Amid towering flames and clouds of toxic air pollution, more than 1 million public school students have been sent home this week throughout California, as districts from San Diego to San Francisco grapple with the impact of wildfires.

The stunning count, compiled by CALmatters from school closure announcements and local press reports, was as of  Thursday evening. It included 180 school districts—with combined K-12 enrollment of more than 1.1 million students—that had either canceled classes earlier this week or planned to on Friday due to hazardous air quality from smoke and/or physical damage from massive wildfires.

The impacted students represent 18 percent of the state’s total public school enrollment, or more than one in six California public school students. That doesn’t include include private schools, parochial schools, colleges or universities, many of which shut their doors as the Camp and Woolsey fires in Butte and Ventura counties spiked air quality indexes to hazardous levels at both ends of California.

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Scott Roark, spokesman for the California Department of Education, called the wave of wildfire-related closures, which mounted throughout the week, “definitely one of the largest in the history of the state.”

The cancellations ranged from the tiny Butte County community of Paradise, where the Camp Fire—the deadliest in state history—has rendered an estimated 80 percent to 90 percent of students homeless, to the Spencer Valley Unified School District in eastern San Diego County, which sent 2,350 students home from school on Tuesday and kept them there all week after San Diego Gas & Electric cut power to its equipment amid red flag conditions. In Southern California, several districts in Ventura County will remain closed until after Thanksgiving break.

Many more districts, particularly in Northern California, canceled classes due to smoke that has blanketed whole counties with a thick brown fog and sent Californians scrambling for breathing masks and inhalers.

Some areas of Sacramento and the Bay Area had an Air Quality Index of more than 460 as of Thursday afternoon, well into the “hazardous” classification. For reference, several county education offices had said in guidance to local school districts that they should consider closing their schools if they have an AQI greater than 275.

Thick brown fog from the Camp Fire clouds the mid-afternoon sun over the Capitol dome in Sacramento on Thursday, as schools across the state closed due to hazardous air quality. Photo for CALmatters by Shawn Hubler.

Thick brown fog from the Camp Fire clouds the mid-afternoon sun over the Capitol dome in Sacramento on Thursday, as schools across the state closed due to hazardous air quality. Photo for CALmatters by Shawn Hubler.

Some of the state’s largest school districts, having struggled to stay open, announced that Friday classes would be canceled. Among them: San Francisco Unified, Elk Grove, Sacramento City, West Contra Costa, and Oakland school districts. In fact, in Alameda, Butte, Sutter and Contra Costa County, all schools were closed.

“We understand that closing schools places a burden on some of our families. We have given this decision careful consideration,” Vincent Matthews, San Francisco Unified’s superintendent, said in a note to parents and community members. “We’ve heard from many staff and families across the city that the indoor air quality has worsened through the week and we want to ensure our staff and students are working and learning in healthy conditions.”

The decision to cancel classes rests in California public schools with officials at the district level. Temporary closures are not unusual in communities impacted by wildfire. What is unusual this week is the scope.

According to state education officials, the prior one-day record for wildfire-related school closures was Oct. 12, 2017, when nearly 600 schools with about 260,000 students closed due to fire danger and hazardous air quality.

The one-day closures announced for Friday “are expected to exceed those numbers,” Roark said.

But it’s the cumulative impact, forcing local officials to weigh all the roles public schools play in their communities, that is most striking. For days, for example, schools in Northern California had exercised caution as smoke from the Camp Fire made its way to Greater Sacramento and the Bay Area. Several districts tried to keep students’ activities indoors and directed teachers and staff to be vigilant about keeping doors closed.

But on Thursday afternoon, as it became clear that the air quality would worsen, districts began to announce closures en masse.

Sacramento City Unified was among those that initially had planned to remain open, seeking to balance the air quality risks against the lost educational time and the needs of the many disadvantaged families in the district for free and reduced price school lunches.

Eventually, however, district officials gave in and said they were closing campuses after all on Friday, having “determined the age of our buildings and HVAC systems will make it more difficult to keep smoke out of classrooms as the air quality worsens leading into tomorrow.”

The district urged parents to keep their children indoors “as much as possible.”

“We deeply apologize to any families who will now have to find last minute child care and meal arrangements,” Sac City added in its announcement. “However, the potential health risks for tomorrow outweigh the need for our students to be in school.”

This story will be updated as it develops.