Air pollution is a massive problem throughout the world, known to cause millions of premature deaths due to respiratory disease, heart disease, stroke and much more including cancer.
The problem is largely caused by the intensive burning of fossil fuels, often in very confined areas with high human populations - but the burning of wood and other materials is also a problem in some areas.
The major causes of air pollution include deisel engines so many large cities are considering banning such engines from their centres, but of course there are difficulties with that strategy as most goods are transported using lorries burning deisel fuel. Domestic heating is another big contributor so governments are encouraging home and business heat efficiency. In less developed countries the priorities are different.
The BBC raises many of these isssues and some of the solutions being trialled in the UK in this series of articles for TV and radio.