Tens of millions of Americans sweltered under furnace-like temperatures Tuesday as central and eastern cities hunkered down for a heat wave set to last through the July 4 holiday weekend. Dozens of local temperature records could be broken, the National Weather Service (NWS) warned, with temperatures in many places to surpass 38 degrees Celsius and high humidity pushing the heat index as high as 115 degrees. It comes as America prepares to celebrate its 250th anniversary of independence, and as the United States along with Canada and Mexico co-host the World Cup soccer tournament amid the extreme heat.