WAITING TO INHALE
High in the stratosphere, ozone (O3) shields us from radiation that can cause skin cancer. But in the troposphere--the layer of atmosphere in which we live and breathe--ozone is a pollutant, formed as a by-product of chemical interactions among other pollutants. And it's very hard to avoid entirely. As a recently published study shows, even hikers in wilderness regions and national parks may suffer impaired lung function from ozone exposure. "There is pollution in areas that are otherwise pristine," says instructor in medicine Susan Korrick '78, M.P.H. '91, M.D., the lead author of the study, which appeared recently in Environmental Health Perspectives. "Even with no local industry, pollutants blown in from elsewhere can have measurable effects."
Korrick's research took place at New Hampshire's Mount Washington on 74 good hiking days during the summers of 1991 and 1992. Her team recruited 530 recreational hikers and tested each of them with a spirometer before they ascended the mountain, then tested them again after their descent. The instrument measured both the volume of air a hiker could exhale in one breath and the volume exhaled in one second.
Meanwhile, scientists from the Appalachian Mountain Club measured levels of ozone, fine particles, and particle acidity in the air. Correlating these air-pollution data with the spirometer tests, Korrick asked if there were a statistical relationship between the extent of air pollution on a given day and diminished lung function for those who had just completed an eight-hour hike. It turned out that even at fairly low exposures--20 to 74 parts per billion (ppb) of ozone--hikers showed decreased lung function. (The 1997 national ambient air quality standard set by the Environmental Protection Agency is 80 ppb over eight hours.) An increase of 50 ppb in ozone was associated with an average decline of 2.6 percent in volume of air exhaled in one second. The 40 hikers who described themselves as asthmatics, or reported episodes of wheezing, showed a 7.5 percent drop in volume--four times as severe as the 1.8 percent change for the 490 non-asthmatics.
Prevailing winds blow pollutants from industries in the northeastern and central states into the Mount Washington area. In 1997, the highest recorded average level of ozone there for an eight-hour period was 100 ppb. "We are generally exposed to significant amounts of ozone when we are outdoors," says Korrick, "and it has a bigger effect on your lungs if you are exercising and breathing hard. This is not to say that people shouldn't continue to hike or work out outdoors. It's very unlikely that a healthy person would notice the degree of change we found, though an asthmatic might. We are looking here only at acute effects, which are usually reversible within 24 hours. Even so, we're finding these changes [in lung function] at ozone levels that in the recent past were thought to be fairly safe."
~ Craig Lambert