Bad Breath

Studies are homing in on which particles polluting the air are most sickening — and why.

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By Janet Raloff, Science News

Tasteless. Invisible to the eye. Air contaminants less than a tenth the size of a pollen grain are nevertheless dangerous.

Even on a clear, sunny day, many tens of thousands — and potentially millions — of tiny particles cloud every breath you take. Some are nearly pure carbon. But reactive metals, acids, oily hydrocarbons and other organic chemicals jacket most of these motes.

Epidemiologists have been calculating human tolls by comparing how many people die when particle numbers in the air are high against mortality figures on cleaner days. Over the past couple of decades, those data have been implicating tiny airborne particles in the deaths of huge numbers of people each year — even where concentrations of these microscopic contaminants never exceed levels permitted by U.S. law.

Based on extrapolations from such data, in China alone an estimated 1 million people die prematurely each year from the toxic inhalation of tiny airborne specks, according to Staci Simonich of Oregon State University in Corvallis and her colleagues in an upcoming Environmental Science & Technology.

Formally referred to as particulate matter, or just PM, these tiny particles from mostly outdoor sources and indoor tobacco smoke together “rival ‘overweight and obesity’ as causes of premature death,” Armistead Russell of the Georgia Institute of Technology in Atlanta and Bert Brunekreef of Utrecht University in the Netherlands report in the July 1 issue of the same journal.

Airborne particles range from windblown dust and grit to nanoscale specks that won’t darken the skies, even when 100,000 particles cloud each cubic centimeter of air. The potential impacts of these little particles also vary widely. Particles big enough to taste and feel can be an uncomfortable nuisance but don’t tend to pose big health risks. Fine particles, especially those 2.5 micrometers in diameter or smaller, are another story.

Studies reported over the past few months indict such specks in contributing to or aggravating a host of conditions, from asthma and stroke to heart disease and premature aging of chromosomes.

Industrial smokestacks aren’t the only villains. Several new studies finger vehicular traffic as a major driver of particulate-triggered illness.

Trafficking in heart disease

No surprise, combustion from cars and trucks is a major source of PM-2.5, also known as “fine” particles — ones 2.5 micrometers and smaller — and especially of “ultrafines,” particles no more than 0.1 micrometers (100 nanometers) in diameter, notes Peter Adams of Carnegie Mellon University in Pittsburgh.

The U.S. Environmental Protection Agency regulates concentrations of PM-2.5 in the air. But by number — not mass — the vast majority of airborne particles are ultrafines. They’re as yet unregulated, but not because anyone thinks they are unimportant. Studies have confirmed that they’re the particles inhaled most deeply into the lungs, from which some pass directly into the blood.

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