NEW YORK, June 22: With more than 100 million Americans facing another day of heat advisories across portions of the Midwest, Northeast, and West on Friday, the National Weather Service is warning people to prepare for dangerously high temperatures.

Last summer, the US experienced record-breaking heat, with over 2,300 death certificates listing "excessive heat" as a cause. As climate change continues to drive global temperatures higher, the limits of the human body's ability to cope with extreme heat are being tested. Recently, over 1,000 people died during a pilgrimage to Makkah in Saudi Arabia, where temperatures soared to 125°F.

In the US, government agencies have long warned that the combination of higher summer heat and high humidity poses increasing dangers. Rising temperatures in regions like the Midwest could lead to "wet-bulb" temperatures that prevent the human body from cooling down through sweating.

According to NASA, "The hotter it is, the more strain our bodies feel, and the more we need to sweat to cool down. But humid air has less capacity to hold additional moisture, so water evaporates more slowly in humid conditions."

The human body cools itself by sweating, but high humidity levels hinder the evaporation of sweat, depriving the body of this cooling mechanism. Scientists calculate "wet-bulb" temperature by covering a thermometer with a wet cloth and measuring how much the evaporation of moisture cools the thermometer.

NASA explains, "Wet-bulb temperature is the lowest temperature to which an object can cool down when moisture evaporates from it."

Long-range weather forecasts predict another record-breaking, steamy July. Fortunately, several websites now help people calculate wet-bulb temperatures by entering the current temperature and relative humidity. For example, the Omni Calculator shows that an air temperature of 95°F combined with 80% humidity results in a wet-bulb temperature of 89.47°F, indicating that sweat evaporation does little to cool the body.

Wet-bulb temperatures above 86°F pose a potentially fatal danger to humans. "In these conditions, you should avoid direct sunlight and drink lots of water," advises the Omni Calculator.

Prolonged exposure to high wet-bulb temperatures can be deadly. "This triggers changes in your body. You dehydrate. Your organs become stressed, especially your heart," NASA states. "Blood rushes to your skin to try to release heat, starving your internal organs. The results can be deadly."

Studies suggest that wet-bulb temperatures of 95°F or above could render some parts of the world uninhabitable. In the meantime, it's crucial to take precautions during periods of high heat and humidity. According to an expert, these precautions include:

• Knowing the symptoms of heat illness, such as muscle cramping, rash, headache, nausea, and dizziness.
• Limiting outdoor exposure during the hottest parts of the day.
• Drinking plenty of water and replenishing with electrolytes if needed.
• If suffering from heat illness, move to a shaded, cool place, loosen or wet clothes to reduce body heat, or take a cool bath if possible.
• Seeking out air-conditioned places to spend time during periods of extreme heat.