chief of NanoMedicine, was funded primarily by the EPA and National Institute of Environmental Health Sciences in an attempt to explore stricter air quality guidelines for vehicle emissions.
The team, which included medical, cardiology and environmental scientists, discovered how these nanoparticles contribute to hardening of the arteries by turning off, or inactivating, the protective characteristics of high density lipoprotein (HDL), otherwise known as “good” cholesterol.
"This is the first study that demonstrates the ability of nano-sized air pollutants to promote atherosclerosis in an animal model," said first author Dr. Jesus Araujo, assistant professor of medicine and director of environmental cardiology at the David Geffen School of Medicine at UCLA, in a UCLA Health Sciences press release.
Sioutas, silhouetted against the brownish haze of the Los Angeles skyline, spends a lot of time collecting samples of air near major freeways.
Pollution particles emitted by vehicles and other combustion sources contain a high concentration of organic chemicals that could be released deep into the lungs or even spill over into the systemic circulation, Sioutas noted.
“The smallest particles are the most toxic because they penetrate into the deep lung, and through the alveoli, they may enter the blood stream due to their small size,” he said.
In the study, the team exposed mice with high cholesterol to one of two sizes of air pollutant particles from downtown Los Angeles freeway emissions over a five-week period and compared them with mice that received filtered air that contained very few particles.
Sioutas and his colleagues developed a complex exposure method to study levels of toxicity in the mice. They found that mice exposed to ultrafine particles exhibited 55 percent greater atherosclerotic-plaque development than animals breathing filtered air, and 25 percent greater plaque development than mice exposed to fine-sized particles.
Pollutant particles are coated in chemicals sensitive to free radicals, which cause the cell and tissue damage known as oxidation, according to Nel, principal investigator of the study. Oxidation leads to the inflammation that causes clogged arteries.
“In the samples from polluted air, we found that ultrafine particles, which are less than 0.1 micrometer in diameter, carried larger concentrations of these chemicals compared to larger particles,” said Sioutas, who has reported similar findings in previous studies.
He noted that earlier studies investigating the links between cardiovascular disease and air pollution have taken place over longer periods of exposure time. The current study suggests that cardiovascular damage can occur more rapidly.
Other researchers on the study included Berenice Barajas, Xuping Wang, Brian J. Bennett and Ke Wei Gong of the David Geffen School of Medicine at UCLA, and Jack Harkema from the department of pathobiology and diagnostic investigation at Michigan State University.
Additional funding was provided by grants from the National Institute of Allergy and Infectious Diseases, the National Heart, Lung and Blood Institute, and the Robert Wood Johnson Foundation.