(HOUSTON) — A relentless heat wave with triple-digit temperatures is continuing to affect millions of people in the South.

Much of Texas and Louisiana will have temperatures at the 100-degree mark by Sunday afternoon, but the heavy humidity will cause heat indexes to soar another 10 to 20 degrees, forecasts show.

More than 33 million Americans are currently under heat alerts from Houston to New Orleans. The temperature in Houston has not dropped below 80 degrees in nearly a week, with no relief in sight. Lack of overnight cooling can strongly contribute to heat-related illnesses.

Numerous records have been set in southeast Texas for warmest low temperature. The heat wave is considered to be much more dangerous than the typical scorching climate the South is known for during the summer and has arrived much earlier in the season than normal.

A persistent ridge of high pressure is what’s causing the heat wave to sit over parts of the U.S., Tim Cady, a meteorologist with the National Weather Service in Houston, told The Associated Press.

It could be more than a week before the region starts to see some relief from the extreme temperatures.

Heat is the No. 1 weather-related killer. More than 600 people die from heat-related illnesses every year in the U.S., according to the Centers for Disease Control and Prevention.

On average, more people in the U.S. die from extreme heat than any other severe weather event.

Mild symptoms of heat-related illness are typically sunburn or a heat rash or heat cramps, with signs including muscle pain and spasms. These symptoms can progress to heat exhaustion — which includes symptoms of headaches, dizziness, muscle cramps, nausea and vomiting — and possibly heat stroke, if the body reaches extremely high temperatures.

Populations including the elderly, infants and young children, outdoor workers and people with low socioeconomic status are at higher risk of heat-related illness.

A perilous heat wave is also currently plaguing India, where nearly 100 people have died in the northern state of Uttar Pradesh and eastern Bihar over the last several days, the AP reported.

Temperatures in parts of the region reached up to 109 degrees Fahrenheit on Sunday — five degrees higher than normal, according to the AP.

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