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As the temperature outside rises, our bodies make adjustments to keep our internal temperature constant to prevent us from overheating through a process called thermoregulation. This includes bodily functions such as sweating and widening of the blood vessels (vasodilation). When we sweat, perspiration evaporates from our skin to cool us down. When the blood vessels under our skin widen, our heart pumps more blood to our skin, which releases more heat from our inner body.

Our bodies are constantly working to hold a steady core temperature around 98-100 degrees Fahrenheit (F). This allows our organs to function properly. But when the temperature outside is extremely hot, our temperature can start to rise. A person with a body temperature above 104 degrees can develop heat stroke. This can cause dizziness, difficulty breathing, confusion, seizures or loss of consciousness. Brain and heart damage—sometimes permanent—can occur when body temperature climbs above 107 degrees F.

Too much summer heat can be unhealthy for everyone, but it can be especially dangerous to firefighters. The incidence of fires increases in the U. S. during the summer months. Firefighters fight almost twice as many fires in the summer compared to the rest of the year. On top of dealing with the extreme heat (sometimes over 700 degrees F!), these first responders face extreme physical exertion, mental stress and smoke inhalation on the job. All of these factors together can place firefighters in immediate danger of heat exhaustion, heatstroke and heart problems. In fact, firefighters are up to 136 times more likely to die of coronary artery or heart disease during or soon after they suppress a fire.

In a study published in Circulation last month, researchers may have uncovered several reasons why putting out fires puts firefighters at risk for heart disease. They discovered that a single, 20-minute session of fire simulation training—where healthy firefighters were exposed to physical activity in the extreme heat (about 755 degrees F)—was enough to injure their blood vessels, even though the firefighters’ core body temperature never reached above 101 degrees F. The problem: Although the firefighters’ bodies did keep their core temperature within a healthy range, their blood vessels did not relax properly immediately after the training. Also, as a result of the training, the firefighters’ blood clotted more easily. Damaged blood vessels and increased clotting of the blood can be very harmful to the heart and sometimes can lead to a heart attack.

This research shows us that even when we are able to keep our body temperature from getting too high, there are hidden dangers of being physically active in extremely hot temperatures. So keep your heart healthy this summer and don’t overexert yourself while outdoors!

Dao Ho, PhD

Dao H. Ho, PhD, is a biomedical research physiologist at Tripler Army Medical Center. The views expressed in this blog post are those of the author and do not reflect the official policy or position of the U.S. Department of the Army, U.S. Department of Defense or the U.S. government.