At least about two dozen people have died amid the perilous climate crisis-driven heatwave that has scorched swaths of the US with record temperatures.

As a huge heat dome sits over the county’s eastern half, extreme heat gripped millions of people in the days leading up to the US’s semiquincentennial on Saturday – and beyond it. More than 20 states experienced have reported stifling temperatures more than 100F (38C), marring celebrations. And more than 140 million people remained under active heat alerts across the US on Sunday.

Officials in New Jersey believe the extreme heat was a factor in the deaths of 22 people across 10 counties there, mostly in central and northern parts of the state. Many of the individuals were found in homes with no air conditioning, outside their residences, on the street and in parked cars, according to officials.

The first of those deaths occurred on Thursday, and the ages of the deceased in question mostly range from their mid-30s to their 80s. Preliminary findings cause investigators to believe the deaths are heat-related, though the chief state medical examiner for New Jersey would later determine the exact cause of death for each.

“This is not a typical summer heatwave,” the New Jersey department of public health said in a statement. “This type of heat can quickly become life-threatening to humans and to animals of all ages.”

The National Weather Service (NWS) has said cool air from the north in the coming days is going to lower some of the most extreme temperatures in the region, including New Jersey. The Fifa World Cup final is scheduled to be held in the New Jersey city of East Rutherford on 19 July.

Elsewhere, a heat-related death was reported in Cook county, Illinois, Natalia Derevyanny, a government spokesperson, told NBC News. The cause of that death was recorded as organic cardiovascular disease – with heat stress as a contributing factor.

Hinds county in Mississippi reported the death of 74-year-old Mitchell Ray Cooley due to heat exposure on Thursday, state officials said. Cooley had been reported missing, and his body was found the next day behind a gas station, the county coroner said in a statement.

“Mr Cooley suffered from a medical condition that impaired his judgment,” the coroner’s office said. “Based on the investigative findings, scene examination, and subsequent evaluation, the cause of death has been determined to be weather-related heat exposure. At this time, there is no indication of foul play.”

Meanwhile, on 27 June, Martha Irene Van Egmond, 83, died in Bolton, Mississippi, after falling in her garden. When her husband, Rick, tried to help her up, he fell too. The couple were unable to get up and spent hours in the heat.

Rick Van Egmond said he and his wife called out for help, and eventually two men from a nearby apartment complex came – but it was too late for Martha. She died surrounded by flowers, doing what she loved, he said to local news outlet WAPT.

Jeramiah Howard, Hinds county’s chief death investigator, attributed her death to the heat combined with her age, WAPT reported.

As Donald Trump spoke during rain-dampened celebrations in Washington DC on Saturday, emergency services there had treated 51 people with heat-related issues as of 8pm ET, with 12 taken to nearby hospitals, according to local emergency response officials.

Other events scheduled for Saturday – including the Independence Day parade in DC – were cancelled amid the blistering heat. Among other weather-related disruptions, Trump’s so-called Great American State Fair on the National Mall also temporarily closed down on Friday after reports that 44 visitors had been treated for heat-related illnesses.

The worst of the heat started moving out of the US’s north-east and midwest regions by Sunday, shifting farther south into the mid-Atlantic and south-eastern parts of the country.

But scientists warn that heatwaves with extreme temperatures are indications that the world must lower the greenhouse gas pollution driving the global climate crisis.

The NWS is urging the public to avoid heat sickness by drinking plenty of fluids as well as staying out of the sun and in air-conditioned environments. Officials have also asked people to check on relatives and neighbors.