We Asked: How can we protect ourselves from summer heat?

Editor’s note: This interview was originally published in Doing Well, a health news outlet from ASU Media Enterprise and ASU Learning Enterprise. Subscribe to Doing Well to get interviews with health experts delivered to your inbox weekly.

By Natasha Burrell

As summer settles in, so do dangerously high temperatures that pose serious health risks. Hundreds of thousands of people die every year from heat illness, making heat the deadliest form of extreme weather. Experts believe the true number of heat-related deaths is likely much higher than we realize. So, how does heat actually affect our bodies, and what can we do to protect ourselves and our families?

I spoke with Dr. Pope Moseley, a physician and biomedical sciences researcher at ASU’s College of Health Solutions, about how heat can amplify underlying health conditions and what to know about heat-related illness. Our conversation has been edited for length and clarity.

Natasha Burrell: You've previously discussed how when there's a heat wave, the hospital fills up not just with heat-related illnesses, but also other conditions like heart attacks, strokes, and dementia. Can you explain how heat makes these illnesses worse?

Pope Moseley: Heat is the first multiplier of illness. There are many conditions that are made worse by heat; heat does a number of things. First, heat can cause you to become dehydrated. You sweat more—but you need to replace that fluid. If you're not replacing fluid, you can pump out liters of fluid to try to cool yourself. When you do that, then your body doesn’t have the blood volume to pump [blood] to various parts.

Not only can dehydration lessen the amount of fluid you have, but we don't have sufficient blood to fill all the tubes. So your body is always making choices of where to send that blood. The brain always gets a big share, heart, kidneys—but when you have, for example, working muscle, there's a lot more blood going to the muscles than normal, and the body has to make choices about where it's taking the blood from. When you're cooling yourself, you can have half your blood volume going to your skin. When you start having demands like cooling through the skin, especially if you're exercising, that pushes blood away from other organs. The biggest organ that's affected is the gut. Your [gut] bacteria produce a lot of compounds that are really important for you to function, including a lot of neurotransmitters. So if you start pulling blood away from the gut, a lot of things happen, including the gut bacteria change, and can cause huge disruptions.

All of these changes that are heat driven affect organs that are not working at tip-top shape. Imagine someone who has a heart that is teetering because it has just enough blood coming into it to pump, to work efficiently. So if you suddenly have to push half your blood volume out to your skin, you can have a failure of that organ. That's why so many diseases are impacted.

There are [also] a whole bunch of medications that can cause you to lose fluid, like diuretics. Many drugs for depression and anxiety, for instance, can make you more heat sensitive.

NB: What is heat illness? What are the signs and symptoms?

PM: There is one group of diseases where you get progressively more dehydrated and your body temperature begins to rise. You develop heat cramps because of electrolyte abnormalities. That's called heat exhaustion. That's one group of diseases.

There's another disease called heat stroke. People like to think of a continuum of heat cramps, heat exhaustion, heat illness, then heat stroke, but that's not the way it works. There's a bunch of dehydration-related diseases, and those are progressive. Heat stroke itself is an explosive, catastrophic illness—you have sudden collapse. It can happen in an exercising person within 15 or 20 minutes. What happens, especially in unconditioned people or people not used to the environment, is you can get this endotoxin leak across your gut. Your liver fails, your kidneys fail; you develop respiratory distress, and your blood no longer clots. This isn't one of those things where you can go hiking and say, Well, I better drink some more water because I'm kind of feeling tired and ill. You can develop an explosive heat illness. The primary problem with heat stroke is loss of mental function, so you don't think well. This is what happens in the Grand Canyon where somebody's hiking and suddenly they go down. They've had this explosive thing and they don't know it's happening until they're too far gone. The mortality range is between 20 and 60 percent. In the best case scenario, you have a one in five chance of dying. So I'm very careful in the heat.

NB: As we head into the hottest months of the year, what advice would you give to someone managing health conditions that can worsen in extreme heat?

PM: When it's a little bit cooler, you can take yourself through heat acclimatization. When you expose yourself to heat on a daily basis for a week or two, you get more heat adapted. I wouldn't recommend going out on a 120 degree day and trying to undergo that process, but I would suggest that you think about taking a walk when it's cooler, or going outdoors when it's in the 80s to let your body slowly get used to heat. Heat acclimatization is the ability to sweat more—you start sweating at a lower temperature, so you start cooling your body earlier than you would if you're not acclimated. Exercise is a great substitute. For heat exposure, exercise generates the same kind of changes. If you decide that you're gonna start a new exercise program in July and go outside and run, that's not so great.

NB: I’ve noticed people hiking up a mountain with just a single bottle of water; does that pose a risk?

PM: Even an acclimatized person can easily sweat well over a liter an hour. You can push all that fluid out very quickly. You need to carry water and you need to realize that in those situations if you don't feel right, you’ve got to stop. You've got to cool yourself because this is a multisystem vascular collapse and you can crash pretty quickly. I've taken care of a number of people where you resuscitate them, but then they die in the ICU over the next two or three days because they have this irreversible inflammatory response.

NB: Here in Arizona, we often hear, “It's a dry heat.” Do different types of heat—dry versus humid—affect our health differently?

PM: You have to transfer heat to the environment. So when your body temperature is 98 degrees, [and] the air temperature is 100 degrees, the only way you're transferring heat is through evaporative cooling [for example by sweating]. If your evaporative cooling gets impaired, you can't transfer heat anymore to the environment. What blocks [heat] transfer? Well, one thing is if you're dehydrated. Another is humidity. [When there is] humidity in the air, you can't rapidly move water from your body up to dump energy, then you can't cool.

NB: What other tips do you have for people who have no choice but to be outdoors, whether for work or daily life? How can they best protect themselves?

PM: Wear a hat, wear long sleeves, seek shelter—even if it's just shade. Continue to drink water because not only does water give you more to evaporate, but when water hits the stomach, temperature drops. Go to cooling centers. It's really hard—heat is not only the first multiplier of disease, it is the first multiplier of social conditions.