Persistence of adaptation to ozone in volunteers exposed repeatedly for six weeks

Eleven generally healthy volunteer subjects were exposed daily to ozone at 0.47 ppm in an attempt to develop adaptation, then reexposed weekly to investigate the persistence of adaptation. Responses were assessed in terms of forced expiratory function and symptoms. Most subjects, when exposed 2 h daily for 4 days with intermittent exercise, developed adaptation as seen in previous studies. The adaptation was partly lost with a 4-day interval between successive exposures, and was more or less completely lost with a 7-day interval between exposures. One subject, who may have had a persistent low-grade respiratory infection, never adapted. Two others showed relatively little response in the initial daily exposures, but showed more severe responses at some point during the later weekly exposures. If these results are relevant to ambient oxidant pollution effects, adaptation may be of relatively little importance in a public-health sense, given that it may fail to develop or may be lost quickly in the absence of very frequent exposures.