With wintertime pollution starting to blanket the valley, the state is publicizing year-old guidelines for when children should stay indoors during recess.

The changes, made last January by the Utah Department of Health and Utah Department of Environmental Quality but only publicized now, are both more conservative and more liberal.

They will keep sensitive children, like those with asthma, indoors at lower levels of pollution than in the past. But they allow healthy children to play outside when pollution levels are much higher.

Steve Packham, toxicologist with the Utah Division of Air Quality, said the guidelines are meant to balance the benefits and risks of playing outside.

"There's a distinct benefit from having students exercise, take a break from studies, and so we don't want to inadvertently lose benefit from those kinds of things unless there's really some evidence that it's seriously going to affect their lungs."

Bad air days are measured by the amount of pollutants called PM2.5 is in the air. When levels reach 35.5 micrograms per cubic meter of air -- a common occurrence during January and February -- "sensitive" students should stay indoors. That includes children with poorly controlled asthma, cystic fibrosis, chronic lung disease, congenital heart disease, or other respiratory problems. The former standard kept them indoors at 40 micrograms.

A new intermediate category was included so that when pollution reaches 55.5, students who are coughing, wheezing and short of breath should stay inside.

And when pollution reaches 90, all children should stay inside. The previous standard for no recess was 65.

Packham said doctors reviewed the new standard, which is also based on a 2006 study of children in Logan. Researchers measured their lung function before and after recess during the winter when pollution levels approached 55 micrograms.

"We just didn't see any affect at all" on lung function, he said.

The pollution levels never reached 90 during the study. The last time that happened was several years ago, Packham said. Still, he said the toxicologists and doctors "felt comfortable" with the change.

The former standard has been in place statewide since 2004, according to the health department. Rebecca Jorgensen, a health specialist in the department's asthma program, said school districts were notified of the changes last year. After the training period, the state decided to publicly launch it this month.

hmay@sltrib.com