An article in the NY Times reported on an EPA study that is to come out in April. It shows that even low levels of ozone contribute to premature mortality.
Ozone's Effects in the Lower Atmosphere
0.003 to 0.010 ppm
Lowest levels detected by the average person (by odor).
0.08 ppm Latest EPA study (to publish April 2006) reports significantly increased risk of premature death in humans. Each 0.01 ppm increase results in a 0.3 percent increase in early mortality.
0.001 to 0.125 ppm
The natural ozone concentration in air.
0.1 ppm
The typical maximum allowable continuous ozone concentration in industrial work areas and public and private spaces.
0.15 to 0.51 ppm
The typical peak concentration in American cities.
0.2 ppm
Prolonged exposure of humans under typical work conditions produced no apparent effects.
0.3 ppm
The threshold level for nasal and throat irritation. Some species of plant life show damage.
0.5 ppm
The level at which Los Angeles, California, declares its Smog Alert No. 1.; can cause nausea and headaches.
1 to 2 ppm
The level at which Los Angeles, California, declares its Smog Alerts No. 2 (1.00 ppm) and No. 3 (1.50 ppm). Symptoms: headache, pain in the chest and dryness of the respiratory tract.
1.4 to 5.6 ppm
Causes severe damage to plants.
5 to 25 ppm
Lethal to animals in several hours.
25+ ppm
Likely lethal to humans in one hour.