Millions swelter as central and eastern US placed under excessive heat watch

Millions of Americans will continue to swelter as Labor Day weekend approaches, with much of the country under some kind of excessive heat watch.

The brutal heatwave the US midwest suffered earlier this week has spread to the eastern half of the country, with more than 20 million people under some kind of a heat alert .

Related: Hot and alone: how US cities work to protect isolated people in heatwaves

But meteorologists predict the weather will cool down, particularly on the east coast just in time for the federal holiday when a cold front moves in. Meanwhile, the south-east may see scattered storms.

Cities across central US states like Arkansas, Missouri, Illinois, Tennessee, Kentucky and Indiana, as well as coastal cities in South Carolina and Virginia are experiencing “major” heat, according to National Weather Service (NWS), with heat index values reaching as high as 105F (40C).

The NWS says: “This level of heat affects anyone without effective cooling and/or adequate hydration. Impacts likely in some health systems, heat-sensitive industries and infrastructure.”

The climate crisis, caused largely by the burning of fossil fuels, forest destruction and methane emissions, means heatwaves are increasing “in frequency, duration, intensity and magnitude”, the World Health Organization says.

Heat is the leading cause of weather-related deaths in the US. Around 1,220 people in the US are killed by extreme heat annually, according to the US Centers for Disease Control and Prevention.

Related: Corn sweat: crop moisture amplifies humidity and heat in US midwest

Those in engaging in physical activity, such as athletes or outdoor workers, where heat advisories are active, are at risk for exertional heat stroke. Other vulnerable groups include children, the elderly and disabled, people experiencing homelessness, emergency responders and incarcerated people.

The American Red Cross urges people experiencing extreme heat to limit outdoor activities and exercise, avoid sugary, caffeinated or alcoholic drinks, and try to stay in a place with reliable air conditioning.