Los Angeles' air sure looks better than it did decades ago. Less lung-stinging ozone hangs over downtown, and the deep bourbon hues of summertime skies are fainter than ever before. But looks are deceiving. A new threat haunts the air we breathe – particles tinier than a virus; so small that, in the diesel-belching ports of Los Angeles and Long beach, more than a million of them fit in a marble-sized chunk of air. These ultrafine particles become lodged in our lungs and hearts and are the culprit behind growing cancer rates. Some 9,600 people will die this year in California because of a smog-related disease. This amounts to a public-health emergency. Why, then, are only a handful of scientists, doctors and public officials responding to the challenge?