Passage FourQuestions 16to 20are based on the following passage.People who live and work in areas with elevated levels of ozone and other airborne pollutants appear to run an increased risk of lung cancer, US researchers report in the December issue of the journal Environmental Health Perspectives. The researchers, Dr Beeson of Texas University and colleagues studied more than 4,000 female and 2,000 male, white, nonsmoking volunteers from 1977 to 1992.At the start of the study, the volunteers filled out questionnaires about their occupations, their exercise patterns, diet and other lifestyle choices, and their family's health history. The Questionnaires also asked whether the volunteers had any respiratory symptoms, how many hours they spent outdoors, and where they lived and worked. The researchers updated this information in 1987 and again in 1992.Using air quality monitoring station data, Beeson and colleagues then determined the levels of particle soot, ozone or "smog", sulfur dioxide, and other pollutants that the volunteers were exposed to, given where they lived and worked. Over the course of the 15-year study, 20 of the women and 16 of the men were diagnosed with lung cancer.Analyzing the relationship between exposure to airborne pollutants and lung cancer risk, the researchers found that both men and women regularly exposed to levels of particle soot that were lower than the National Ambient Air Quality Standard of 50 microgram per meter cubed ran an increased risk of lung cancer. And both men and women exposed to elevated levels of sulfur dioxide ran an increased risk of lung cancer.In addition, men regularly exposed to ozone levels of 80 parts per billion (ppb) ran more than three times the risk of lung cancer as men exposed to lower levels. The Environmental Protection Agency (EPA) limit on ozone is 120 ppb, Beeson and colleagues report. Women, however, did not appear to run an increased risk of lung cancer if exposed to high levels of smog."This gender difference may be due to the males spending much more time outdoors than females," they write. "This was especially true for the summer when ozone levels are higher." The difference may also have been due to hormonal differences, they add. Some research findings also suggest that the female sex hormone estrogen may partly offset the consequences of exposure to high ozone levels."Our findings suggest that the current EPA standard of 120 ppb for ozone may not adequately protect the large portion of the US male population who live or work in communities where the current standard for ozone is frequently exceeded," Beeson and colleagues conclude. "More research with a larger number of incident cases of lung cancer is needed to better understand the observed gender difference in regard to ozone exposure as well as to better separate the independent effects of ozone, airborne particulate matter sulfur dioxide, and other airborne pollutants."