There is a silent killer radiating through cities all over the world. It leaves no path of destruction. It is invisible to the human eye. It doesn’t flood homes, rip trees from the ground or topple buildings over. But it kills more people than any other weather-related disaster.

The culprit is heat, and it’s responsible for hundreds of deaths each year. While a sunny day might feel nice after a long winter, too much heat can be harmful to your health.

“Human beings are like all animals – they have a range of temperatures at which they can function best,” says Diana Petitti, a medical doctor and professor at Arizona State University. “If it’s so hot that you can no longer adapt, all sorts of bad things can happen.”

Heat and the body
You may have heard of the term “heat stroke.” A heat stroke happens when the human body gets too hot. But what’s going on in the body during a heat stroke?

On a hot day, your body goes through changes to help it adapt. First, you sweat. Your body produces sweat because it’s trying to stay at a comfortable 98.6 degrees. As liquid sweat molecules appear on the skin, they change to gas molecules. This process is called evaporation. As the gas molecules leave your body, they take a little bit of heat with them, cooling you down.

When your brain registers the loss of fluid from sweat, you become thirsty. You might reach for a bottle of water to quench your thirst. But water isn’t enough to replace the lost fluids, because sweat also contains important salts and sugars. If you drink too much water without taking in those other nutrients, you can dilute the calcium and sodium in the fluid outside your cells. This can cause the cells to swell up—including cells in your brain.

This kind of “water poisoning” is rare. But it’s best to have juice or a sports drink when you’ve been sweating more than usual.

If you ignore the feeling of thirst and fail to replace the fluid lost from sweat, your body will send out more serious warnings. You might start to feel tired, dizzy and lightheaded. These are signs that you’re becoming dehydrated.

“Your body is trying to tell you to get out of the heat, drink more, get some salt and rest,” Petitti says.

Dehydration means your body doesn’t have enough fluid to work properly. This can lead to hyperthermia, when your body temperature rises so much that sweating alone cannot bring it back down. Hyperthermia causes fever, and if that fever gets too high, it can cause a type of seizure called a convulsion. “That can lead to shock, coma and sudden death,” Petitti says.

Severe heat strokes are rare, however. Doctors can usually treat patients before their symptoms become too serious. But certain groups of people are more at-risk from the heat than others.

Age is one factor in heat risk. Elderly people and infants are both more likely to suffer bad effects from heat. As the body ages, it has a harder time regulating temperature. Elderly folks also don’t always realize they’re thirsty. This makes them prone to dehydration. Infants are another high-risk group because they can’t tell anyone when they need something to drink.

Certain medications can also make a person more likely to get dehydrated. Some drugs prevent sweating, while others make users less aware of the heat so they’re less likely to drink fluids they lose from sweat.

People living alone without air conditioning are also more at risk of getting heat stroke, especially if they have no one to check on them. When a heat wave strikes, this group is especially vulnerable.

Measuring heat’s impact
In 1995, 700 people died during a heat wave in Chicago. In 2003, a massive heat wave that spread across Europe killed more than 30,000 people. These disasters sparked a movement to protect people against the dangers of heat.

“Public health officials realized that these deaths are preventable,” says Sharon Harlan, a sociologist at ASU. Harlan is leading a project to figure out which neighborhoods in Phoenix are the hottest, and how the heat affects people who live there.

Global climate change is making the Earth hotter. In the future, we will probably see more frequent heat waves with even higher temperatures. In order to plan for these events, public health officials must be able to predict how great of an effect the heat wave will have. This is hard to do, because the number of people who die from heat-related causes is often underestimated. That’s where scientists like Gerardo Chowell-Puente come in.

Chowell-Puente is a mathematical epidemiologist. He develops mathematical models to study the impact of different ways of dealing with events like epidemics or natural disasters. He is working with Harlan and Petitti to develop a statistical model for estimating heat-related deaths and hospitalizations in Phoenix.

Chowell-Puente uses data from death certificates from Maricopa County, Arizona. All death certificates include one or more death codes to show the primary or secondary causes of death. Sometimes, Chowell-Puente says he can tell by the death code whether the person’s death was related to heat. The code will say something like “heat stroke” or “dehydration.”

But other times, the code might say the cause of death was “heart attack.” It’s possible that the heart attack was brought on by exposure to extreme heat, but the death code alone is not enough to know for sure. By using a computer to scan the death certificates, researchers can search descriptions written by physicians for key words like “hyperthermia.” This helps determine if heat played a role in the cause of death.

Chowell-Puente also uses climate data from different neighborhoods over a number of years. He compares the data sets and plots them on a graph to see if there is a connection between death rates, location and temperature. And in Phoenix, there is.

Heat and inequality
All over the United States, families who earn similar amounts of money are likely to live in the same neighborhoods. This is partly based on the types of houses people can afford, Harlan says. Neighborhoods are usually separated by race and ethnic group, as well. In Phoenix, Hispanics, African Americans, Native Americans, and other people living in poor areas are most likely to feel the unwanted affects of heat.

Low-income families may not be able to afford a home with air conditioning, making it hard to cool off inside during the summer. Their neighborhoods have fewer trees, plants and open spaces like public parks that help to cool the outdoor environment. People in minority populations are also more likely to have jobs that expose them to the heat, like landscaping and construction.

“A large number of deaths from heat exposure in Phoenix happen outdoors, especially with people who work outdoors,” Harlan says. The models Chowell-Puente creates will help find the hottest – and deadliest – neighborhoods in Phoenix. With this information, public health officials can better predict how many people are in danger when a heat wave strikes, and what needs to be done to save lives.

“I’m hoping that by pointing out the connections between social and economic inequalities and exposure to extremely hot weather, policy makers are going to be conscious of the idea that this has a real impact on people’s health and people’s lives,” Harlan says.