WASHINGTON -- The heat that has gripped the Midwest is not only unpleasant. It is deadly.
The weather is suspected of contributing to a number of deaths across the nation.
At least six more fatalities were reported Thursday, including a Michigan restaurant cook who suffered a heart attack after being sent home from his job and a teenage boy who drowned while swimming at summer camp in the same state.
Missouri officials confirmed five heat-related deaths since June. Kansas City authorities were investigating at least 13 others in which heat was suspected.
Emergency room visits were way up, according to public health officials, mainly because of people suffering from heat exhaustion and heat stroke.
A hot air mass that has plagued the Plains for days continued to spread eastward Thursday, roasting residents of the Ohio Valley and the East Coast under a sizzling sun that made people sick, closed schools, and prompted cities to offer cooling centers and free swimming.
Forecasters issued excessive heat warnings for a huge section of the country, from Kansas to Massachusetts.
The temperature approached triple digits in Philadelphia and much of central and western New York. Philadelphia school officials sent students home early and canceled summer school for Friday.
In South Carolina, a heat index is expected to spike Friday at 115.
The Ohio Health Department said emergency room visits have been three or four times higher than usual this week, mainly because of heat exhaustion and heat-stroke cases.
An unrecognized factor in heat-related illnesses may be hot nights.
"Everybody kind of gets fixated on how hot it gets: 'Did we break 100?' " observed Illinois state climatologist Jim Angel. "But the nighttime temperatures can be just as important."
For folks without air conditioning, a nighttime respite from the worst of the heat gives the body a vital chance to recover from the stresses of the day.
But while the current heat wave has recorded 12 all-time daily highs so far this month, it also has registered 98 all-time overnight highs, the National Oceanic and Atmospheric Administration reported.
And that's just all-time highs.
When it comes to a record high for a particular date, 1,279 locations have tied or broken daytime records this month, while 3,128 night time highs have been tied or broken.
For example, on Wednesday Eppley Airfield in Omaha had an overnight low of 82 degrees. That was a full 5 degrees warmer than the previous warmest overnight on that date, set in 2002.
Litchfield, Minn., also posted an overnight low of 82, besting a warm nighttime record for that date of 74 degrees set in 1964, and the overnight low of 82 at Lambert Field in St. Louis edged out the 1998 mark by 1 degree.
When temperatures overnight do not cool to levels that provide relief, it increases stress on people without air conditioning, on livestock, and on crops, said Deke Arndt, chief of the Climate Modeling Branch at NOAA's National Climatic Data Center.
In general, both daytime and nighttime temperature increases "are consistent with what we would expect in a greenhouse-warmed world," he added.
High overnight readings also increase energy consumption as air conditioning units run deeper into the night and start earlier in the morning, he said.
This problem was a major factor in the 1995 heat wave that struck Chicago, claiming more than 700 lives, Mr. Angel said.
There were reports of elderly people who died in apartments because they couldn't afford to run their air conditioning or were afraid to open windows because of the fear of crime, he noted.
And unlike people, livestock and crops have little recourse from the heat. July is a sensitive time for corn, Mr. Angel said, possibly reducing its yield because of the heat.
The long-range U.S. forecast for August calls for warmer than normal conditions across the southern tier of states from Arizona to North Carolina, with the hottest conditions concentrated in Texas and Louisiana.