What is Heat Exhaustion?

Heat exhaustion is a set of symptoms caused by prolonged exposure to heat, often during physical exertion. It's the middle stage of heat illness, which progresses from heat cramps to heat exhaustion and finally to heat stroke. It doesn't only happen during exercise, though. Spending a long stretch in high heat, like sunbathing or gardening, can bring it on too.

Preventing heat exhaustion is far easier than treating it, and a few practical habits go a long way:

• Stay hydrated in the heat, even when you're not exercising.
• Avoid the sun during the hottest part of the day. Save outdoor work for early morning or evening.
• Pay attention to your body and rest when you start to tire.

Heat exhaustion develops in stages. When heat stresses the body, it responds in normal ways: seeking cooler surroundings, sending more blood to the skin, and sweating to shed heat. When those responses aren't enough, the body starts losing too much salt and water, leading to dehydration and strain on the heart and circulation. If the process isn't stopped, it can progress to heat stroke, where circulatory collapse and organ failure become life-threatening.

Heat exhaustion can sneak up on you before it becomes dangerous. Common signs include heavy sweating, cool or clammy skin, fatigue, a rapid but weak pulse, headache, nausea, and sometimes vomiting. Dizziness and faintness can set in as well. Catching these early is key to keeping the condition from worsening.

Heat exhaustion can often be treated at home in its early stages. Move to a cooler area and stop any strenuous activity right away. Drink water or an electrolyte drink, and change into light clothing. Cool towels, a cool shower, or a cool bath can help bring body temperature down faster. If symptoms don't improve within an hour, or if they get worse, seek medical attention, since heat exhaustion can progress to heat stroke.

Heat exhaustion appears rarely in the historical record, but heat stroke shows up often. Around 24 B.C., the Roman commander Aelius Gallus lost much of his army during a campaign in Arabia, and the soldiers' symptoms, later recorded by the senator Cassius Dio, match what we'd now recognize as heat stroke. Accounts like these show that extreme heat has been a danger to people for thousands of years.

Some people are more vulnerable to heat exhaustion than others:

• Children, whose bodies absorb heat from the environment faster than adults' do.
• Older adults, whose cardiovascular systems may struggle to shed heat.
• Athletes, who push themselves hard even in the heat.
• Outdoor workers, who can't avoid heat exposure.

Others at higher risk include people with chronic illnesses or other health conditions, those taking certain medications, and anyone without access to a properly ventilated, cool home.

There are two types of heat exhaustion: one from losing too much salt, and one from losing too much water. They share similar symptoms, and both respond to prompt electrolyte intake. With the salt-depletion type, plain water can actually dilute sodium levels further, so an electrolyte drink is the better choice.

Heat exhaustion and heat stroke are often used interchangeably, but they're different stages of heat illness. Left untreated, heat exhaustion can progress to heat stroke, which is far more dangerous. Heat stroke signs include hot, dry skin, rapid shallow breathing, and failure of the organs and circulation, and it can be fatal. If you suspect heat stroke, call emergency services immediately.

Animals get heat exhaustion, too. Cats and dogs can move from heat exhaustion to heat stroke within minutes, and because they can't cool themselves as efficiently as people do, they rely on us to keep them safe. Keep pets in cool, well-ventilated or air-conditioned spaces in hot weather. Their symptoms often go unnoticed, but the danger is just as real, so make sure they always have plenty of water and a cool place to rest.