About 150 million people expected to experience temps above 90 F

Like yesterday, the hottest place in the U.S. today was the Rio Grande Village in Texas, according to the National Weather Service. The Big Bend campground hit 112 degrees and was under a Category 4, or extreme, heat risk.

While the campground in Big Bend will be downgraded to a Category 2 “moderate” heat risk tomorrow, it will nonetheless hit a sweltering 105-degree high.

New Mexico Gov. Michelle Lujan Grisham has declared a state of the emergency as wildfires threaten communities in the southern part of the state.

The South Fork fire, which forced the evacuation of nearly 7,800 people Monday night from the village of Ruidoso, more than doubled in size to nearly 14,000 acres in less than 24 hours.

It is 0% contained, according to the New Mexico State Forestry Division.

"This is a full suppression fire, and all available resources are responding to help secure line and perform point protection for homes in and around Ruidoso," fire officials said in a Tuesday afternoon update.

A neighboring fire southwest of Ruidoso has scorched nearly 5,000 acres in the Mescalero Apache Reservation and is also 0% contained.

The Tribal Council declared a state of emergency for the reservation.

Air quality alerts are in effect for parts of six states: Connecticut, Massachusetts, New Hampshire, New Jersey, New York and Ohio.

Additional alerts are expected to go into effect on Wednesday for areas in Indiana, Maine and Michigan.

When heat reacts with air pollution, it accelerates the formation of ground-level ozone, which can cause coughing, shortness of breath and lung irritation. When ground-level ozone levels are high, the CDC advises people to spend more time indoors and avoid strenuous activities outside.

In Chicago, where the 97-degree weather broke records on Monday, the heat wave is just getting started, with another seven days of high temperatures and humidity expected.

At one of the city's public beaches, Jason Lach with the Chicago Fire Department warned about lack of hydration even while people sought to cool off in the lake.

“You don’t realize you’re sweating and dehydrating as much as you are — even spending 20, 30 minutes in 63-degree water like we have right now, you can get dehydrated and tired quickly and become over exerted,” Lach warned.

Dr. George Chiampas, attending physician of emergency medicine and sports medicine at Northwestern Medicine, advised that a sports drink with electrolytes and sugar may be better to replenish lost fluids than just water. For people who exercise or work outdoors, it’s critical to stay at a “balanced hydration level,” Chiampas said.

By the fourth or fifth day of high temperatures, the body can begin to lose weight from lost fluids and become even more vulnerable to the heat. Chiampas' guidance: “Check your weight in the morning, and the next and then at night. If you’re down three or four pounds, drink to get back to your baseline body weight.” 

A key sign of serious dehydration is the color of urine, Chiampas added: It "should never be dark like iced tea,” he said — a well-hydrated state is indicated by a pale, “light lemonade color.”

Amid the severe heat forecasts, 911 systems across Massachusetts went down for almost 2 hours on Tuesday, preventing emergency calls from going through statewide.

Service was restored at 3:49 p.m.

During the outage, members of the public were advised to call direct lines of local district police stations and use call boxes to reach fire departments or to report other medical emergencies.

Boston Mayor Michelle Wu addressed the issue during a news conference meant to focus on the Boston Celtics' NBA Championship win.

It’s not yet clear what caused the outage. State officials said they continue to investigate what led to the disruption.

Leaving pets or children inside hot cars can lead to tragedy. When it’s 80 degrees outside, a car can reach 109 degrees in 20 minutes and 123 degrees in an hour, according to the CDC.

Consumer Reports explains the dangers and shares tips on how to prevent a death from happening, no matter the weather.

The heat wave coming for the Northeast will hit harder in states where air conditioning is less common. 

Most of New Hampshire, Maine and Vermont are under heat advisories or excessive heat warnings. The forecast in Manchester, New Hampshire, calls for temperatures of 99 degrees F tomorrow. But each of these states lags behind the U.S. average for air conditioning coverage, according data collected in 2020 by the U.S. Energy Information Administration. 

About 89% of U.S. households had some kind of air conditioning and most — about two-thirds — featured central air as of 2020. But only 67% of households in Vermont had A.C. In Maine, that figure was 70%. New Hampshire came in at 78%. 

The figures took stock of which households had room air conditioners and which had central air. Fewer than one in 10 households in Maine and Vermont had central air. Around 21% had it in New Hampshire.

Google searches for “air conditioning” and related terms were spiking in Vermont, New Hampshire and Maine, according to the search engines’ trend-tracking tool.  

The National Weather Service’s heat-risk rating tool suggests parts of these states will experience “major” or “extreme” heat risk during the next three days. “Extreme” risk means forecasters expect long-duration heat with little overnight relief, which is expected to impact health systems and infrastructure.

Workers across the U.S. are enduring scorching temperatures this week, a factor that has become top of mind for many economists who say that climate change could become a long-term drag on production in addition to a serious health risk.

Economists at the Federal Reserve Bank of San Francisco wrote in a recent letter that the construction industry faces particular risks.

“Extreme heat is most likely to affect economic outcomes through the construction sector for two reasons,” the economists wrote. “First, construction makes up a larger share of economic output than other vulnerable sectors, like agriculture. Second, decreases in construction productivity slow capital accumulation and therefore have long-lasting effects on macroeconomic outcomes.”

Beyond the economic downsides, heat waves are expected to become an increasingly serious risk to the health of many laborers. The Adrienne Arsht-Rockefeller Foundation Resilience Center found that injuries due to severe heat total about 120,000 every year and are expected to hit about 450,000 annually by 2050. High temperatures can cause workers to slow down and be clouded with fatigue, which can lead to mistakes, injuries and in extreme cases death.

The Occupational Safety and Health Administration, the federal agency tasked with overseeing worker safety, has guidelines about how much physical work can be done safely outside in hot conditions. In construction, heat stress becomes a concern at a wet bulb temperature of 77 degrees F, and is not safe when above 91 degrees. A wet bulb reading measures heat risk in sunlight, factoring in temperature, humidity, wind speed and solar radiation.

Labor, environment and health care advocates have signed a petition requesting that FEMA consider extreme heat a major disaster. NBC News’ Maura Barrett reports on the increasing number of deaths related to heat in the U.S. over the last 30 years.