Californians may notice more days in bad air categories. Here’s why

UPDATE: Late-season spring storms had a rare impact on Northern California air quality

On May 6, updated and stronger national air quality standards went into effect from the Environmental Protection Agency for fine particulate matter pollution. Based on 2020-2022 data, half of California’s counties fail to comply with the new standards.

Pollution isn’t increasing but the reconfigured standards are tighter. This means some days that would have previously been categorized as healthy, or carrying little risk, are now considered to have some level of negative health impacts.

• Air quality tracker: Check real-time conditions near you

The Bay Area Air Quality Management District says the number of Spare the Air days won’t change, but “Bay Area residents can expect to see more moderate air quality levels on our website,” according to spokeswoman Kristina Chu.

The update from the EPA reflects evolving science on the effects of fine particulate matter pollution, known as PM2.5, on human health. PM2.5 particles are more than 20 times smaller than the width of a human hair. They are dangerous to the heart and lungs, and can result in a variety of short- and long-term health effects.

The EPA has “reset the standard to try to protect the most sensitive members of the population,” said Michael Kleeman, a professor in the Department of Civil and Environmental Engineering at UC Davis.

The table below shows how the air pollution thresholds have changed.

There is no change to the “unhealthy for sensitive groups” AQI category, the threshold for Spare the Air days in the Bay Area. However, break points between “unhealthy,” “very unhealthy,” and “hazardous” air quality have been tightened. Pollution events such as wildfires could result in more “very unhealthy” and “hazardous” air quality days due to the stricter standards.

Although events like wildfires, which can seriously impact air quality levels, even when a blaze is burning hundreds of miles away, have become more frequent, the U.S. has made significant progress in reducing fine particulate matter. Since 2000, PM2.5 concentrations have decreased by 42% nationally, according to the EPA.

California is making progress but still chasing the EPA target. Inland counties tend to have the worst air, especially in the Central Valley, due to agricultural and vehicular emissions.

California “has one of the biggest economies in the world. If you put that in a mountain-valley system where the air is trapped for days, or even weeks at a time, then you get a recipe for pollution events, and there’s no way around that,” Kleeman said.

Under the previous fine particulate pollution standards, many Northern California and Bay Area counties were within annual thresholds, but now half of California’s 58 counties fail to meet the revised standards.

Bay Area counties that do not meet updated annual standards are Alameda, Contra Costa, Santa Clara and Solano. 

By 2032, the EPA projects some California counties will comply with the regulations, but 23 counties are expected to remain above the annual particulate matter pollution threshold, including many in the Bay Area and San Joaquin Valley.

In an April 2023 letter to the EPA, the San Joaquin Valley Air Pollution Control District said “there is no clear path forward on how the (San Joaquin) Valley or other regions of California can attain this new proposed standard.”

Still, Kleeman noted the strides California has made in the recent decades. Particulate pollution has gradually declined throughout the state with the adoption of electric vehicles and stricter regulations on wood burning, he said.

“If you measure it in the relative scale, the standards keep dropping and we keep chasing that standard,” Kleeman said. “That might seem frustrating, but on an absolute scale, things are much better than they were 20 years ago. We’ve made a lot of progress.”

Reach Anthony Edwards: anthony.edwards@sfchronicle.com