For the third time in a month, Pima County's air crossed the line Friday into the unhealthy realm because of windblown dust.
All afternoon Friday, a county air quality monitor in the city's heart was rated "unhealthy for sensitive groups" for large particles, due to east-to-west winds that reached up to 29 mph before dying down late in the day. Both Thursday and Friday, the dust obscured and at times blocked views of the mountains from the city.
The unhealthy rating - shown as an "orange day" on the federal air quality index - means that people with heart disease and lung problems should stay indoors. Particulates can block the exchange of oxygen in the bloodstream, making it harder for the body to get and use the oxygen it needs.
The county's top environmental official said officials worry about these pollution levels but aren't sure they can do anything about them because the dust may have come from natural causes - high winds.
"My first thought is always public health. We have noticed a lot of impact from the weather on air pollution lately, not just true here but everywhere," said Ursula Kramer, director of the county Department of Environmental Quality.
When the air is rated unhealthful for sensitive lungs, that increases the odds that very young children, older adults and those with heart or respiratory problems will have their lung conditions aggravated or will exhibit symptoms of respiratory ailments such as asthma, says the county's "Air Info Now" website.
The general population isn't likely to be affected, the U.S. Environmental Protection Agency says.
The air quality index at the county's monitor near Grant Road and North Stone Avenue ranged from 101 to 104 Friday afternoon for large particles, also known as PM 10 particles. It was in the 90s most of the morning.
The air is rated unhealthy for sensitive lungs when the index ranges from 101 to 150.
For smaller particles, known as PM 2.5 particles, the air quality index was in the 90s all afternoon, putting it at the upper end of the moderate rating. The smaller particles are considered more damaging because they can lodge more deeply in the lungs.
The high particulate levels came a little less than a month after the county's air exceeded EPA standards for large particles on April 8 and 9, and exceeded standards for fine particles April 9. Until then, the county's air had never exceeded the 2.5 standard.
The air hadn't gone above the standard for large particles since July 2009.
It won't be known until later today whether the air formally exceeded the EPA standards on Friday because officials must average the air quality over a 24-hour period, from midnight to midnight.
In the April dust storms, much of the dust was blown here from the Imperial Valley of Southern California, west of El Centro.
On Friday the winds were blowing from the east, and dust could have come from the Willcox area, said Beth Gorman, a DEQ program manager.
Also, unlike in the April event, the county got no complaints about dust blowing in from mine tailings south of Tucson.
If it's determined that Friday's bad air was caused by a natural event, the county can seek a ruling from the EPA to keep any exceedance of air quality standards from counting toward a violation, Kramer said.
The air pollution must exceed the standards four times in a year at a single monitor for a violation to occur.
On StarNet: Read the transcript of Tony Davis' live blog of Thursday's Southwest Climate Assessment meeting at live.azstarnet.com
Contact reporter Tony Davis at firstname.lastname@example.org or 806-7746.