West coasters are experiencing a dangerous heat wave which broke record temperatures over the weekend and is expected to stay through midweek. Large parts of California and southern Oregon are under excessive heat warnings as temperatures across the states have broken the 100 degree mark.

Over 150 million Americans were under an excessive heat watch and heat advisory as of Friday, forecasters said. Palm Springs, California saw its all-time high broken Friday when the mercury hit 124 degrees.

Many rely on fans and air conditioning during extreme heat events. But some West Coast cities including Seattle, Portland and San Francisco have low rates of in-home air conditioning units, leaving many residents to struggle during heat waves.

These cities have the lowest rates of in-home air conditioning units:

Which cities are the least air conditioned?

San Francisco is the least air-conditioned city, according to data from the U.S. Census Bureau's American Housing Survey. In 2021 - the most recent year of data available - nearly 55% of homes did not have air conditioning units in San Francisco. The American Housing Survey looks at the 15 largest American Metro areas to determine which have the lowest rates of in-home AC.

Seattle ranked second on the list with 46% of homes functioning without AC.

Vivek Shandas, the founder of the Sustaining Urban Places Research Lab at Portland State University previously told USA TODAY, "In the Pacific Northwest, in these northern latitudes, we’re really ill-prepared and arguably most vulnerable to these heat waves coming through because we just don’t have a history of planning for it."

Scientists say introducing more AC units is not the solution to the underlying causes that contribute to an increase in heat waves, but it can offer relief to at-risk residents living in areas where extreme heat is not common.

"The cities that have the least amount of air conditioning, it's often because their outside, ambient environments don't really break 90 degrees very often and historically we design a lot of infrastructure based on historical patterns of what is it that happens in an environment," said Shandas.

Previously, Seattle was the least air conditioned city in the U.S. The amount of homes in the Seattle metro area with air conditioning nearly doubled between 2013 and 2021. About 31% of Seattle homes had air conditioning in 2013, compared to 53% in 2021.

Staying cool during a heat wave without AC

If you live somewhere without air conditioning, experts recommend opening your windows at night and closing them before the afternoon heats up. 

Typically, the peak hours when most areas see the hottest weather is from 1 p.m. to 6 p.m. That's when it's most important to try to keep cool, even if that means finding refuge in a business or restaurant or carrying around a fan or spray bottle to cool off. 

Dr. Fred Campbell, a professor at the University of Texas Health Science Center in San Antonio previously told USA TODAY that having multiple fans "directly blowing" on you can make a big difference, when it comes to regulating your temperature. This is especially helpful for older Americans and young children who can't regulate their temperature as well.

Keeping cool:Here's how to keep cool and stay safe during this week's heat wave hitting millions

Contributing: Claire Thornton, Olivia Munson and Sarah Elbeshbishi, USA TODAY