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P1Hazardous levels of air pollution have become a defining feature of life in many of the world's largest cities. This essay examines the principal causes of the decline and outlines the measures most likely to reverse it.

P2The deterioration stems largely from two interlinked sources. The first and most visible is road traffic. As urban populations swell, the sheer number of petrol and diesel vehicles releases vast quantities of nitrogen oxides and fine particulates at street level, precisely where people breathe. The second source is industrial and domestic energy use, much of it still derived from coal and other fossil fuels, which adds sulphur dioxide and soot to the urban atmosphere. Rapid, poorly regulated construction compounds the problem by throwing dust into already saturated air.

P3A problem with several roots demands action on several fronts. The single most effective step would be to curb vehicle emissions, both by investing heavily in clean, frequent public transport so that driving becomes a choice rather than a necessity, and by progressively restricting the most polluting vehicles from city centres, as London has done through its low-emission zone. Alongside this, governments must accelerate the shift from fossil fuels to renewable electricity, since cleaner generation reduces pollution at its source. Stricter enforcement of emissions standards on factories and building sites would reinforce these gains.