Bad Breath

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.