How heat waves can kill...and how to stay safe

“Heat waves are getting hotter, longer, more frequent, and you’re getting less relief at night,” said Ken Graham, director of the National Weather Service, in April. As the Earth keeps blowing past earlier heat records, here’s what to know and how to protect yourself.

1. How heat stroke can kill you

“Heat is a silent killer,” says Kristie Ebi, professor of global health at the University of Washington.

“People typically have a low awareness that heat can be deadly, so they don’t take actions to protect themselves.”

Heat stroke, the most dangerous heat-related illness, is a medical emergency. More than half of older heat stroke victims die.

To understand how heat kills, it helps to know how the body cools itself off:

• Sweating. When sweat from the body’s 2-to-4 million sweat glands evaporates, it cools the skin (but may hasten dehydration).

• Moving blood to the skin. “You want to get that hot blood from the body’s core to the skin,” says Robert Meade, research fellow at the Harvard T.H. Chan School of Public Health. Why? When tiny blood vessels in the skin dilate—that is, expand—they release heat like a radiator.

But if enough blood gets shifted to the skin, too little is left for internal organs, including the GI tract.

“The intestines start to become more permeable,” explains Meade. “So toxins from the gut start to flood into the bloodstream.”

And that can set off another crisis.

“The toxins can trigger a systemic inflammatory response,” says Meade. “In many ways, heat stroke is similar to sepsis.”

That’s an extreme body-wide immune reaction to an infection.

“The systemic inflammatory response changes how the blood clots, so it can cause disseminated intravascular coagulation—blood clots in the blood vessels,” Meade explains.

“And when the brain gets hot enough, we start to see neurological effects, so people become very confused.”

The end result: Cells die and critical organs fail. “The heat just wreaks havoc on the entire body,” says Meade.

2. How heat strains the heart

Though heat stroke can be deadly, cardiovascular disease takes the most lives in a heatwave.

“Heart disease puts you at greater risk because your heart is less able to tolerate the higher heat,” says Ebi. “And many people who have underlying heart disease don’t know it.”

How does heat strain the heart?

“When the blood vessels in the skin open up, you have the same amount of fluid in a larger container, so blood pressure drops,” explains Meade.

Sweating makes it worse. “When we sweat, we’re losing fluid, so we can become dehydrated,” he adds. Like a hose with less water, that also lowers blood pressure.

“To maintain sufficient blood pressure to feed vital organs, the heart needs to work harder and beat faster.”

“The heart isn’t working as hard as it does when you’re exercising. But during a heatwave, we’re talking about constant stress over a day or multiple days. In a person who has underlying cardiovascular issues, that cumulative strain could be dangerous.”

3. Who is at higher risk in a heatwave

It’s not just heart problems.

“There’s a long list of vulnerabilities that put people at higher risk from heat,” says Ebi. Among them:

Diabetes 

“Our lab has shown that type 2 diabetes reduces the body’s capacity to thermoregulate—that is, to boost sweating and blood flow to the skin,” notes Meade, referring to research done by his former colleagues at the University of Ottawa.

“And diabetes often comes with a host of cardiovascular and kidney complications that also raise risk.”

Kidney disease 

“As we get older, our kidneys’ ability to filter is already reduced,” says Meade. “So heat is another stress on top of that. And if someone already has weak kidneys, heat can precipitate kidney failure.” How?

“If heat stress is extreme, it can reduce blood flow to the kidney and reduce how much fluid the kidney is processing,” explains Meade. That can cause acute kidney injury.

Lung disease

Heat stress can worsen respiratory conditions like asthma and chronic obstructive pulmonary disease (COPD), though spikes in air pollution triggered by heat waves are also partly to blame.

Older age

In one University of Ottawa study, after two hours in 98°F heat, 12 people mostly aged 60 to 70 gained more body heat than 12 people mostly aged 18 to 24.

Why do older people gain more heat? They may sweat less, their blood vessels dilate less, and their hearts are less able to pump harder and faster.

“But there’s a huge amount of variation in how people respond to heat stress,” notes Meade.

Staying fit may boost your resilience. His team compared 38 physically fit adults aged 18 to 66 to 35 less-fit similar adults during three 30-minute bouts of cycling in 104°F heat.

“Those who had high levels of physical fitness had less age-related decline in thermal regulation,” says Meade.

“Being physically fit may help during heat stress, because some responses—like an increased heart rate—are similar to our response to exercise.”

That said, more studies testing fit and less-fit older people are needed.

One caution: “For the same degree of dehydration, older people are not as thirsty,” notes Meade. So you have to drink even if you’re not thirsty.

“Staying hydrated is very important,” he adds. “But don’t overdo it if you’re only drinking water. That can severely dilute sodium levels in the blood.”

A good rule of thumb: Your urine should be the color of lemonade. If it’s darker, you may be dehydrated. If it’s colorless, you may be overhydrated.

Most packaged foods can help restore sodium. That includes yogurt, milk, bread, and other foods that may not taste salty. (Check the label.)

“Heat may also reduce appetite, so remember to eat,” Meade advises.

4. Many medications can worsen heat stress

Dozens of drugs may impair the body’s ability to handle excess heat. Some may be surprising.

“Antihistamines like Benadryl have anticholinergic properties,” says Meade. “They block the receptor that activates sweat glands, so you may have a reduced sweating response.”

Antihistamines may also constrict small blood vessels in the skin, making it harder to release body heat.

“Diuretics are easier to wrap your head around because they reduce the amount of fluid in your body,” says Meade. Other drugs that lower blood pressure may also boost heat risk.

“Beta-blockers reduce heart rate,” notes Meade. And calcium-channel blockers curb the heart’s contractions.

Many drugs that promote mental health—like selective serotonin reuptake inhibitors, tricyclic antidepressants, and lithium—may also alter the body’s response to heat.

Check with your doctor or pharmacist if you’re taking any prescription meds.

That said, “We know what systems these medications affect,” says Meade, “but we need more research looking at how they actually affect the body’s response to heat stress.”

5. Check your area's risk at cdc.gov/heatrisk

“Last year, heat-related illness caused over 120,000 emergency department visits,” said Mandy Cohen, director of the Centers for Disease Control and Prevention, at a press conference in April.

“To be prepared this summer, CDC has developed a new dashboard that includes the National Weather Service’s heat risk map and EPA’s AirNow air quality index.”

The HeatRisk results depend not just on temperature but also on humidity.

“When the humidity is high, sweat may drip off your body, but you get no cooler if the sweat doesn’t evaporate,” explains Meade.

And keep in mind that a heat wave is riskier if it hits early in the summer because we’re not yet acclimated.

“The more we’re exposed to heat, the better the body is at dealing with it,” says Meade.

The CDC’s HeatRisk website also has links to local cooling centers for people who have no air conditioning.

“Once your indoor temperature tops 95°F, an electric fan no longer cools your body,” notes Meade.

“If you feel poorly, reach out to someone. Don’t wait.”

And expect more records to fall.

“‘Insane’ Heat Has Been Scorching Miami,” ran the New York Times headline in May. “It’s Not Even June.”

Miami’s heat index—which takes humidity into account—reached 112°F on May 18 and 19, “breaking the previous daily record by an astonishing 11 degrees,” noted the Times.

Yet in April, the paper added, Florida enacted a law banning local governments from requiring heat and water breaks for outdoor workers.

“High temperatures can be deadly, but almost all of those deaths are preventable,” says the University of Washington’s Kristie Ebi. “Nobody has to die in a heatwave.” 

In heat stroke, dehydration and a shift of blood to the skin leave too little blood (and oxygen) for the gut, allowing toxins to enter the bloodstream.

What to know about heat-related illnesses

Heat stroke (most severe) 

What to look for

What to do

Heat exhaustion

What to look for

What to do

Heat cramps

What to look for

What to do

Sunburn

What to look for

What to do

Heat rash (least severe)

What to look for

What to do

Source: Adapted from the Centers for Disease Control and Prevention.