A yellow haze of smoke from the County Fire near Napa Valley is filling the central San Joaquin Valley with small particles of soot and ash that can make the air unhealthy to breathe.
And the smoke from the fire couldn't be drifting into the Valley at a worse time.
"Tomorrow we're going to have a spike in air quality because of fireworks," said Dr. John Gasman, a pulmonologist at Kaiser Permanente Fresno.
Fourth of July fireworks alone can increase levels of air pollution four to five times higher than limits set by health-based federal standards. And in combination with fire smoke, the pollution levels can become extremely dangerous.
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Late Monday, air quality measured at the very unhealthiest category in the Valley because of smoke from the County Fire. Tuesday morning, the level had dropped to the moderate range but it could peak again, air quality officials warned.
The bowl-shaped Valley is notorious for capturing pollutants. Smoke filtering in the Valley can get caught in stagnant air. "It's amazing how it spreads and it gets trapped in the nooks and crannies of the Valley," Gasman said.
Plumes of smoke from the fire and fireworks contain particle pollution called PM 2.5. The microscopic pieces of soot, ash (and metals in fireworks) are so tiny they can be inhaled and get into the bloodstream. The particles have been linked to heart attacks and stroke, and they irritate the lungs of most people — but anyone with asthma or other respiratory problems is at risk for breathing problems.
And there's a third air pollution factor to consider: This time of year, the Valley has spikes of ozone, a pollutant that irritates lungs. The levels for ozone have been moderate the past few days, but they are creeping up, said Heather Heinks, spokeswoman for the San Joaquin Valley Air Pollution Control District.
But for now, the biggest concern is sooty particles from the fire and fireworks. The elderly, very young and anyone with respiratory issues should avoid breathing smoke-filled air, Heinks said. And someone who plans to exercise outdoors should check the air quality.
A simple safety tip: "If you're seeing smoke, that is a distinct indicator that it's having an impact on you," Heinks said.
Residents can check the district's wildfire page for information about any current wildfires and whether they are affecting the Valley. The Real-time Air Advisory Network (RAAN) allows Valley residents to track pollution at any address. Residents can also follow air quality conditions by downloading the free "Valley Air" app, available in the Apple store or Google Play.