她避免在尾气排放的环境中行走，不使用杀虫剂，购买有机食物，关闭窗户避免外部污染空气进入，在她房间里用空气过滤器。
She avoids walking through exhaust, doesn't use pesticides, buys organic foods, shuts her Windows to keep air pollution outdoors and USES air filters in her home.
当然，如果市场参与者的活动带来额外的(不由本人、而由社会所承担的)“外部”成本或效益，譬如污染，那么竞争就无法实现最优化了。
Of course if the activity of the market participants produces external costs or benefits-costs or benefits to nonparticipants, as in the case of pollution, competition will not optimize output.
外部环境致癌因素是指环境中导致罹患癌症的危险升高的因素，如空气污染、紫外线辐射及室内的氡等。
External environmental causes of cancer are factors in the environment that increase risk of cancer such as air pollution, UV radiation and indoor radon.
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