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August 31, 1999

Traffic Exhaust Poisons Home Air

Scientists have demonstrated that where you live -- and, specifically, the traffic outside your front door -- affects the quality of the air inside your home. A study showed that homes situated near major highways had worse indoor air pollution than those in more rural settings, with respect to PAHs (polycyclic aromatic hydrocarbons), a class of compounds that contain known cancer-causing toxins. Exhaust fumes from motor vehicle traffic seeped inside two urban dwellings, peaking during morning and evening rush hours, whereas PAH levels peaked later and were lower in a suburban residence further away from traffic.

The study, which appeared in the August issue of the Journal of Exposure Analysis and Environmental Health, also showed that cooking contributes substantially to PAH concentrations indoors. Individuals with underlying respiratory disease, such as, asthma or emphysema, are susceptible to the effects of air pollution.

PAHs are a family of molecules produced by incomplete combustion, often from open burning, incineration, industrial power generation, and motor vehicles. Because they can be carcinogenic and are dispersed throughout the environment, PAHs pose a significant public health concern, said senior author Timothy J. Buckley, PhD, assistant professor, Environmental Health Sciences, Johns Hopkins School of Public Health. "And," he noted, "since people spend most of their time indoors, it's important to evaluate both the indoor and outdoor sources of PAH that contribute to indoor exposure so we can determine effective ways they can be controlled and lessened."

During the summer of 1997, for approximately two weeks at each of three nonsmoking residences -- one urban, one semi- urban, and one suburban -- the scientists used sensitive monitors to measure minute-by-minute PAH concentrations so they could profile daily variations in indoor levels. Each of the three residences had a different proximity to, and density of, traffic sources.

In addition to the outdoor traffic source, study investigators noted that indoor PAH concentrations were also strongly influenced by cooking and other indoor combustion related activities such as candle burning, as became apparent from daily records kept by residents.

The urban dwelling was situated nearest to major roadways carrying an average of 233,000 cars and trucks on weekdays and 220,000 on weekend days. The semi-urban residence was slightly farther from major roadways that carried 231,000 vehicles each weekday and 218,000 each weekend day. The suburban dwelling was further away still, and was exposed to roughly 25 percent of the urban locations' burden.

Traffic emerged as the dominant outdoor source of indoor PAHs, its effect most pronounced during the morning rush hour. The urban location's indoor PAH levels began to rise at 5:30 a.m. and reached a peak at 8:30 a.m. The semi-urban location's indoor PAH levels also peaked at that time, yet its PAH concentrations were somewhat lower. The suburban dwelling's morning peak lagged behind, appearing at about 9:30 a.m., and was lower still. All three locations exhibited a lull around noon, after which the urban and semi-urban locations once again exhibited a slow increase in indoor PAH levels, which peaked again at approximately 5:00 p.m. Evidence of an evening rush hour at the suburban location was far weaker. A third peak at 10:00 p.m. was detected inside the urban dwelling.

Indoor concentrations at all three locations were two to three times less on weekend days than on weekdays, and weekend PAH levels didn't fluctuate like those during the week, further evidence that traffic was indeed the predominant outdoor source of PAH levels inside the residences.

Cooking also contributed significantly to indoor concentrations of PAHs, with frying and sautéing generating peak exposures roughly ten to 20 times greater than those from any other source. "If there is a threshold Level above which PAHs' toxic effects can thwart the body's metabolic or repair processes," said Dr. Buckley, "this research is important in identifying the peak indoor concentrations that may overwhelm these protective processes. High peak concentrations from cooking superimposed upon high pollution levels from traffic in high traffic areas could create indoor levels of health concern." The authors cautioned, however, that their observations regarding the contribution of cooking to indoor PAH concentrations should be considered as preliminary evidence of a significant source.

This work was supported in part by the National Institutes of Environmental Health Sciences.

Public Affairs Media Contacts for the Johns Hopkins Bloomberg School of Public Health: Tim Parsons or Kenna Brigham @ 410-955-6878 or paffairs@jhsph.edu.