In extreme heat, the first symptoms of exposure can happen in as little as 15 minutes. Signs of heat exhaustion can start with a headache, nausea, muscle spasms, dizziness or shortness of breath. When it’s above 90 degrees, people of all ages need to be careful, not just those who are older or very young. 

Certain medications can also make people more vulnerable to heat's effects. Some drugs for blood pressure can aggravate dehydration, while beta blockers that slow the heart rate can make it more difficult to detect warning signs.

Many drugs taken for mental health disorders, including antipsychotics and antidepressants, can affect the body’s thermostat and increase the risk of a heatstroke. Scientists think this is because of the medications’ effects on the hypothalamus, the part of the brain that regulates our body’s temperature. Antidepressants in particular can interfere with internal temperature control and prevent sweating, which is critical for the body to cool itself. 

Stimulants for ADHD can also raise body temperature.

Doses of some drugs may need to be adjusted temporarily to avoid deadly side effects during heat waves, according to the Centers for Disease Control and Prevention.

A wildfire along the busy Interstate 5 corridor in northern Los Angeles County is 24% contained after growing by more than 1,000 acres overnight.

The Post Fire, which had previously prompted the evacuation of some 1,200 campers and recreationists near Gorman, California, has scorched more than 15,600 acres since erupting Saturday.

A red flag warning indicating high-risk fire conditions remains in effect until Tuesday evening because of low humidity and strong winds.

In a briefing Monday, Daniel Swain, a UCLA climate scientist, warned that the upcoming fire season could be especially active after two years of heavy rain and snow. Vegetation fed by the heavy precipitation will begin to dry out during the hot summer months — a process called fuel loading that could trigger more fires later in the season.

“The good news is increasingly in the rear-view mirror,” Swain said. “The bad news is that I think that the back half of this season is going to be much more active, with a lot more concerning level of wildfire activity in a lot of areas, than the first half.”

Across California, firefighters are battling some 2,103 wildfires in total, which have burned more than 66,000 acres, according to the California Department of Forestry and Fire Protection. At least 22 structures have been damaged or destroyed. No fire deaths have been reported yet this season.

With the heat index reaching over 100 F in some areas, public schools in New York and Massachusetts have changed their schedules like they would for a snow day.

In Worcester, Massachusetts, one of the largest public school districts in the state, summer break will start a few days early.

"Because of the age of our buildings, many do not have air conditioning. We regret that we are ending the school year in an abrupt manner, but we want to ensure our students and staff are safe in what are predicted to be unusually hot temperatures," Rachel H. Monárrez, the superintendent of Worcester Public Schools, wrote in a letter to families and staff.

When it's hot out, students learn less in classes without AC, according to a study from Harvard University's Kennedy School of Government. Each degree higher than the school year's average temperature showed a 1% decline in learning for the entire year when a school did not have AC, the study found.

Schools in multiple New York districts, including Buffalo, Rochester and Syracuse, will let students in pre-K through 8th grade leave after lunch.

This week’s extreme heat is caused by a “heat dome” parked over much of the Midwest and Northeast. A heat dome is a high-pressure system in the upper atmosphere that traps warm air in place, similar to how a lid traps heat over a pot of boiling water.

Heat domes can build and intensify over a region by blocking cooler winds or preventing clouds from forming, thus fueling persistent high heat and humidity.

Scientists say that climate change is increasing the likelihood of heat domes, making extreme heat events both more frequent and more intense in a warming world.

In Chicago, Pittsburgh and Washington, D.C., the heat index value — a measure of the temperature the body feels with humidity factored in — is likely to exceed 100 degrees. Humidity levels in those cities are expected to range from around 60% to 73%.

For people in New York and Boston, meanwhile, temperatures could feel like they're in the high 90s, given forecast humidity levels around 63%.

The heat index is a crucial measure of extreme heat, since the sweat our bodies make to cool us evaporates less quickly in high humidity, making us feel hotter.

According to data from the Centers for Disease Control and Prevention, 1,168 people died of heat in 2023 — the highest toll in 30 years. That figure does not include deaths in which heat was a factor but not the cause, which would make the total much higher.

While much of the Midwest and the East Coast will be sweltering under a heat dome, other areas of the country are bundling up.

Parts of southern Oregon and Northern California faced a freeze warning early this morning. 

Yesterday, Kirk, Oregon, had a low temperature of 19 F, making it the coldest place in the U.S. Today, forecasters are calling for a low of 38 in Kirk.

In Texas, by contrast, the country's highest temperature of the day was recorded at 114 degrees in Rio Grande Village. The area remains under extreme heat risk today.

Tens of thousands of people in the Pittsburgh area lost power last night because of storm damage, a particular concern because the city reached 100 F for the first time in 30 years.

Some 35,000 people in Pennsylvania were still without power this morning, according to PowerOutage.us.

The residents were among the 76 million people under heat alerts today, with temperatures in some places expected to be than 25 degrees above average. About 100 daily records are likely to be shattered this week.

The heat dome causing these extremes is set to move south around the weekend, providing some relief to the Northeast, but cities including Washington, D.C., could still experience up to eight days of severe heat.

The news comes as several health care and labor groups filed a petition to get FEMA to recognize extreme heat as a major disaster.

Parts of New York state will reach beyond 100 degrees today, the weather service has warned.

In Rochester, on the shores of Lake Ontario, it will reach 102 by 3 p.m., with a heat index — a measure of how hot it feels that factors in humidity — of 104.

The weather service urged people to find air-conditioned spaces, avoid strenuous work outside and to check on family members and neighbors.