Heat Waves and Health: What Makes Heat Dangerous and How to Stay Safe
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Heat waves do not knock down buildings or flood streets, so they can seem less dramatic than hurricanes or tornadoes. Yet in the United States, the National Weather Service describes heat as the leading cause of weather-related deaths in recent decades. The good news is that heat illness is largely preventable. Knowing what makes heat dangerous, and who is most at risk, can save lives.
Key points
- Your body cools itself mainly by sweating. High humidity slows evaporation, which is why humid heat is especially dangerous.
- The heat index combines temperature and humidity into a "feels like" number measured in the shade.
- Older adults, young children, people with chronic illnesses, outdoor workers and people without air conditioning face the highest risk.
- Heat stroke is a medical emergency. Call 911 and start cooling the person right away.
- Heat waves have become more frequent and intense as the climate warms, according to the IPCC.
What is a heat wave?
There is no single global definition. In general, a heat wave is a period of unusually hot weather, often with high humidity, lasting two or more days. "Unusually hot" depends on the place: a temperature that is normal in Phoenix could be a dangerous heat wave in Seattle, where fewer homes have air conditioning and people are less used to heat.
That local context matters for health. People and buildings adapt to the climate they usually experience, so heat that is extreme for a particular area tends to cause more harm there.
How your body handles heat
Your body works hard to keep its core temperature close to 37 °C (98.6 °F). In hot weather it does this in two main ways:
- Sweating: as sweat evaporates from your skin, it carries heat away.
- Moving blood to the skin: your heart pumps more blood to the surface, where heat can escape.
Both strategies put strain on the body. Sweating causes loss of water and salt. Pumping extra blood makes the heart work harder, which is one reason heat waves increase heart attacks and other cardiovascular problems, not just heat stroke.
When the air is humid, sweat evaporates more slowly, so it cools you less. When nights stay hot, the body never gets a break to recover. Hot nights are a major factor in heat-related deaths.
Understanding the heat index
The heat index, used by the National Weather Service, estimates how hot it feels when humidity is factored in with the air temperature. For example, an air temperature of 32 °C (90 °F) with high humidity can produce a heat index well above 38 °C (100 °F).
Two important details are often missed:
- The heat index is calculated for shady conditions. According to the National Weather Service, full direct sunlight can raise heat index values by up to 15 °F.
- Wind, physical effort and heavy clothing also change how much heat stress your body experiences.
The National Weather Service issues heat advisories and extreme heat warnings based on local criteria. Taking those alerts seriously is one of the simplest ways to stay safe.
Who is most at risk
Anyone can develop heat illness, but some groups are much more vulnerable, according to the Centers for Disease Control and Prevention (CDC):
- Adults 65 and older, whose bodies adjust to heat less efficiently and who may not feel thirsty
- Infants and young children, who depend on others to keep them cool
- People with chronic conditions such as heart disease, lung disease, diabetes, kidney disease or mental illness
- People taking certain medications, including some for blood pressure, depression and other conditions, which can affect sweating or hydration
- Pregnant people
- Outdoor workers and athletes, who generate extra body heat through exertion
- People without air conditioning or reliable shelter, including people experiencing homelessness
- People living alone, who may have no one to notice early warning signs
Where you live matters too. Cities are often several degrees warmer than surrounding areas, especially at night, because pavement and buildings absorb and release heat. This is called the urban heat island effect. Neighborhoods with fewer trees and more pavement tend to be hotter.
Signs of heat illness
| Heat exhaustion | Heat stroke | |
|---|---|---|
| Skin | Cool, pale, clammy; heavy sweating | Hot, red, dry or damp |
| Other signs | Nausea, dizziness, weakness, headache, muscle cramps, fast weak pulse | High body temperature (103 °F or higher), confusion, fast strong pulse, possible loss of consciousness |
| What to do | Move somewhere cool, loosen clothing, sip water, use cool cloths. Get medical help if symptoms worsen or last more than an hour | Call 911. Move the person somewhere cooler and cool them with water or cool cloths. Do not give them anything to drink |
Heat stroke can cause permanent damage or death if not treated quickly. If you are unsure which one you are seeing, treat it as an emergency.
How to stay safe
Stay cool
- Spend the hottest part of the day in air-conditioned spaces. If you do not have air conditioning at home, look for public cooling centers, libraries or shopping centers.
- Electric fans help move air, but the CDC notes that at very high temperatures, fans alone may not prevent heat-related illness.
- Take cool showers or baths, and wear lightweight, loose, light-colored clothing.
- Close blinds or curtains on sunny windows during the day.
Stay hydrated
- Drink water regularly, before you feel thirsty.
- Limit alcohol and very sugary drinks.
- If you have a medical condition that limits fluids, ask your doctor how much to drink during hot weather.
Plan activities wisely
- Schedule outdoor work or exercise for early morning or evening.
- Take frequent breaks in the shade and pace yourself.
- Never leave children or pets in a parked car, even for a few minutes. The temperature inside a closed car can rise dangerously fast, even on a mild day.
Check on others
Check on older neighbors, relatives and anyone living alone, at least twice a day during a heat wave if possible. A short phone call or visit can catch problems early.
Tip: Look up your local heat alerts and cooling center locations before summer starts, and save them on your phone. During a heat wave, it is easier to act on a plan you already have.
Heat waves and climate change
Of all types of extreme weather, heat waves have the clearest link to climate change. The IPCC's Sixth Assessment Report concluded that hot extremes have become more frequent and more intense across most land regions since the 1950s, and that human-caused climate change is the main driver. Some recent heat waves would have been extremely unlikely without human influence, according to attribution studies.
A single hot day is weather, not climate, as explained in Climate vs weather. But the long-term trend toward more frequent and severe heat is clear, and further warming will make dangerous heat more common. That makes both preparedness and efforts to limit warming, discussed in What can one person do?, increasingly important.
The bottom line
Extreme heat strains the body, especially when humidity is high and nights stay warm. Older adults, young children, people with health conditions and people without cooling are most at risk. Staying cool, drinking water, timing activities and checking on others are simple steps that prevent most heat illness. And if someone shows signs of heat stroke, call 911 right away.