Pollutants in the air we breathe have been classed as a leading environmental cause of cancer by the World Health Organization.
It said the evidence was clear they cause lung cancer.
Sources of pollution include car exhausts, power stations, emissions from agriculture and industry - as well as heating in people's homes.
The WHO said the classification should act as a strong message to governments to take action.
The International Agency for Research on Cancer (IARC), a part of the WHO, has now classed air pollution in the same category as tobacco smoke, UV radiation and plutonium.
It said air pollution had been know to cause heart and lung diseases, but evidence had now emerged that it was also causing cancer.
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The air we breathe has become polluted with a mixture of cancer-causing substances'
The IARC said the most recent data suggested 223,000 deaths from lung cancer around the world were caused by air pollution.
More than half of the deaths were thought to be in China and other East Asian countries. Rapid industrialisation has led to smoggy skies in cities such as Beijing.
However, it is a global problem and concerns about air pollution were raised in Europe again this week.
Data suggests there may also be a link with bladder cancer.
Dr Kurt Straif, from IARC, said: 'The air we breathe has become polluted with a mixture of cancer-causing substances.
'We now know that outdoor air pollution is not only a major risk to health in general, but also a leading environmental cause of cancer deaths.'
