Ozone — three oxygen atoms in a single molecule — is a powerful anti-oxidant that damages lungs. Recent research has shown that even low levels cause respiratory problems. As a result, the Environmental Protection Agency has lowered the acceptable level for ozone.
Ozone is mainly a summer problem. Producing it requires the right combination of heat, sunlight and pollutants. Like wintertime particulates, it accumulates in our soup-bowl valleys when the air stagnates. But, unlike the winter inversions, ozone levels decline overnight when the sunlight goes away.
That means early mornings — particularly before the roads fill with morning commuters — are the peak time for outdoor breathing.