RENO, Nev. (AP) — Thick smoke from a massive wildfire at Yosemite National Park more than 150 miles away kept Reno school children indoors Friday as the air quality index approached the rare “very unhealthy” zone and county health officials expanded pollution warnings into the weekend.

The air quality index moved into the “unhealthy” category in the region Friday afternoon, a concern for not only those with respiratory problems, but healthy people too, Washoe County Health District officials said.

Visibility was reduced to a mile in some areas around Lake Tahoe and along the Sierra’s eastern front from south of Carson City to north of Reno.

High school football players were kept off the practice field and an outdoor food festival was canceled in Reno Friday night. The 24th annual Lake of the Sky Air Show scheduled Saturday at Lake Tahoe was scrubbed due to FAA regulations.

The Washoe County Health District issued a red alert for “unhealthy” conditions for all residents — broadening earlier warnings for those most vulnerable like the elderly and young — when the air quality index hit the 171 mark Friday morning on a scale where good is 0-50 (green), moderate 51-100 (yellow) and unhealthy for sensitive groups 101-150 (orange.).

The dense bitter haze with the smell of a campfire worsened with a mid-afternoon reading of 200 a few blocks off the main downtown casino drag, the worst it’s been in five years, officials said.

The AQI was just one point shy of the “very healthy” purple zone (201-250) that triggers an automatic health alert because everyone may experience more serious health effects, district officials said.

“As the pollution gets worse it affects more people and the effects become more serious,” said Daniel K. Inouye, the branch chief for monitoring and planning in the district’s division of air quality management.

The AQI slipped back to 170 by 5 p.m. on Friday, but officials forecast unhealthy conditions into Sunday.

“We’re looking for more of the same,” Inouye said.

Dr. Kevin Brown, emergency medicine chair at Northern Nevada Medical Center in Sparks, said his hospital was starting to see an increase in the number of people coming in with respiratory problems.

“It’s irritating certainly, but it can become serious if you get wheezing or shortness of breath,” he told KOLO-TV, adding the bad air also is causing some people to experience heartburn.

The California wildfire raged out of control on Friday, growing from 99 square miles to more than 165 miles as it spread inside the border of Yosemite National Park. The flames have forced the evacuations of hundreds from homes in communities near the park.

The National Weather Service in Reno issued a dense smoke advisory effective through 7 p.m. Saturday for California’s Alpine County and the southern part of the Lake Tahoe basin. Visibility in Markleeville and along parts of U.S. Highway 50 was expected to drop to one-quarter mile.

Washoe County issued an initial orange alert Thursday for sensitive populations in the Reno-Sparks area. On Friday, health officials urged everyone to reduce prolonged or heavy exertion, and said that such activities should be avoided altogether by the elderly, young and people with heart or lung disease.

The last time the AQI surpassed 200 in Reno was in 2008 when a string of lightning fires in Northern California immersed the Reno area in smoke and the reading peaked at 211, Inouye said.