A disproportionate number of Pittsburgh's dirtiest air days occur when the region's prevailing southwest winds turn and blow from the southeast, carrying emissions produced in the Monongahela River valley into the city, according to a new Carnegie Mellon University study.

The study found that when the wind is from the southeast, emissions from U.S. Steel's coke works in Clairton and its Edgar Thompson steel plant in Braddock push fine particle soot levels above the national health standards in Oakland and Squirrel Hill.

"Those southeast winds are helping pollutants from the Clairton and the Edgar Thompson works hitch a ride right into our backyards," said Cliff Davidson, professor of civil and environmental engineering and engineering and public policy at Carnegie Mellon University and a study author.

Dr. Davidson said the study, which appeared in last month's Environmental Science & Technology magazine, also indicates that low smokestack heights at Edgar Thompson and Clairton contribute to the air quality problem in those neighborhoods.

"If the stacks were higher, the soot pollutants would still be getting into the atmosphere but would be diluted before reaching the ground," Dr. Davidson said. "Both Edgar Thompson and the Clairton Coke Works have emissions that are close to the ground, and the data strongly suggests that the shorter stacks are one reason this is happening."

According to the study, done over 14 months ending in September 2002, on the 96 days that the wind was blowing from the southeast there were 17 days, or 18 percent, when the air quality standard was exceeded. On the 226 days when prevailing westerly winds were blowing, there were seven days, or 3 percent, when airborne particle levels were too high. The monitors collecting data on soot levels and wind direction were located in Schenley Park, about six miles east of Downtown Pittsburgh.

U.S. Steel said it believes there are "many contributors" to the high soot readings in the East End of the city.

The steelmaker is spending $1 billion for a coke oven replacement and upgrade project in Clairton that is scheduled to come on line in December 2011 and will reduce soot emissions from 751 tons a year to 368 tons, or 58 percent.

Dr. Davidson said the Edgar Thompson steel mill, which emits 472 tons of fine particles a year, "stands out as the closest big emitter," located just five miles from the East End neighborhoods.

The fine airborne particles are called PM 2.5 because they are 2.5 micrometers in size or smaller -- so small that several hundred could fit on the period at the end of this sentence. The particles can be emitted directly from power plants, industries or vehicles, or form in the atmosphere from chemical reactions involving sulfur dioxides and nitrogen oxides emitted from those same sources.

Such fine soot pollution can be breathed deeply into the lungs and cause serious health problems, including asthma, heart attacks, pulmonary disease and premature death, as well as increased hospital admissions and emergency room visits for respiratory and cardiovascular problems.

The World Health Organization estimates that 1.4 percent of all deaths are the result of particulate air pollution.