How Hot Is Too Hot? Here’s What Temperature Is Considered Too Hot for Humans

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The Earth is getting hotter, and these changes can affect human health. Heat-related injuries like heatstroke, heat exhaustion, and even heat-related deaths have been on the rise over the past 30 years.

Overheating can happen to anyone. That's why it's important to understand how the heat can affect your health. Here's what you should know about the heat index and how to adjust when it's too hot for you to be outside.

When is it too hot to be outside?

If you think it's too hot to be outdoors, you're probably right. Most people get a pretty quick sense of when it's too hot and time to look for shelter.

But temperatures that may be comfortable for you may be dangerous for others. That's because some people are more likely to develop heat-related injuries, including:

• Children and babies

• Adults over 65 years

• Pregnant women

• People who work outdoors

• Athletes, especially those training outdoors

• People with underlying medical conditions

That's why it's helpful to have a concrete temperature cutoff. A 2010 study suggested that a heat index of 95°F (35°C) is the highest temperature that humans with no underlying medical conditions can safely tolerate for long periods of time. But more recent data suggests that this number is actually much lower. A heat index of 77°F to 88°F (25°C to 31°C) may actually be what humans can safely tolerate for long periods of time. And this number is likely even lower for people who are at higher risk for heat injury.

Right now, experts urge people to take caution starting at heat indexes of 90°F (32°C).

What is a heat index?

The heat index is a measurement that takes into account not just the temperature but also the humidity. The heat index tells you what temperature it actually "feels like" when you're outside.

Higher humidity makes your body "feel" hotter. That's because when you get hot, your body starts to sweat. As sweat evaporates from your skin, you start to cool down. But when it's humid, your sweat can't evaporate easily. This means you lose the cooling effects from sweating, which means your body temperature can't cool down.

The heat index doesn't take into account other factors that can affect your body temperature, such as:

• Whether you spend time in direct sunlight

• If you're exposed to cooling effects from the wind

• Whether you're working or exercising outdoors

All of these things can also affect how hot you get when you're outdoors. But a heat index can give you an idea of whether it's too hot to be outdoors.

What is the hottest temperature humans can safely tolerate?

The hottest temperature humans can safely tolerate depends on:

• Their overall health

• How much time they're spending outdoors

• What activities they're doing outdoors

People can survive temperatures of 104°F to 122°F (40°C to 50°C) at least for short periods of time. But just because humans can survive these temperatures doesn't mean these temperatures are safe.

Exposure to these temperatures can lead to heat exhaustion and heatstroke, even in people who aren't at risk for heat-related injuries.

Instead, you want to use the heat index to give yourself a starting point for safety. The heat index chart has a color-coded grid that tells you how likely you are to develop a heat-related injury based on the air temperature and humidity level.

Extreme heat index

An extreme heat index - sometimes called extreme heat warning - means that people are very likely to develop heatstroke or sunstroke if they spend time outdoors. Heatstroke is the most serious type of heat injury. An extreme heat index starts at 130°F (54°C).

This risk is even higher for people who are already more vulnerable to heat-related injuries.

During an extreme heat warning you should:

• Find air conditioning and stay indoors.

• Avoid any strenuous activity or exercise.

• Drink plenty of fluids.

• Wear light clothing.

You should also check on those who are at higher risk for heat injury and never leave people or pets in a car.

Dangerous heat index

When the heat index is in the danger zone it means that people are likely to develop heatstroke and heat exhaustion if they spend time outdoors. You are even more likely to develop heatstroke if you spend a lot of time outdoors or you're physically active outdoors.

People who are more likely to develop heat injuries can develop heatstroke in these conditions.

A dangerous heat index ranges from 105°F to 129°F (41°C to 54°C). In this heat index you should:

• Limit time spent outdoors.

• Stay indoors with the air conditioning on as much as possible.

• Avoid strenuous activity or exercise while outdoors.

• Drink plenty of fluids.

• Wear light clothing.

People at risk for heat injury should stay indoors with air conditioning as much as possible.

Extreme caution heat index

When the heat index is in the extreme caution range, people are likely to develop heat exhaustion if they spend a lot of time outside or are physically active while outdoors. People who are at risk for heat injury are at higher risk of developing heatstroke in these conditions.

An extreme caution heat index ranges from 90°F to 105°F (32°C to 41°C). In this heat index you should:

• Take frequent breaks and spend time indoors in the air conditioning.

• Avoid strenuous activity or exercise while outdoors.

• Drink plenty of fluids.

• Wear light clothing.

Caution heat index

When the heat index is in the caution range, people can develop fatigue and muscle cramps if they spend a lot of time outdoors or are physically active while outdoors.

A caution heat index ranges from 80°F to 90°F (27°C to 32°C). In this heat index you should:

• Avoid strenuous activity or exercise while outdoors.

• Drink plenty of fluids.

• Wear light clothing.

How cool should you keep your home during heat waves?

There's no single answer, but experts at the Department of Energy recommended keeping your home at 78°F (25°C).

This might not be cool enough for older adults, children, and people who are pregnant or have certain medical conditions. If you, or someone in your home, is at risk for heat-related injuries, you may want to keep your thermostat at 72°F (22°C). You can set your thermostat lower at night since cooler temperatures can help you sleep better.

Most homes in the U.S. have air conditioning. But air conditioning might not be enough to keep your home at an ideal temperature during periods of extreme heat. Air conditioners usually lower the inside temperature of a home by about 20°F.

So you'll need to take additional steps to cool your home:

• Keep blinds, shades, and curtains shut.

• Close the doors to unused rooms.

• Insulate around air conditioning window units.

• Open the windows at night to let cool air in, close them when temperature rises.

• Turn on attic fans (ventilators).

• Place bowls of ice in front of portable fans.

• Minimize kitchen appliance use.

• Minimize use of electronic devices and lamps.

• Weather strip doors and windows.

• Use window reflectors designed to reflect heat outside.

Utility costs can be a burden for many people. But it's important to know that most states will not disconnect power services when severe weather is expected. If you're facing an electricity shutoff, you may be able to apply for an exemption. Here's information on weather-related disconnect policies for each state.

When should you go to a cooling station?

A cooling station is an air conditioned public space, such as a shopping mall, library, or other public facility. Cooling stations are open to everyone, and many are open 24 hours a day, 7 days a week.

You should go to a cooling station when the heat index is high and you don't have access to air conditioning. This can help you avoid overheating and heat-related injury. Call 211 to get a list of local cooling stations. You can also check this directory to find a cooling station near you.

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