The Silent Killer

It's only April and the warm temperatures of 2015 are already grabbing headlines. But I doubt anyone is out there quaking at the prospect of sweltering through another "hottest year ever." You see, as extreme weather events go, heat waves don’t have much flair for the dramatic. Tornadoes leave a violent trail of flattened houses in their wake. Flotillas of kayaks take to the streets during major floods. Even cold waves inspire BuzzFeed listicles showing how boiling water can freeze in midair. But heat waves just…make us sit around and sweat.

Though unassuming, extreme heat is deadly, which is why it’s often referred to as a silent killer. The Centers for Disease Control and Prevention estimates that between 2006 and 2010, heat contributed to 666 deaths per year (hot as hell, indeed).

“People have a sense that ‘There’s always been heat waves, what’s the big deal?’ ” says Kim Knowlton, a senior scientist with NRDC’s health and environment program (disclosure). “The big deal is there’s a lot more of them, and they kill people.”

Extreme heat is insidious in more ways than one. Not only does its health impact take people unawares, it’s also not always obvious when heat factors into someone’s death. In the summer of 2003, Europe sweated through its hottest temperatures in 500 years. At the time, it was reported that about 20,000 people had died from the heat—15,000 in France alone. A later epidemiological study that compared the number of people who actually died that summer to the number of expected deaths, however, put the toll closer to 70,000. That’s no minor rounding error.

That so many heat-related deaths can fly under the radar seems remarkable, but the relationship between extreme heat and health is…well, it’s complicated. Heat doesn’t exist in a vacuum. Several environmental conditions beyond the number on the thermometer can interact to worsen peoples’ health.

Air pollution, humidity, and cloud cover all play a role in how a heat event affects public health. Duration matters, too. “You can go to a hot yoga class for about an hour of extremely hot temperatures and emerge from that and you’ll be fine,” says George Luber, who is chief of the climate and health program at the CDC. “But if you extend the exposure to a couple hours or a couple days, it becomes dangerous.”

Heat stroke—and its milder cousins heat exhaustion and heat cramps—is probably the most obvious example of a heat-related illness, and the one most likely to show up on a death certificate. The very young and the very old, who are less able to regulate their body temperature, are particularly susceptible. Extreme heat and its cronies, says Luber, can also make many chronic health problems worse. People with cardiovascular, respiratory, and renal diseases are at greater risk during heat waves, as are people with diabetes and psychiatric conditions.

In order to prevent some of those death certificates in the first place, some health departments around the country are beginning to use syndromic surveillance to collect information in real time. Originally developed to detect bioterrorism outbreaks, syndromic surveillance systems work by probing data from electronic medical records for trends that raise red flags. For instance, if suddenly there is an uptick in local reports of fever, cough, and sore throat, it might be an early indication that flu season is kicking off. During a heat wave, authorities would be on the lookout for a rise in the number of hospital visits from patients complaining of keywords like “sun,” “heat,” and “dehydration.” Such information would let health officials know where to direct resources during a heat event and measure how well preventive steps are working.

Being prepared is key and can be as simple as spreading the word that heat is, in fact, dangerous. Physicians, public health officials, and even weather forecasters can remind people to crank up the AC, hydrate, and check up on loved ones and neighbors—especially those at greater risk—when sweltering days are ahead.

Communities can also open cooling centers to give those without access to air-conditioning a place to chill out. New York City’s Cool Roofs program is painting city rooftops white to reflect sunlight. This can lower temps within a building by 30 percent: Collectively, those pale roofs can reduce the urban heat island effect. Adding more green spaces to cities cools them off, too.

Whatever our strategy, we should get to it, because the prognosis is not good. Climate change has already started to spawn more frequent and intense heat waves. The U.K.’s Met Office predicts that by 2050, the kind of heat Europe saw in 2003 could happen every other year. According to a 2012 report by NRDC, the total number of excessive heat days in the United States will increase to more than eight times the baseline level by the end of the century.

So remember, folks, drink plenty of water and cut those carbon emissions already. Because bringing down the planet’s fever could keep you out of the sickbed, too.