单项选择题

American researchers say they have found the strongest connection between ozone(臭氧) pollution and damage to health. Their findings show that short-term increases in ozone lead to higher death rates in cities.
Ozone is form of oxygen. The gas is produced naturally in the upper atmosphere to protect the earth against radiation from the sun. But human activities can also create ozone in the lower atmosphere. Gasses from cars and industry react with sunlight to form this ozone. Levels usually increase in the warmer months. Ozone is the main chemical in smog, the air pollution that is a combination of fog and smoke.
Ozone has been connected with heart and lung problems especially, and with higher rates of hospital cases. Researchers from Yale University and Johns Hopkins University did the study. Michelle Bell of Yale was the lead researcher. The Journal of the American Medical Association published the results. The researchers collected information on 95 American cities. These contain about 40 percent of the national population. The study compared death rates to ozone levels between 1987 and 2000. The research suggests that even a small increase in ozone, 10 parts per thousand million, can lead to higher death rates the following week. The study found that the average daily number of deaths rose point-six percent. And deaths among older people rose point-seven percent.
The study is one of the largest ever done of ozone and death rates. The researchers note that ozone is widespread in the United States and many other countries. The United States Environmental Protection Agency is re-examining its air pollution rules. The limit at present for ozone is 80 parts per thousand million for an eight-hour period. Limits were higher in the past. But the researchers say they found an increase in deaths even below the present levels.
According to the results of the study from Yale University and Johns Hopkins University, we
know that ______.