* Cancer agency ranks polluted air alongside tobacco smoke
* Some 23,000 lung cancer deaths in 2010 due to pollution
* Transport and power generation are major air polluters
By Kate Kelland
LONDON, Oct 17 (Reuters) - The air we breathe is laced with cancer-causing substances and should now be classified as carcinogenic to humans, the World Health Organisation's (WHO) cancer agency said on Thursday.
The International Agency for Research on Cancer (IARC) cited data indicating that in 2010, 223,000 deaths from lung cancer worldwide resulted from air pollution, and said there was also convincing evidence it increases the risk of bladder cancer.
The WHO is a Geneva-based agency of the United Nations focused on international public health matters.
Air pollution, mostly caused by transport, power generation, industrial or agricultural emissions and residential heating and cooking, is already known to raise risks for a wide range of illnesses including respiratory and heart diseases.
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.
"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," said Kurt Straif, head of the IARC's monographs section, which is tasked with ranking carcinogens.
"The air we breathe has become polluted with a mixture of cancer-causing substances."
In a statement released after a week-long meeting of experts reviewing the latest scientific literature, IARC said both outdoor air pollution and "particulate matter" - a major component of it - would now be classified among its Group 1 human carcinogens.
That ranks them alongside more than 100 other known cancer-causing substances in IARC's Group 1, including asbestos, plutonium, silica dust, ultraviolet radiation and tobacco smoke.
IARC's monographs programme, sometimes known as the "encyclopaedia of carcinogens", aims to be an authoritative source of scientific evidence on cancer-causing substances.
It has already classified many chemicals and mixtures that can be components of air pollution, including diesel engine exhaust, solvents, metals and dusts. But this is the first time that experts have classified outdoor air pollution as a cause of cancer.
"Our task was to evaluate the air everyone breathes rather than focus on specific air pollutants," said Dana Loomis, deputy head of the section. "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."
Although both the composition and levels of air pollution can vary dramatically from one location to the next, IARC said its conclusions applied to all regions of the world.
IARC's director, Christopher Wild, said the agency's decision to classify outdoor air pollution as carcinogenic to humans was an important step towards alerting governments to its dangers and potential costs.
"There are effective ways to reduce air pollution and, given the scale of the exposure affecting people worldwide, this report should send a strong signal to the international community to take action," he said. (Editing by Eric Walsh)