Around 160 million Americans were under either major or extreme heat warnings as the country prepared to mark the 250th anniversary of its independence, according to the National Weather Service.Celebrations on the National Mall -- the vast lawn stretching from Congress to the Washington Monument -- -- were partially postponed in the early afternoon on Friday because of the heat.A young woman apparently suffered from heat exhaustion and was evacuated by paramedics from the Great American State Fair, being held on the Mall."It's like the 30th person," said an event staffer. "They might have to shut this (event) down."Less than 90 minutes later, organizers did just that.In New York City, the heat index -- the apparent temperature when humidity is factored in -- stood at 105F (41C) in mid-afternoon, shy of the 115F that forecasters had predicted. Still, the brutal heat prompted a massive governmental response, with cooling centers opening across the city and public swimming pools extending hours of operation.Across the country, "numerous daily temperature records are expected today and Independence Day, with some consecutive-day, monthly, and all-time records possible," the National Weather Service said.Heated celebrations