How can extreme heat impact health?

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Understand how heat can pose serious health risks.  

Increases in extreme heat conditions create health concerns. Extreme heat can lead to dehydration, fatigue, heat stroke, heat exhaustion, and heat cramps, which occur as a result of the body’s inability to cool itself relative to the surrounding environment when heat indexes exceed certain physical thresholds. Because extreme heat is the deadliest of all natural events year over year, an important goal of the First Street Foundation Extreme Heat Model is identifying heat events that pose a threat to human safety. 

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NWS heat safety thresholds 

Using thresholds informed by the National Weather Service (NWS), days of “Caution” threshold temperatures associated with relatively minor consequences, such as fatigue, to be around 80 degrees. Threshold temperatures above 125 degrees are considered the “Extreme Danger” threshold and are associated with life-threatening effects such as heat stroke. Threshold temperatures are based on Heat Index Values which is a measure of how hot it feels outside when incorporating relative humidity. For example, if the temperature is 96 degrees with 65%  humidity, the heat index, how hot it feels, is 121 degrees.

Screen_Shot_2022-08-03_at_5.39.00_PM.pngNWS Heat Caution Index

Caution (80℉): Fatigue and decreased productivity

At a heat index of 80 degrees, prolonged exposure to extreme heat or physical activity can increase the body’s risk of fatigue. While anyone spending prolonged amounts of time outside of an air-conditioned environment can experience these effects, individuals who work outdoors or perform physical labor in settings without air-conditioning may experience additional strain and possible decreases in productivity. 

Health Caution (90℉): Heat stroke, cramps, and heat exhaustion are possible

Once the heat index exceeds 90 degrees, heat-related illnesses such as cramps, heat exhaustion, and heat stroke become possible from extended exposure to extreme heat, particularly for vulnerable populations such as the very young, the elderly, people who are pregnant, individuals with certain medical conditions, and those who are taking certain medications. Care should be taken to drink plenty of fluids, avoid excessive alcohol or caffeine consumption, avoid direct exposure to sunlight, and wear light, breathable clothing. 

Danger (>100℉): Heat stroke possible, cramps and heat exhaustion likely

When the heat index exceeds 100 degrees, early-stage heat-related illnesses such as cramps and heat exhaustion go from being possible to likely for anyone who is exposed to heat for a prolonged period of time. The risk of heat stroke also increases, particularly for high-risk groups (young children, elderly, pregnant, and some chronically ill individuals). Avoid extended periods of time in the heat, drink plenty of fluids, and familiarize yourself with the symptoms and treatments for these conditions.

Extreme Danger (>125℉): Heat stroke likely, high-risk individuals at lethal risk

 While heat-related illnesses can cause death at any heat index, this risk is particularly pronounced at a heat index above 125 degrees. Heat stroke is likely to occur for any individual exposed to these temperatures for a prolonged period of time, and care must be taken to avoid any extended amount of time in this level of heat. Stay indoors as much as possible, drink plenty of fluids, and wear light, loose-fitting clothing to cover your body from direct exposure to the sun, which will only worsen the effects of heat.

Screen_Shot_2022-08-04_at_1.32.30_PM.pngThe symptoms associated with heat exhaustion vs heat stroke. (Source:

Dangerous heat events on Risk Factor 

Because extreme heat is the deadliest of all natural events, an important goal of our Extreme Heat Model is to identify heat events that threaten human safety. For this reason, Risk Factor shows the number of hot days, this year and 30 years from now, that exceed safety thresholds informed by the National Weather Service. Use Risk Factor to find property-specific heat risk assessments for any U.S. property.

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