A recent spike in the air pollution of California’s Central Valley can be attributed to the state’s ongoing drought, warmer seasonal temperatures, and increased levels of a certain air pollutant known as fine particulate matter (PM 2.5).

According to the New York Department of Health, PM 2.5 refers to the tiny particles or droplets in the air capable of reducing general visibility, traveling deep into an individual’s respiratory tract, and even causing short-term health effects such as eye, nose, throat, and lung irritation.

KQED News reports that Central Valley’s PM 2.5 particles are being formed when gases from tailpipe and other sources react with sunlight and water vapor. Additionally, the San Joaquin Valley Air Pollution Control District’s real-time monitoring website has had hourly readings of PM 2.5 in the “unhealthy” and “extremely unhealthy” zones since last week.

According to the Air District, warmer-than-normal November temperatures and the current state drought are primarily responsible for this spike in air pollution. “Low overnight temperatures, high afternoon temperatures, so the inversion layer is getting fairly close to the ground overnight. It’s mushing all that pollution down,” said the Air District’s Jaime Holt.

Deputy air pollution control officer Samir Sheikh added that “extreme stagnation, long dry spells, with very little air movement,” were also to blame. “Just the perfect conditions for PM 2.5 to form in the atmosphere.”

According to KQED News, air quality activists are urging the Air District to publicly ask schools to cancel outdoor athletic events for the time being, in addition to asking residents not to burn their fireplaces or idle their cars at drive-thru restaurants.

“We do want to keep this relationship open with the schools,” said Holt. “What we don’t want them to feel is that we are coming down with a hammer on them in such a way that it makes them not want to participate in the [air-monitoring] program at all.”