The downtown Fresno skyline, cloaked in thick haze, on Jan. 17. The amount of fine particles known as PM-2.5 in the air is now so high that even otherwise healthy adults are at risk. (Craig Kohlruss, Fresno Bee / January 17, 2014)

FRESNO — On bad-air days here in the Central Valley, school officials hoist red flags to warn parents and pupils that being outside is officially deemed “unhealthful for all groups.”

This winter, though, the most polluted on record, schools have not only raised red flags. On several days, they have had to send out notices saying the red flags should really be purple—indicating “very unhealthful” air — if only they had them. But such warnings have been be so rare that schools don't even have the flags designating the most extreme conditions.

Of course, parents could just look at the sky itself.

From Stockton to Bakersfield, a haze of chemical-laced particles has tinted the air a rusty gray all winter. In the evenings there's a charcoal stripe across the horizon. The Sierra Nevada hasn't been visible for more than a month.

A high-pressure ridge, four miles high, sits off the West Coast, blocking Pacific storms from cleaning the air in the Central Valley. Pollution levels have spiked across California, but nowhere is it as bad as in this agricultural region.

With no rain since Dec. 7, fine particles that can embed in lungs and enter the bloodstream build up in an ever-darkening sky. Meteorologists don't expect the weather to shift until at least the end of the month.

When Kellie Townsend returned from her Christmas vacation at the coast, she knew right away something was wrong.

"As soon as I drove into the valley, I could feel a burning in my throat," she said.

Townsend, who works in the Earth and Environmental Sciences program at Fresno State, heeded air board warnings to stay inside. Her neighbors seemed to do the same. The only people she saw out were gardeners with leaf-blowers. For exercise there was her Lindy Hop dance class. One weekend she went to the mountains for a dose of fresh air.

But after three weeks, on a recent balmy day, the 42-year-old returned to running up and down hills near a walking trail. She purposely didn't check the air rating — which was a red alert with about three times the amount of fine particles found in air considered healthful.

"I'm scared. I can feel that something isn't right. I can feel the tightness in my chest," she said. "But I get tense when I'm inside too long. I told my husband, 'My head feels chaotic inside.' I know what will happen — I will be coughing tonight. Maybe the damage is long term. But what do I do?"

People who live in the Central Valley are used to bad air. Surrounded by mountains on three sides, home to industrial agriculture and oil fields, and with most of the state's long-distance big-rig traffic driving through on Interstate 5 and state Highway 99, the region historically has had some of the worst pollution in the nation. 

Warnings about spikes usually go out in the summer and are directed at sensitive groups: children, older people and those with respiratory problems in a region where the asthma rate is three times higher than the national average.

Now the amount of fine particles — known as PM-2.5 — in the air is so high that a new group is affected: outdoorsy adults with no health problems. On many days, the air district, tracking hourly readings, sends out an alert: "Real Time Activity Risk Warning."

As the weeks stretch on, people are ignoring the warnings.

Chris Bouk, a Kingsburg junior high P.E. teacher, spent a recent school week teaching his classes inside. Students were limited to only necessary movement between classrooms and school buses.

It didn't stop Bouk, 54, from his weekend bike ride.

"I have a schedule and I want to keep it," he said. "It's not only cardio that's important; you have to think abut your muscles. And it's a mental issue — just being outside is good for you. I mean, it's what, 72 degrees? This feels good." 

Bouk, a serious cyclist, did notice a difference.

"You can almost taste the air," he said. "And, well, you've had a chest cold? It feels like that — the tightness."

For almost two months, Dr. Pat Golden, a Fresno cardiologist and internist, has told his patients to stay inside.

On a recent afternoon, a 91-year-old woman asked for permission to just take a walk outside. He told her no. Then he went for his own brisk daily walk.