"The air we breathe has become polluted with a mixture of cancer-causing substances," said Kurt Straif of the WHO's International Agency for Research on Cancer (IARC).
"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."
The IARC said a panel of top experts had found "sufficient evidence" that exposure to outdoor air pollution caused lung cancer and raised the risk of bladder cancer.
Although the composition of air pollution and levels of exposure can vary dramatically between locations, the agency said its conclusions applied to all regions of the globe.
Air pollution was already known to increase the risk of respiratory and heart diseases.
The IARC said pollution exposure levels increased significantly in some parts of the world in recent years, notably in rapidly industrialising nations with large populations.
The most recent data, from 2010, showed that 223,000 lung cancer deaths worldwide were the result of air pollution, the agency said.
In the past, the IARC had measured the presence of individual chemicals and mixtures of chemicals in the air -- including diesel engine exhaust, solvents, metals, and dust.
But the latest findings were based on overall air quality.
"Our task was to evaluate the air everyone breathes rather than focus on specific air pollutants," said the IARC's Dana Loomis.
"The results from the reviewed studies point in the same direction: the risk of developing lung cancer is significantly increased in people exposed to air pollution," he added.
The predominant sources of outdoor air pollution were transport, power generation, emissions from factories and farms, and residential heating and cooking, the agency said.