t levels of air pollution. He said the results suggested that "the effect of long-term exposure to relatively low concentrations of fine particulate matter in air may lead to a reduction in life expectancy of more than a year". Brunekreef's calculations were done for a World Health Organisation (WHO) working group re-evaluating air quality guidelines for Europe. His report was published in the journal Occupational and Environmental Medicine. He stressed that his estimate depended on the accuracy of the two U.S. studies. But Brunekreef said he did not expect future studies or a re-evaluation of the U.S. reports to produce different findings. One of the U.S. studies followed 8,000 people in six American cities for a period of 15 years. The second study, by the American Cancer Society, involved more than 550,000 people over eight years. His estimates were based on long-term exposure to very small amounts of fine particulate matter, as little as 10 micrograms per cubic metre.