Permissible Ozone Levels in Houses

Formation

• Typically, ground-level ozone is produced by chemical reactions caused by oxides of nitrogen and volatile organic compounds like those found in car exhaust, gasoline vapors and various solvents. Some vendors sell so-called air cleaners that are intended to emit ozone. According to the EPA, these devices are unlikely to result in cleaner air around the home; in fact, they may instead lead to ozone concentrations exceeding safe levels.

Current Standards

• Beginning in 2008, the EPA standard for ambient air sets the maximum safe exposure level at 75 parts per billion averaged over eight hours. In other words, if you measure concentration over an eight-hour period, the average should be below 75 parts per billion. The maximum one hour safe exposure level is 120 parts per billion. The eight-hour standard is actually lower than the original 1997 standard, which set the safe level at 80 parts per billion.

Workplaces

• The Occupational Health and Safety Administration (OSHA) sets standards for safe exposure levels in workplaces. Currently OSHA has set a maximum time-weighted average level of 100 parts per billion; the maximum permissible short-term exposure level is 300 parts per billion. According to OSHA, experiments have shown levels as low as a few parts per million may be lethal to mice, so concentrations in excess of these standards could very well be dangerous.

Considerations

• There is some evidence to suggest levels even lower than those in the NAAQS could pose a threat to human health. A 2009 study in the American Journal of Respiratory and Critical Care Medicine looked at lung function in a small group of healthy nonsmoking volunteers exposed to low levels of ozone during exercise. On average, a 6.6-hour exposure to levels of 70 parts per billion caused a statistically significant decrease in lung function compared with the control group. A level of 60 parts per billion, on the other hand, did not cause a significant change.