(CNN) -- Take me out to the ball game? For some dealing with this relentless heat wave, that idea could make you think twice.
The heat wave that has taken hold of much of the upper Midwest over the past few days is taking its toll on just about everyone -- including those who may be used to working up a sweat outdoors.
Case in point: Monday's Philadelphia Phillies vs. Chicago Cubs baseball game. Phillies pitcher Roy Halladay was forced to leave the game early as temperatures at Chicago's Wrigley Field soared into the 90s. The heat index during game time was far more than 100 degrees.
Halladay, his face beet-red and his off-white jersey soaked, left the game in the fifth inning.
"You could tell the heat was getting to him a little bit," said Phillies' pitching coach Rich Dubee, in an article on the team's website. "I talked to him after the fourth, and he said he was somewhat lightheaded, but he wanted to go back out there. Of course, he went out there in the fifth and just had a tough time staying focused and seeing the signs."
The team said Halladay felt better after the game.
Chicago isn't alone. The "dangerous" heat wave baking the central United States is expected to extend to the East Coast by the end of this week, the National Weather Service said Tuesday.
The weather service on Tuesday declared "excessive-heat" warnings in 13 states -- Illinois, Indiana, Iowa, Kansas, Kentucky, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Oklahoma, South Dakota and Wisconsin -- through Friday.
Parts of six other states -- Alabama, Arkansas, Mississippi, Ohio, Tennessee and Texas -- are under heat advisories through at least Wednesday, the weather service said.
Cities already are under heat watches for the rest of the week include Grand Rapids, Michigan; Memphis, Tennessee; Taunton, Massachusetts; Wilmington, Ohio; Detroit and Pontiac, Michigan; State College, Pennsylvania; New York City; Baltimore and Washington.
"Heat-index values" -- how hot it feels outside -- have been running more than 125 degrees in the worst-hit areas. The scale designed to describe how intense the heat feels takes relative humidity into account along with temperature.
Two factors contribute to making this current heat wave especially dangerous: the lack of a significant drop in temperatures overnight to allow people's bodies to cool and relatively high humidity, which makes the air feel appreciably hotter than the thermometer may indicate, said Jacob Beitlich, a Des Moines, Iowa-based meteorologist for the National Weather Service.
In Iowa, for instance, he noted that the impact of mid-90s temperatures has been compounded by dew points -- or saturation temperatures -- in the upper 70s and low 80s. These combine to make the heat index spike so that it feels as hot as 126 degrees, the weather service said.
"That takes a toll on your body," Beitlich said. "When it's more humid, it's more difficult to cool down from sweating."
The National Weather Service notes that extreme heat typically is the biggest weather-related killer in the United States, taking about 115 lives each year. That's why the weather service and other government agencies urge people to minimize their time outdoors during periods of extreme heat, to drink plenty of fluids, and to keep especially close tabs on the elderly and young people.
CNN's Ted Rowlands and Meteorologist Jacqui Jeras contributed to this report.