Published on July 1, 2026 11:20 am
As a practicing internal medicine physician, I face some version of the same question every summer: Is it safe to be outside right now? This week, the answer is tilted towards tough No. National Integrated Heat Health Information System (NIHIS) has Dangerous heat wave building marked across much of the USHeavy to extreme heat is expected through the Fourth of July weekend and beyond.
The temperature of your phone is only half the story. Without checking the heat index, you’re missing the number that actually predicts how hard your body will have to work to stay cool, which could be the difference between a good day and a trip to the emergency room. Here’s how it works, when to worry, and what calms you down.
Temperatures are expected to rise to dangerous levels in US regions and states
according to NIHHIS Interactive Heat MapCentral, eastern and south-eastern states are bearing the brunt of this heat. Some of the most affected states include: Arizona (especially in the southwestern part of the state), Texas, southeastern Colorado, Florida, Tennessee, Virginia, and Mississippi. The temperature is expected to rise from high 90s to low 100s.
The reason this heat wave has been particularly brutal: Humidity is accumulating on top of already high temperatures, causing the heat index to rise rapidly. This means that elevated temperatures, combined with thick, oppressive humidity, will push the heat index above 110 degrees Fahrenheit in some places.
What’s worse is that the details we often forget happen overnight. When daytime lows hover in the 70s, your body never completely cools, and temperatures rise rapidly for a few days.
What is heat index?
heat index “Feels like” is temperature. It combines air temperature and relative humidity to estimate how hot the air feels. Humidity drives this because our main cooling system is sweat evaporating from the skin. When the air is already saturated, sweat has nowhere to go, which forces our body’s core temperature to rise. That’s why a seemingly small number can still be feel We feel the burning sensation when we step outside.
How to Read a Heat Index Chart
On the National Weather Service (NWS) heat index chart below, temperature values (in Fahrenheit) are plotted at the top. The left side of the chart shows the relative humidity percentage.
For example, let’s take a 90-degree afternoon with 70 percent humidity. Look for 90 degrees at the top, then 70 percent on the left side of the chart. Where those two numbers intersect on the chart is the heat index, which in this case would be 105 degrees Fahrenheit. In other words, 90-degree weather may seem tolerable until you add 70 percent humidity, and suddenly the air feels sticky and more than unpleasant to be inside. That’s because a heat index of 105 degrees falls into the orange block of the chart, aka the danger zone. (We will learn what this means in the next section.)
Keep in mind that the chart also considers shade and light wind. But if we step out into the open with the sun’s rays, the temperature may rise by an additional 15 degrees. So, a 90-degree day can easily feel like triple digits on an exposed path.
What heat index should I worry about?
The NWS divides the heat index into four regions. Each zone indicates the severity of heat-related health risks. The danger zone – orange or red on the chart – indicates heat that poses a significant risk to overall health.
Here is a description of each heat index zone (all are in degrees Fahrenheit and colors refer to the corresponding color zone on the heat index chart):
- caution (light yellow): : 80-90 degrees; Fatigue appears from sustained effort during physical activity
- extreme caution (dark yellow): : 90 to 103 degrees; heat cramps and heat exhaustion started
- danger (orange): : Heat index of 103-124 degrees; Heat exhaustion is likely, and heat stroke becomes possible
- extreme danger (red): : above 125 degrees; There is a high possibility of heat stroke
Once the heat index crosses 103, strenuous efforts (long climbs, fast work, big mileage) become a bad bet. Heat stroke is more than overheating: Once your core exceeds 104 degrees, you may experience confusion or altered thinking; This is a true emergency area, and requires immediate medical care.
How to stay safe outside during a heat wave
Based on my expertise as a doctor, here is my best advice for staying safe when entertaining or exercising outside in extreme temperatures.
cold quickly and often
regular breaks Reduce heat stress more effectively than waiting until you feel hotter in air conditioning or in the shade. Move to the shade, pool, or nearby safe water or air conditioning before you feel suffocated.
Use fast-cooling zone
Due to the high blood flow, heat in the hands, forearms, face and neck decreases rapidly. Place your forearms in cold water For a few minutes your core drops faster than you expect.
remove excess clothing
Fabric traps heat and prevents sweat from evaporating, which is the body’s main pathway for cooling; Bare skin dissipates heat faster.
Treat fans as a backup, not a solution
Moving air helps if the skin is moist, but once the air gets hot, a fan alone will be quite useless. Pair it with mist. Fill a spray bottle with cool water and spritz your face and body if you feel hot.
Respect Customization Window
If you have just moved to the city or the heat has just increased, your body needs about ten days to adjust. The first few days are when people get into trouble. To get used to hotter temperatures, gradually increase Time spent outdoors and intensity of workout over time.
drink water frequently
Water is enough for small, easy efforts. On long, sweaty days, water won’t be enough. adding electrolytes It replaces sodium lost in your sweat.
Train indoors when the index is in the orange or red zone
If the map shows “danger” or “extreme danger”, postpone big trips and go to an air-conditioned gym. Indoor miles still count, and the trail will be there when the heat rises.
know when to stop
Heavy sweating, cramps, nausea, dizziness, or a pounding headache are signs you should pass out. Get in the shade and cool off.
Many of us believe that during a heat wave we can avoid drinking less water indoors, but that is not the case. So, keep drinking water even when you are at home, as we still lose fluids through breathing and low-level sweating. And don’t forget to check the “feels like” temperature on your weather app to find out if it’s safe to entertain outside.
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