The World Health Organisation has classified outdoor air pollution as a leading cause of cancer.
"The air we breathe has become polluted with a mixture of cancer-causing substances. We consider this to be the most important environmental carcinogen, more so than passive smoking," said Kurt Straif, head of the WHO's International Agency for Research on Cancer.
The agency evaluates cancer-causing substances.
Previously, air pollution had been found to boost the chances of heart and respiratory diseases and the agency had deemed some of the components in air pollution, such as diesel fumes, to be carcinogens.
But this is the first time it has classified air pollution in its entirety as causing cancer.
Straif said the risk to the individual was low, but the main sources of pollution were widespread, including transport, power plants and industrial and agricultural emissions. Research suggests that in recent years, exposure levels have risen significantly in some parts of the world, particularly countries with large populations going through rapid industrialisation such as China.
The most recent data, from 2010, showed that 223,000 lung cancer deaths worldwide were the result of air pollution.
A recent study commissioned by Greenpeace found air quality is taking a heavy toll in Hong Kong. It linked 3,600 deaths and 4,000 cases of child asthma in 2011 alone to pollution from 96 coal-fired power plants in Guangdong and Hong Kong.
Watch: How to deal with Hong Kong's smog problem
The expert panel's classification was made after scientists analysed more than 1,000 studies worldwide and concluded there was enough evidence that exposure to outdoor air pollution causes lung cancer.
Straif said some of the most polluted metropolises were in China and India, where people frequently don masks on streets to protect themselves.
"I assume the masks could result in a reduction of particulate matter, so they could be helpful to reduce personal exposure," Straif said. But collective action by governments was necessary to improve air quality.
"People can certainly contribute by doing things like not driving a big diesel car, but this needs much wider policies by national and international authorities," Straif said.
Other experts emphasised the cancer risk from pollution for the average person was very low, but virtually unavoidable. "You can choose not to drink or not to smoke, but you can't control whether or not you're exposed to air pollution," Harvard University biostatics professor Francesca Dominici said. "You can't just decide not to breathe."
Additional reporting by Agence France-Presse