In June a massive “heat dome” smothered the famously temperate Pacific Northwest, subjecting parts of Washington State, Oregon and western Canada to blistering and unprecedented temperatures. Lytton, British Columbia, set an all-time Canadian record with a searing 121.3 degrees Fahrenheit (49.6 degrees Celsius). A day later most of that village was destroyed by a huge wildfire. During another western heat wave in early July, California’s Death Valley reached a scorching 130 degrees F (54 degrees C)—just shy of its record of 134 degrees F (57 degrees C), which was reported in 1913 (and is somewhat disputed now). A third heat wave blanketed the U.S. and Canadian West in recent days.

It is virtually impossible that heat waves like the Pacific Northwest’s June scorcher would have occurred without climate change, according to a recent analysis by the World Weather Attribution collaboration. Scientists estimate it was a one-in-1,000-year event, says Kristie L. Ebi, a professor of environmental and occupational health at the University of Washington and a co-author of the report. “And that’s an ‘at least,’” she notes. “It could be more rare than that, because it was so far outside where the climate model said temperatures would get to in this region.” If warming reaches two degrees C above preindustrial levels—the threshold that most national governments have agreed to try to avoid in hopes of reducing climate change impacts—“that event could occur every five to 10 years,” Ebi says.

These heat waves pose a major risk to public health. “In an average year in the U.S., heat kills more people than any other type of extreme weather,” says Kristina Dahl, a senior climate scientist at the Union of Concerned Scientists. Hundreds of people died in the recent Pacific Northwest heat wave, according to estimates: there were at least 486 deaths in British Columbia, 116 in Oregon and 78 in Washington (by comparison, hurricanes have killed an average total of 46 people a year in the U.S. over the past 30 years). A recent U.S. Centers for Disease Control and Prevention report found there were more than 3,500 emergency department visits for heat-related illness this past May and June in a region that includes Alaska, Idaho, Oregon and Washington State. Nearly 80 percent of these visits occurred between June 25 and 30, when Oregon and Washington were experiencing the worst of the wave.

The human body functions best at 98.6 degrees F (37 degrees C). When it overheats and becomes dehydrated, the blood thickens. The heart has to pump harder, and it and other organs can be seriously damaged. The body has mechanisms to rid itself of excess heat—most notably sweating. But at a certain point, that fails to work, especially if humidity is high and perspiration cannot evaporate. “Once your thermal stress or heat gain becomes too much, even sweating is not going to keep up with getting rid of the additional heat,” says JohnEric Smith, an associate professor of exercise physiology at Mississippi State University. This situation can result in heat exhaustion (a dangerous condition characterized by symptoms that include nausea, muscle cramps and dizziness) and the deadlier heat stroke, which can cause delirium, hot and dry skin, and loss of consciousness.

People can eventually acclimatize to some level of heat. If you live in a hot climate or work in hot conditions for a period of weeks or months, your body becomes more efficient at sweating and cooling itself down, Smith says. This process takes time, however. When severe heat hits places where most people are unaccustomed to it, such as the Pacific Northwest, it can be especially deadly. Elderly people, children and those with already existing conditions such as heart, respiratory or kidney disease are particularly vulnerable, according to Smith. Furthermore, common medications (beta-blockers, for example) can affect the body’s ability to sweat.

Farm laborers, construction workers and others who toil outdoors can be exposed to potentially fatal heat for many hours a day. A farm worker in Oregon died while working in the extreme heat on June 26. Dehydration is among the dangers—there have been cases of farm workers developing severe kidney disease after hours of sweating in the sun. Many outdoor workers are paid by the hour and may feel that they cannot afford to take a day off because of the weather. Only two states—California and Washington—have permanent heat protection standards for outdoor workers, according to the Union of Concerned Scientists’ Dahl (Oregon has adopted emergency ones). At the federal level, versions of a bill recently introduced in the U.S. House and Senate would direct the Occupational Safety and Health Administration to issue national standards protecting workers from heat-related illness.

Athletes are also at increased risk because their body produces excess heat from muscle activity. In 2001 National Football League (NFL) player Korey Stringer died from heat stroke during training in Minnesota. The institute that bears his name now studies ways to prevent heat-related illness and death among athletes, members of the military and physical laborers.

There are well-known ways to mitigate the risks of extreme heat. Staying in cool buildings with air-conditioning is a great option for those who have it. For those who do not, either because they cannot afford it or because they live in places known for gentle summers (such as Seattle), some cities have established cooling centers. But people need to be able to access these resources. That is not always easy if they have to take public transit, which heat waves can also disrupt. If you lack access to air conditioning or a cooling center or have to work outside, at least try to seek shade. Direct sun warms the skin and makes you even hotter, so wear long sleeves and loose-fitting clothing to cover up. If you have to exert yourself, take frequent breaks and drink plenty of water.

Communities should have heat action plans, says the University of Washington’s Ebi. She adds that these plans should include an early-warning-and-response system. In addition to forecasting extreme heat events, such systems should detail appropriate ways to deal with them—including how to help the most vulnerable people, who are often disproportionately affected. State and federal agencies could help support communities, but heat response systems should be locally based, Ebi says.

As the planet warms, heat waves like those that have occurred in the U.S. and Canadian West this year are becoming frighteningly common—and catching climate scientists off guard. “Even a lot of our climate models that project out how frequent extreme heat will be in the future wouldn’t have necessarily predicted this level of heat for that part of the country,” Dahl says. “But then to realize that I am seeing it in my lifetime, and living it right now, is really terrifying.”