When city skies are thick with smog, people who are sick, injured, elderly or young often die at higher rates.
A University of Delaware scientist says air pollution threatens healthy adults, too, because tiny particles can zoom through human lungs up to two times faster and penetrate deeper than previously assumed.
"Smog kills," says air-pollution expert Anthony S. Wexler, a professor of mechanical engineering at UD, "perhaps partly because pollutant particles are so deeply deposited in our airways."
A study by Ramesh Sarangapani and Wexler, expected to appear in the next Journal of Aerosol Science, reveals how pollutant particles smaller than 2.5 micrometers--a size identified by the U.S. Environmental Protection Agency (EPA) as hazardous--penetrate buildings and people's airways.
From car exhaust and power-plant emissions to fumes from household fireplaces and wood stoves, various human activities fill the air with tiny pollutant particles. In the atmosphere, fine particles also can be generated by gases such as sulfur dioxide, nitrogen oxides and volatile organic compounds.
When they float solo, these particles are so small, they can't be seen without a microscope. Yet, clusters of fine particles produce clouds of dust, black soot and gray haze. And, when ground-level ozone mixes with air pollution, choking smog can blanket U.S. cities, reducing visibility by 70 percent in some regions, the EPA reports.
"Tens of thousands of elderly people die prematurely each year from exposure to ambient levels of fine particles," according to official EPA information. And, because children breathe 50 percent more air per pound of body weight, compared to adults, they're more susceptible to tiny bits of air pollution, especially if they suffer from asthma.
To combat the human health effects of fine pollutant particles, EPA Administrator Carol M. Browner signed new air-quality standards July 16, 1997, recommending lower levels of exposure. On an annual basis, the EPA says, levels of very small particles (>2.5 microns) shouldn't exceed 15 micrograms per cubic meter of air, on average. And, on any given day, levels of such particles should never spike higher than 65 micrograms.
The National Ambient Air Quality Standards "were established to protect human health," Wexler notes. "They were put in place because of an apparent correlation between concentrations of particulate matter and data on people getting sick and dying. But, the physiological connection between particulate matter and health has never been fully explained. Our research is a first step toward better understanding how these tiny particles affect us."
While preparing his doctoral dissertation, Sarangapani has teamed with Wexler to prepare several recent journal articles. Their first study examined fine particles in upper airways, while the current paper describes how particles disperse in the lungs. A third article, submitted to Fundamental Applied Toxicology, "will make it clear why some people die from breathing too many of these particles," Wexler says.
A fast and invisible threat
Clearly, whenever levels of fine pollutant particles rise in major cities such as New York and Philadelphia, an increased number of people die or enter hospitals, according to Wexler. Epidemiological studies, comparing weather and air-quality data with mortality and morbidity statistics, "tend to suggest a connection," he says.
But, exactly how do fine particles impair breathing capacity? Researchers have long known that tiny pollutants penetrate airways via three biomechanical events: (1) gravitational settling; (2) the random collisions of gaseous molecules, known as diffusion; and (3) impaction, which occurs when particles can't turn corners and hit respiratory walls.
Sarangapani says, however, that these mechanisms don't tell the whole story about fine particles and human health. Now, he and Wexler have more fully described two other activities of tiny particles zooming through airways: dispersion and expansion resulting from contact with moisture.
"As people breathe," Wexler explains, "a clump of fine particles called a bolus will rapidly disperse throughout the lungs. At the terminal alveoli-little sacks at the end of each respiratory branch, where oxygen and carbon dioxide trade places with blood-these particles take up water and expand, much like a sponge, because of hydroscopic effects."
Mathematical models of these physical events (dispersion and hydroscopic expansion) suggest that "the smallest particles can sometimes penetrate almost two times farther into airways than we had suspected," Wexler says.
That's because air in the center of a lung tube flows faster than the surrounding stream, he explains. And, particle-laden air mixes with clean air at each intersection of the respiratory branches. All that secondary mixing "dramatically speeds the movement of these fine particles through the respiratory system," Wexler reports.
The next step, Sarangapani says, is to further investigate why fine particles can be toxic in the lungs. "With the current amount of knowledge available to us," he says, "I think that the EPA's current standards are a reasonable response. But, additional research is needed to identify the precise mechanisms involved in particulate toxicity."
To model fine particles moving through airways, the researchers analyzed data from clinical studies of people whose lungs were injected with a bolus of particles. Their study helps explain the impact of air pollution on human health. And, Sarangapani says, it should prove useful to clinicians who use the bolus-injection technique to non-invasively assess the lung function of people with asthma and others with impaired respiratory function.
Along with Sarangapani and Wexler, this research team includes UD undergraduate Mark Mathre, whose work also is supervised by Ajay K. Prasad, an associate professor of mechanical engineering.
This research was sponsored by the Electric Power Research Institute.
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