FRESNO (AP) — The winter air is so bad in the San Joaquin Valley that even healthy people are being urged to stay indoors.

Such warnings typically are broadcast in the summer and are aimed at the elderly, children and people with breathing problems. But a haze of fine particles has cloaked the skies from Stockton to Bakersfield this winter because a stubborn high-pressure ridge parked off the West Coast has prevented storms from cleaning the air.

The state’s agricultural heartland is no stranger to air pollution. It is home to farms and oil fields, and trucks rumble through the region daily. But this winter is the most polluted on record.

Skies are filled with microscopic, chemical-laden particles that can get lodged in the lungs and absorbed into the bloodstream to create health risks in everyone. Children rarely ventured out for recess. Doctors urged patients to stay inside. Even outdoors types are told to limit their activities.

Kellie Townsend, who works at Fresno State, initially exercised indoors. But after several weeks, she took up running again.

“I’m scared. I can feel that something isn’t right. I can feel the tightness in my chest,” she told the Los Angeles Times in Saturday’s editions (http://lat.ms/1jvOebR ). “But I get tense when I’m inside too long.”

Dr. Pat Golden, a Fresno cardiologist, told his patients to stay indoors for nearly two months. But he recently took a walk outside.

“I prefer not to see what I’m breathing, and you can see this air,” he told the newspaper. “But I don’t know how to quantify the risk. I balance it against my peace of mind from being outside and moving my leg muscles. It’s a terrible choice to have to make.”

The San Joaquin Valley Air Pollution Control District board said the region would have clean winter air if it weren’t for the drought.

Earlier this month, the board’s executive director, Seyed Sadredin, noted that other regions in the state have seen dirty air that exceeded federal health standards. The group has long maintained that the Central Valley should not be penalized for failing to meet federal clean air standards, in part because some of the pollution is wafted from elsewhere, including China.

Air quality advocates said the Central Valley needs to stop blaming others.

“It’s building up and not blowing away because we need rain. But we’ve created every bit of what we’re breathing right now. We can’t blame San Francisco or China,” clean air activist Tom Frantz said. “What we’re seeing, is what’s ours to change.”