Record-breaking heat wave threatens millions across U.S.

More than 24 million people from the Northern Plains to New England to parts of Texas are under a heat advisory, with at least six states setting record highs on Monday. 

Boston is under a heat emergency as temperatures rose above 90 degrees for the third consecutive day. Many flocked to cooling centers and neighboring parks. Tuesday is expected to be another scorcher in the city, with temperatures expected to be above 90 degrees for a fourth straight day.  "In the last couple of days we've had 18 calls that we've flagged as directly heat-related. But on top of that we've seen a 31% increase in call volume for the last 48 hours and today's on track to do that as well," said Jim Hooley, the chief of Boston Emergency Medical Services.