Record-breaking heat grips much of the U.S. ahead of July 4

Record-breaking heat spread across much of the United States on Wednesday, pushing more than 160 million people under extreme heat warnings or advisories as the country headed toward the July 4 holiday weekend. Dangerous heat was intensifying across most of the central and eastern United States, with heat indices likely topping 100 degrees and, in some areas, climbing far higher.

The blast furnace conditions were already changing daily routines in places like Hill City, Kansas, where mail carrier Sabrina Hooper was walking long delivery routes through punishing heat. In Chicago, teacher Michelle Klein stocked up early, filled her gas tank and watered her plants before the hottest stretch. Property investor Amy Kaspar's tenant had an air conditioner running but still could not cool an apartment enough, with humidity and wind making the air feel like exhaust from a bus.

The National Weather Service issues heat products when an extreme heat event is expected within the next 24 to 36 hours and poses a significant threat to life. Extreme heat has killed more people in the last 10 years than any other weather phenomenon. Employers should provide cool water, rest and shade, and the Labor Department's proposed federal heat rule would help protect about 36 million workers in indoor and outdoor jobs if finalized.

Those conditions can quickly cause heat-related illness, especially where cooling access is limited and where people are spending hours outside in direct sun.

PJM Interconnection, the nation's largest electric grid operator, warned of price spikes, transmission congestion and record demand as electricity use climbed with the heat. The U.S. Department of Energy issued an emergency order under Section 202(c) of the Federal Power Act, finding a statutory emergency in PJM's region and authorizing the grid operator to tap backup generation at data centers and other large electricity users during the heat wave.