If you find yourself short of breath on your next hike, it may be more than just exertion. AMC researchers and others have studied how hikers' health is affected by ozone, a gas that is beneficial in the earth's upper atmosphere where it occurs naturally, but is a health threat at ground level where it forms as a product of pollution.

The Environmental Protection Agency categorizes children and adults who are active in the outdoors as people who are "sensitive" to ground-level ozone because their exercise causes them to breathe in polluted air faster and more deeply than other people. In the early 1990s, the AMC contributed to a hiker-health study looking at the effects of ozone and other pollutants on the lung function of adult hikers. The findings showed that hikers' lung function was impaired by ozone — even at concentrations considered to be "safe" by the EPA.

The Study
The study was conducted from 1990 to 1992 at the Pinkham Notch Visitor Center, located at the base of New Hampshire's Mount Washington, by researchers from the AMC, the Harvard School of Public Health, and Brigham and Women's Hospital. Hikers between the ages of 18 and 65 volunteered to answer a questionnaire with demographic information such as age and gender, as well as information about their aerobic fitness and history of respiratory illness and smoking. Hikers were also asked to participate in three to eight measurements of lung function (as forced expiratory volume) before and after their hike.

In this "double-blind" study, hikers and researchers were both unaware of ambient air-quality conditions that were being measured concurrently. As the hiker study was going on, ozone concentration was monitored at the base and summit of Mount Washington by the AMC. In addition, other factors that affect air quality — such as fine-particulate matter — were measured near the base of the mountain, so that these effects could be accounted for as well.

The Findings
The study found a decline in hikers' lung function related to ozone exposure during the hike, even after adjusting for hikers' age, gender, hours hiked, weight carried, smoking and respiratory health history, and ambient temperature. For each incremental increase in ozone of 50 parts per billion there was a 2.6 percent decline in the hikers' measured lung function.

Participants with a history of pulmonary disease had a significantly greater ozone-associated decline in lung capacity (7.5 percent for each 50 parts per billion increase). In addition, hikers who hiked for a longer time period, thus exposing themselves to more ozone, also experienced greater lung impairment. On the other hand, a hiker's level of work (as measured by pounds carried in a backpack or maximum pulse rate) was not a significant predictor of decrease in lung function.

Hikers should be aware that even ozone levels considered safe for the general public may affect hikers differently. Negative effects of ozone on hikers' lungs were found at an average one-hour ozone level of 40 parts per billion, which is well below the revised EPA standard of an average 85 parts per billion over an eight-hour period.

The Significance
This study, the first to include field measurements of people representing a broad range of age and fitness levels, found greater ozone effects than in most previous field and chamber studies. These findings underscore the special concern of ozone to outdoor recreationists. As the report concluded:

Physicians, public health officials, and the general public should be aware of the potential acute impacts of relatively low-level pollutants not only among residents of urban and industrial regions but also among individuals engaged in outdoor recreation in certain wilderness areas.

Learn more about the steps you can take to protect yourself.