Tiny particles in air deadly, engineer says

When ground-level ozone mixes with air pollution, smog can blanket cities, reducing visibility by 70 percent in some regions.

August 17, 1999
Web posted at: 12:34 p.m. EDT (1634 GMT)

"Smog kills," said Anthony S. Wexler, a professor of mechanical engineering at the University of Delaware, "perhaps partly because pollutant particles are so deeply deposited in our airways."

A study conducted by Wexler and colleague Ramesh Sarangapani shows how pollutant particles smaller than 2.5 micrometers — a size identified by the Environmental Protection Agency as hazardous — penetrate buildings and people's airways.

Pollutant particles are found in car exhaust, power-plant emissions and the smoke from fireplaces and wood stoves. Clusters of the particles produce clouds of dust, haze and smog. "Tens of thousands of elderly people die prematurely each year from exposure to ambient levels of fine particles," according to EPA.

In a paper to be published in the Journal of Aerosol Science, the scientists explain how particles penetrate human airways through dispersion and expansion resulting from contact with moisture.

"As people breathe," said Wexler, "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 suggest that "the smallest particles can sometimes penetrate almost two times farther into airways than we had suspected," said Wexler.

That's because air in the center of a lung tube flows faster than the surrounding stream, he said. 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," he said.

The next step, Sarangapani said, is to further investigate why fine particles can be toxic in the lungs. "With the current amount of knowledge available to us," he said, "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."

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