Remember Wednesday, July 22, when the mountains disappeared behind a wall of dust?

The Pima County Department of Environmental Quality reports that on that day, the department's air quality monitors recorded the highest levels of particulate matter ever monitored in Pima County.

Dusty air that day was attributable to a natural event, a dust storm near Casa Grande. An air quality advisory was issued that morning by the department. It collected filters from monitoring sites the next day to begin the process required by the U.S. Environmental Protection Agency to determine the accurate weight of the particulate matter captured on the filters.

The PM10 (particulate matter that is 10 microns or less in diameter) reading at the Orange Grove monitoring site for Wednesday, July 22 was 270 micrograms/cubic meter of air. The EPA health standard for PM10 is 150 micrograms/cubic meter.

"In 2008, 96 percent of the time, our levels of PM10 are in the healthy range," said Beth Gorman of PDEQ. "The days when there are high winds or forest fires tend to be the times that we have unhealthy levels of particulates in our air and those at risk need to be cautious."

Scientific studies have linked particle pollution exposure to a variety of health problems.

PDEQ currently monitors for five different types of air pollution.

Information on particulate matter (PM10 and PM2.5), ground level ozone, and other air pollutants can be obtained from special instantaneous monitors at the PDEQ website or by calling 882-4AIR.

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