Understanding Heat Risk

The warm-season months — May through September — of 2023 were the hottest ever recorded in the United States, according to the Centers for Disease Control and Prevention (CDC), which also reports that heat-related emergency room visits nationally hit a six-year high last year. This year also appears to be headed for the record books, with hundreds of cities in nearly every state on track for one of their 10 warmest summers on record. In times of extreme heat, it’s important to know how heat impacts the body and where to get up-to-date information on risk levels where you live.

The Effects of High Heat and Humidity on Your Body

Most of the time, your body compensates for heat by sweating. When the sweat on your skin evaporates, it takes some of the heat from the blood just under your skin with it. As this cooled blood travels through your body, it brings your body temperature down. In times of high heat, however, your body temperature may rise faster than it can cool itself by sweating.

High heat is even more dangerous when combined with high humidity. Humidity is a measure of water vapor in the air. There are two ways to measure it: absolute humidity, or the actual amount of water vapor in the air, and relative humidity, or the amount of water vapor in the air relative to the temperature of the air. When humidity is high, the air is already so saturated with water vapor that the sweat on your skin is less likely to evaporate, inhibiting your body’s ability to cool itself. Your body may also react by producing even more sweat, which can lead to dehydration.

Tracking Heat Risk

Meteorologists use the term “heat index” to refer to the apparent temperature, i.e., what the heat feels like to your body when the effect of humidity is factored in. The chart below can be used to calculate the heat index based on the temperature and the relative humidity. For example, if the temperature is 96 degrees and the relative humidity is 50%, the heat index — how hot it feels to your body — is 108 degrees.

It’s important to note that the values in the chart above are for shady locations. In direct sunlight, the heat index can be up to 15 degrees higher.

This table shows how different heat indices can affect your body:

Earlier this year, an interactive, color-coded heat risk map debuted on both the National Weather Service and CDC websites. The map shows not only the current levels of heat risk across the U.S. but also the forecast over several days.

Keeping Safe

According to the CDC, certain populations are disproportionately affected by extreme heat. These include:

• Older people
• Children and adolescents
• People with preexisting health conditions
• Pregnant women
• Outdoor workers
• People with limited access to cooling resources
• People living in low-income communities

Everyone, but especially members of these vulnerable populations, should take precautions against heat-related illnesses when temperatures are high. We offered tips in a previous post.

For more, see our related posts:

• Rising Temperatures Increase Risk of Hot Car Deaths
• How Your Meds Can Change How You Weather the Weather
• Keeping Cool in a Power Outage