As the Bay Area bakes under extreme heat this week, many residents are sweating in homes without air conditioning. Nationwide 92% of homes have some kind of air conditioning. But thanks to the Bay Area’s normally mild Mediterranean climate and summer fog, the rate is less than half that in parts of the region.

The larger San Francisco metropolitan area, including Oakland and Hayward, has the lowest rate of air conditioning of any major metro area in the country, according to census data from 2021, which includes two Bay Area metros. But despite having the lowest rate, 182,000 more homes have added air conditioning since 2015. And the percent of housing units with air conditioning in the San Francisco metro area has grown from 36% to 45% of homes in that time.

While most people were hiding from the heat this week, Paul Massone and the HVAC technicians at his Concord-based company, MechPros Heating and Air Conditioning, were zooming around the Bay Area in high demand. Sometimes they needed a few extra minutes in the air-conditioned work truck to cool down between appointments in hot attics and stuffy apartments, fixing air-conditioning units.

“We’re fighting this heat — it’s been a bit crazy,” Massone, who has been in the industry 23 years, said. “In the last 10 years, more people from Oakland, Berkeley, El Cerrito… have been calling for air conditioning.”

That tracks with Leslie Ogar’s experience in the Bay Area. She used to live in Hayward and did not have air conditioning. “It was very hot, but luckily, we’re fortunate to have air conditioning where we live now, in Oakland.”

On Tuesday, she was at Lake Merritt, in the midst of the heat, selling burgers and tacos at a pop-up food stand. She was keeping cool in the shade, like the gaggles of geese also seeking refuge around the lake, as a few people braved the heat for their daily walk.

With climate change, warming temperatures and changing standards, air conditioning has become more popular in places like San Francisco and Seattle. Seattle used to have the lowest rates of air conditioning, but San Francisco now has that distinction.

Nationwide, 72% of all housing has central air, and an additional 20% have room air-conditioning units. Just 8% of housing units in the country do not have any form of primary air conditioning.

In the San Jose metro area, which includes Sunnyvale and Santa Clara, 23% of housing has no air conditioning. The weather is generally hotter in the South Bay, so air conditioning rates are higher than neighboring cities to the north, but San Jose has slightly less air conditioning than California in general. And California’s rate is lower than the country’s, with 22% of housing units having no air conditioning.

In San Jose, the percentage of households with central air conditioning has grown from 45.9% in 2011 to 57.8% in a decade, reflecting rising temperatures and demand.

Data collected by the U.S. Census Bureau’s American Housing Survey shows how many households use different types of heating and cooling systems, and the data is also broken down by income levels, and whether the household is renter- or owner-occupied. It paints a picture of regional and economic differences in who has air conditioning at home and who doesn’t.

Renters in the San Jose metropolitan area are much less likely to have air conditioning than owners. Overall, 67% of owner-occupied housing has central air conditioning, compared to 52% of rentals.

Income matters, too. Generally, households in higher-income brackets are more likely to have air conditioning, but the gap between renters and owners persists even for those with the highest incomes.

As for households with under $50,000 annual income? Renters in that group are slightly more likely to have some kind of air conditioning than people who own their homes, though less likely to have central air.

If you are one of the millions without air conditioning during this long weekend of excessive heat warnings, or if you have to be outside for long periods, it is important to take basic precautions, said Dr. Joel Levis, an emergency physician with Kaiser Permanente in Santa Clara.

The basics? Avoid direct sunlight when possible, take frequent breaks in the shade, wear loose light-colored clothing and stay hydrated. “Those are the most important things you want to do to prevent some of the more serious effects of these heat waves.”

While many will be able to avoid the worst of the heat, nearly a quarter of San Jose residents have no air conditioning, including Kimberley Stanley.

She went to the Dr. Martin Luther King Jr. Library in the middle of the day to cool down Wednesday because she doesn’t have air conditioning at home.

“It’s just very hard to stay cool,” she said. “The library allows me to escape the heat.”

Cameron Duran contributed reporting.