By Kate Kelland and Stephanie Nebehay

LONDON/GENEVA (Reuters) - The air we breathe is laced with cancer-causing substances and is being officially classified as carcinogenic to humans, the World Health Organization's 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.

Depending on the level of exposure in different parts of the world, the risk was found to be similar to that of breathing in second-hand tobacco smoke, Kurt Straif, head of the agency's section that ranks carcinogens, told reporters in Geneva.

"Our task was to evaluate the air everyone breathes rather than focus on specific air pollutants," deputy head Dana Loomis said in a statement. "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."

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 exposure levels have risen significantly in some parts of the world, particularly countries with large populations going through rapid industrialization, such as China.

IARC reviewed thousands of studies on air pollution tracking populations over decades and other research such as those in which mice exposed to polluted air experienced increased numbers of lung tumors.

In a statement released after reviewing the literature, the Lyon-based agency said both 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.

CARCINOGEN ENCYCLOPAEDIA

Air pollution is highly variable over space and time.

Loomis said there was relatively high exposure in Asia, South Asia, eastern North America, some places in Central America and Mexico, as well as North Africa.

But 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.

"Our conclusion is that this is a leading environmental cause of cancer deaths," Dr. Christopher Wild, director of IARC, told the news briefing in Geneva.

IARC's ranking monographs program, sometimes known as the "encyclopedia 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 air pollution as a cause of cancer.

Wild said he hoped the comprehensive evidence would help the WHO, which is revising its global 2005 guidelines on air quality. The U.N. agency makes on recommendations on public health issues to its 193 member states.

Asked why it had taken so long to reach the conclusion, he said that one problem was the time lag between exposure to polluted air and the onset of cancer.

"Often we're looking at two, three or four decades once an exposure is introduced before there is sufficient impact on the burden of cancer in the population to be able to study this type of question," he said.

(Editing by Alison Williams)