An "ongoing extensive heat wave" was set to bake a wide-swath of the East Coast from Maine to North Carolina through the July 4th holiday, forecasters warned Monday.

"Hot and humid weather like what we’re experiencing this weekend can cause heat illness, and even death,” said New York Health Commissioner Mary Bassett in a statement. “If you have air conditioning and you have not used it up until now, now is the time to turn it on."

Temperatures were expected to reach 10 to 20 degrees above normal and are forecast to stick around until the end of the week, according to the National Weather Service.

In New York City, families gathered at public fountains, with children running in and out of the spray to keep cool over the weekend. The high temperatures prompted New York City to put out an excessive heat warning, issued when a combination of heat and humidity makes it feel like it is 105 degrees or greater.

In Chicago, high temperatures broke on Monday after a sweltering weekend. It was so hot on Saturday that three different Minnesota Twins players were forced to leave a baseball game against the Chicago Cubs due to heat-related illness, according to NBC Chicago. The players were treated with IV’s to help replace lost fluids on the scorching hot day.

“I just couldn’t catch my breath,” player Bobby Wilson, NBC Chicago reported. “My heart was fluttering. I felt dizzy. Started getting a headache. Couldn’t even hold a conversation in the dugout.”

Water was sprayed onto the Michigan Avenue Bridge to cool it down. When there’s extreme heat, the steel on movable bridges like it expands, Mike Claffey with the Chicago Department of Transportation said. The bridge moves to allow boats to pass.

The potentially record-setting highs prompted the weather service to issue excessive heat advisories and warnings for the rest of the week. Even the nights will offer little relief, with temperatures predicted to stay above 75 degrees in urban areas, the weather service said.

A woman holds a drink on a hot summer day in Central Park, New York, on Sunday. Eduardo Munoz / Reuters

If the high temperatures hold until Friday, it will be the first seven-day heat wave for New York City since 2013, NBC New York reported.

In Philadelphia, officials issued a Heat Health Emergency through Wednesday, and urged the public to check in on older friends, relatives and neighbors.

The high temperatures marked the start of an official heat wave in Boston, but there are still days to go until it beats the city's longest, which lasted nine days in 1912, according to the National Weather Service.

Animal shelters reminded pet owners of the importance of keeping pets cool and hydrated.

Meanwhile, the Ohio Valley was set for wetter weather with heavy rain and thunderstorms forecast, but temperatures were still set to reach triple digits in parts of Ohio, Kentucky and Indiana through Independence Day, according to the National Weather Service.

Residents were warned of the potential for flooding from the storms.