The National Weather Service has baked biscuits inside a hot car, in a safety message about the peril of leaving children or pets inside a vehicle.

As a heatwave takes grip of large swathe of the US, with up to 200m people expected to be affected by a heat index of up to 115f degrees (46c), the officials performed the experiment inside a car in Nebraska to show how hot vehicles can become when left unattended.

To demonstrate the dangers, the NWS staff set about baking the biscuits in the city of Omaha, using only heat from the sun.

“If you are wondering if it’s going to be hot today, we are attempting to bake biscuits using only the sun and a car in our parking lot,” NWS Omaha said on Twitter. “We will keep you posted with the progress.”

CNN said four biscuits were placed on a baking sheet on the dashboard of a car and left to sit in the sun. After 60 minutes, the pan had reached 175.2f (80c) and the tops of the biscuits reached 153f. The back seat registered 120.4f in the shade.

It said, four hours later, the tops of the biscuits were nearly finished baking, but the bottoms remained doughy.

The car had to be turned around to adjust for the changing angle of the sun.

The experiment was carried out to warn people about the dangers of leaving children or pets inside vehicles, even for for a short period of time. US summers frequently come with stories of tragic deaths as a result of a toddler or baby being left in a car.

CBS News said six people had died in connection with the heat – four people in Maryland, one in Arizona, and another in Arkansas.

Several events were cancelled in New York City, including OZY Fest and the NYC Triathlon.

The NWS said the east coast and midwest are likely to see temperatures in the upper 90s, combined with high humidity. Experts are urging people to limit their time outside and drink lots of water.

Cities in Vermont and New Hampshire are opening shelters where people can cool off. Some power outages have been reported in Philadelphia and after storms in Michigan’s Lower Peninsula.

Police in Braintree, Massachusetts, asked residents “to hold off” all criminal activity until the extreme heat is over.

“Folks. Due to the extreme heat, we are asking anyone thinking of doing criminal activity to hold off until Monday,” the department wrote on Facebook. “It is straight up hot as soccer balls out there. Conducting criminal activity, in this extreme heat is next level henchmen status, and also very dangerous.”