本句是called动词过去分词做后置定语，修饰a long chain of compounds. 后面是reactive(易反应的) components的定语从句

　　链接2 . （2012陕西卷）

　　Eating too much fatty food, exercising too little and smoking can raise your future risk of heart disease. But there is another factor that can cause your heart problems more immediately: the air you breathe.

　　Previous studies have linked high exposure(暴露) to environmental pollution to an increased risk of heart problems, but two analyses now show that poor air quality can lead to heart attack or stroke (中风) within as little as a few hours after exposure. In one review of the research, scientists found that people exposed to high levels of pollutants (污染)were up to 5% more likely to suffer a heart attack within days of exposure than those with lower exposure. A separate study of stroke patients showed that even air that the U. S. Environmental Protection Agency (EPA) considers to be of "moderate" (良好) quality and relatively safe for our health can raise the risk of stroke as much as 34% within 12 to 14 hours of exposure.

　　The authors of both studies stress that these risks are relatively small for healthy people and certainly modest compared with other risk factors such as smoking and high blood pressure. However, it is important to be aware of these dangers because everyone is exposed to air pollution regardless of lifestyle choices. So stricter regulation by the EPA of pollutants may not only improve environmental air quality but could also become necessary to protect public health.

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