Already this summer, at least nine all-time temperature records have been broken and 10 records have tied in the United States. One of them was Chino, California, near Los Angeles, which hit a blistering 120 degrees Fahrenheit on July 6. Meanwhile Death Valley in California just had the hottest July ever measured.

Overall, 2018 is on track to be the fourth-warmest year on record. And many other countries are suffering from the heat too. A village in Oman saw temperatures linger above 108°F for 51 hours straight, which likely broke the world record for highest minimum temperature ever. In July, it reached 106 degrees Fahrenheit in Kumagaya, Japan, the highest-ever recorded temperature in the country.

As #Japan is gripped by a #heatwave, Japan Meteorological Agency issues hourly weather analysis maps showing temperatures. Details at https://t.co/ONqr7JQgpL pic.twitter.com/VMjkY78ABv — WMO | OMM (@WMO) July 23, 2018

Sydney, Australia saw temperatures rise to 76.5°F for more than two days last month, which is astounding since it’s the middle of winter there.

And here in the US, more heat is in store for the West, according to the National Weather Service:

A new week starts with an old pattern. Triple digit heat continues to bake the west and provide little relief for firefighting efforts while heavy rainfall threatens the east with more flash flooding concerns on already saturated ground. pic.twitter.com/5OfhW5dnBj — NWS (@NWS) July 30, 2018

These heat waves comport with what scientists expect from climate change. The body of evidence shows that the world will face longer, more intense heat waves as average temperatures go up, and that they will be deadly.

Already, at least 70 people have died in Canada from the recent heat. Record temperatures in recent weeks killed more than 90 people and injured more than 57,000 in Japan. In May, a heat wave killed 65 in Karachi, Pakistan.

But it turns out that heat waves are often most dangerous not necessarily where it’s hottest, but where it’s hardest to cool off.

Heat waves are not an equal-opportunity threat

The common denominator in the recent heat-related deaths and hospital visits in Canada and Japan is that many occurred among people who were already facing health risks and who didn’t have access to cooling. We saw this play out in Quebec, where many of Canada’s recently heat-related fatalities occurred, according to NPR:

Most of the people who died as the region reached temperatures up to 95 degrees are elderly men and women living alone in apartments with no air conditioning, and many had chronic health conditions. David Kaiser, a physician manager at the Montreal Regional Department of Public Health, confirmed to NPR that 34 of the deaths occurred in the city from June 29 through July 7. With few exceptions, he said, the people were over the age of 50, many between 65 to 85. About 60 percent were men and most had an underlying medical or mental health condition, Kaiser added.

In Pakistan, many of the deaths during its heat wave came during a power outage that swept Karachi that left people with no way to escape the heat. The heat wave in Japan arrived last month after massive flooding and landslides that also knocked out power to tens of thousands of homes the week before.

Without air conditioning, homes can overheat, putting people at higher risk of heat stroke, where the body’s internal temperature reaches 104°F or higher.

People with cardiovascular problems like high blood pressure also suffer in the heat since they often take medications that can dehydrate them. High temperatures can also accelerate the formation of pollutants like ozone, which can inflame the lungs. At night, if temperatures don’t decline, people can’t cool off enough to cope. And as the climate changes, nights are warming faster than days.

Even healthy people suffer under these conditions, but the elderly face the highest health risks from heat.

These events are a reminder that cooling isn’t a luxury; it can be a matter of life and death.

Temperatures don’t have to reach extremes before turning deadly

As I’ve written about before, scientists have figured out that when it comes to health risks from high temperatures, the key factor to pay attention to is the deviation from the norm.

A 105°F day in Phoenix may barely register for Arizona residents, but 90°F weather in Portland, Oregon, could send people to the hospital.

Heat waves are often most pronounced in dense, urban areas. Asphalt, concrete, steel, and glass soak up the summer weather and create a heat island, which can make a city upward of 22°F warmer than its surroundings. And the climate itself is becoming more erratic, with parts of the world seeing major temperature swings over the course of a few days, making it harder for people to adjust.

The World Meteorological Organization says that heat-related deaths and illnesses have risen steadily since 1980, and now 30 percent of the world’s population lives in regions vulnerable to heat waves. With climate change exacerbating heat waves, we need to prepare for more deadly heat across the US and around the world.