The World Health Organization considers fine particulate matter pollution levels higher than 10 micrograms per cubic meter to be unsafe. The majority of American cities are in the safe zone, with the average pollution level at 9.6. Thirty-three percent of cities are above the W.H.O. standard. Those cities tend to be geographically dispersed throughout the United States, but are predictably cities with heavy industry and driving, like Cleveland, Chicago, Los Angeles and Philadelphia. Outside of the W.H.O., the United States has its own particulate matter standard of 12 micrograms per cubic meter. The pollution in 13 percent of American cities is higher than that.