As the world is getting more and more densely populated, air pollution has become a severe problem. Air pollution mainly derives from four major human activity sectors: industry, energy, transportation and agriculture. Energies are needed to run factories, to power trains, planes and buses. Nearly all of these energies are produced by burning fuels, which would cause air pollution. Scientists are studying new ways of generating electricity that can be less damaging to the environment. The increased public environmental awareness has led to the advent of activities such as recycling.
随着世界人口越来越密集（densely populated），空气污染已经成了严重的问题。空气污染主要来源于四个主要的人类活动领域：工业、能源业、交通运输业以及农业。经营工厂，为火车、飞机和公共汽车提供动力都需要能源。几乎所有这些能源都是通过燃烧燃料产生的，这就会造成空气污染。科学家们正在研究能减少环境破坏的新发电方式。增强的公共环保意识使诸如回收利用（recycling）等活动产生。
