Tiny particles in air deadly, engineer says

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http://cnn.com/NATURE/9908/17/particles.enn/

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)

Tiny particles of air pollutants can zoom through human lungs up to two

times faster than previously thought and pose a risk to healthy adults,

according to a university scientist.

" Smog kills, " said 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. "