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Heat kills in three main ways, Jay said. The usual first suspect is heatstroke—critical increases in body temperature that cause organs to fail.

When inner body temperature gets too hot, the body redirects blood flow toward the skin to cool down, Jay said. But that diverts blood and oxygen away from the stomach and intestines, and can allow toxins normally confined to the gut area to leak into circulation.

"That sets off a cascade of effects," Jay said. "Clotting around the body and multiple organ failure and, ultimately, death."

But the bigger killer in heat is the strain on the heart, especially for people who have cardiovascular disease, Jay said.

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