Live air quality readings provider on Monday said San Francisco Bay Area’s Air Quality Index readings surpassed 100 on Purple Air despite the absence of any major wildfires in the state.

Orange dots were scattered around the region over the weekend and even on Monday mornings, SFGATE reported.

An improved reading of air quality in the region – which was in the yellow “moderate” rate and mostly below 100 – was recorded by the Bay Area Air Quality Management District. Several residents, however, are curious about how the air quality does not improve in winter even after a rainy period.

The management district spokesperson Aaron Richardson explained that it is not uncommon for the worsening of air quality in the winter, from November until February, because of the smoke coming from fireplaces. The district record said fireplaces in the region reach about 1.4 million.

“We have a temperature inversion layer over the area with hot air over cool air and that creates a lid that traps the fireplace smoke,” Richardson said.

Apart from this, the offshore winds bringing particulate material from the Central Valley and into the coast contribute to the weekend and Monday’s conditions, which makes the Bay Area’s air quality worse.

“This is the season when we start to see some buildup of fine particulate pollution, and we have a little more than normal because of the offshore flow coming in from the Central Valley,” Richardson said.

Another management district spokesperson, Kristine Roselius furthered that the winds are blowing in moisture. The humidity-sensitive Purple Air sensors are being pushed up by these conditions.

“We have fog from the Central Valley and the humidity can throw off the Purple Air sensors,” Roselius said. “Right now, I’m looking at Purple Air, numbers in San Francisco and Marin are over 100. On our site, San Francisco is 85. Purple Air uses a different technology. They run about 30% higher [than the sensors used by the district].”