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托福阅读材料推荐:空气污染问题

2016-07-20 14:18:44来源：网络

　　本文是Air Pollution Facts, Air Pollution Effects, Air Pollution Solutions, Air Pollution Causes - National Geographic托福阅读材料。托福阅读材料推荐:空气污染问题希望对大家托福备考有所帮助。更多精彩尽请关注新东方在线托福网!

Air Pollution

托福阅读材料推荐:空气污染问题

　　Comes From Many Sources

　　Here's what you need to know about the warming planet, how it's affecting us, and what's at stake.

　　Smog hanging over cities is the most familiar and obvious form of air pollution. But there are different kinds of pollution—some visible, some invisible—that contribute to global warming. Generally any substance that people introduce into the atmosphere that has damaging effects on living things and the environment is considered air pollution.

　　Carbon dioxide, a greenhouse gas, is the main pollutant that is warming Earth. Though living things emit carbon dioxide when they breathe, carbon dioxide is widely considered to be a pollutant when associated with cars, planes, power plants, and other human activities that involve the burning of fossil fuels such as gasoline and natural gas. In the past 150 years, such activities have pumped enough carbon dioxide into the atmosphere to raise its levels higher than they have been for hundreds of thousands of years.

　　Other greenhouse gases include methane—which comes from such sources as swamps and gas emitted by livestock—and chlorofluorocarbons (CFCs), which were used in refrigerants and aerosol propellants until they were banned because of their deteriorating effect on Earth's ozone layer.

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