A prolonged heat wave kept its grip on the US South on Tuesday as dangerously high temperatures, rising to well above 38oC, and oppressive humidity were on tap across a wide swathe of the region through the holiday weekend.

Meanwhile, in Chicago, children, the elderly and people with respiratory diseases were being cautioned to stay indoors for a wholly different reason – polluted air.

Some 62 million Americans in central Arizona, across Texas and the Deep South and into Florida’s panhandle were under excessive heat watches, warnings and advisories that were expected to last until the Fourth of July, the National Weather Service said.

“There may be more danger than a typical heat event due to the longevity of elevated record high nighttime lows and elevated heat index readings during the day,” the NWS said in an advisory. “It is essential to have a way to cool down and interrupt your heat exposure.”

Heat index temperatures were forecast to reach 43.3oC in Dallas, 43.8oC in New Orleans and 41.6oC in Mobile, Alabama, on Tuesday, the service said, urging people across the region to stay out of the sun and drink plenty of fluids.

“This is a health threat,” New Orleans Mayor LaToya Cantrell said during a news conference on Tuesday, noting that the city had opened cooling centres. “This is unprecedented. We are living in unprecedented times. We are going to do everything necessary to meet the needs of the most vulnerable.”

The stationary high pressure system across the South that is trapping the heat and humidity, known as a heat dome, has been lingering for the last few weeks, causing the sweltering weather.

The heat wave claimed the life of a 14-year-old boy who was hiking in the Big Bend National Park in Texas on Friday when the temperature reached 48.3oC. His stepfather was killed in a car crash when he rushed to get help, the park said in a statement.

In several upper Midwestern cities, air-quality alerts were in effect as a cloud of hazy smoke from wildfires in Canada hovered above the region. Chicago, the third-largest city in the United States, had the worst air quality of any large city in the world due to the smoke, according to IQAir, a website that tracks pollution.

The growing frequency and intensity of severe weather across the United States is symptomatic of human-driven climate change, scientists say.

As of midday on Tuesday, no widespread power outages were reported in the South. However, some 110,000 homes and businesses remained without electricity in Arkansas, Tennessee and Oklahoma after strong storms over the weekend took down power lines, according to poweroutage.us.

(Reporting by Brendan O’Brien in Chicago; Editing by Chizu Nomiyama and Mark Porter)