插图  Sandstorms and the dust and gases from volcanoes pollute the earth's atmosphere naturally. But the most serious kind of air pollution comes from people. Factories, power stations, and rehicles exhausts[1] pump harmful gases into the air, contributing to global warming, causing acid rain[2], and destroying the ozone layer. On a much smaller scale, air pollution causes all sorts of breathing problems. There are no simple solutions to air pollution, but people could reduce air pollution by saving energy and reducing harmful emissions from vehicle exhausts and power stations.
  
When a volcano erupts, dust is blasted[3] high above the troposphere and may take weeks to be carried around the weather zone, so cannot be washed out of the air by rain. It will eventually fall to the earth after a few years.
  
Air pollution is at its worst over big cities where millions of people live, work, and travel about in cars, buses, and trucks. Factory chimmeys can be fitted with devices to cut down[4] air pollution, but this is expensive. If better pubic transport and cycle lanes were available in towns and cities, people would be less likely to use their cars, reducing the overall amount of pollution.
  
Smoking cigarettes adds to the general levels of air pollution as well as damaging our health. Cigarette smoke contains chemicals that can cause cancer, a gas which stops oxygen being taken into the blood and a substance called nicotine, which raises blood pressure and makes the heart beat faster. No-smoking areas cut down on this pollution and save non-smokers breathing in other people's cigarette smoke.

  沙暴和火山爆发喷出的尘埃和有毒气体自然而然地对地球大气造成的污染,然而最严重的空气污染却来自人类。工厂、火力发电站和车辆将有害废气排放到空气中,是造成全球气候变暖,出现酸雨,破坏臭氧层的罪魁祸首。就更小的范围来看,空气污染会引发各种呼吸道疾病。没有解决空气污染的简单办法,但人们可以通过节约能源、减少车辆和火力发电站的有害气体排放来达到降低污染的目的。

  火山爆发时,尘埃被高高喷射入对流层以上的空间,需要几个星期才飘浮移至气象层,因而雨水无法将其从大气中冲走。几年之后,尘埃将最终漂落在地球上。

  好几百万人在城市里工作、生活,还以轿车、公共汽车和卡车为交通工具,因而这种百万人口的城市空气污染最为严重。工厂烟囱可以安装滤清装置以减少空气污染,但成本昂贵。如能改善城镇公共交通,并设置自行车车道,人们就不再会热衷于使用自家汽车,从而达到全面减少污染的目的。

  吸烟不仅对健康有害,而且增加空气污染程度。香烟雾含有多种有害物质:不仅有致癌化学物质、有某种能阻止氧气进入血液的气体,还有引起血压增高、心速加快的被称为尼古丁的物质。设立禁烟区有助于减少污染,并能使不吸烟者免受被动吸烟之害。