Don't let anyone kid you. It's the heat AND the humidity.
The two factors have combined to make weather conditions in Western Pennsylvania miserable to the point of being dangerous.
And it's even worse over the center of the nation where trapped heat and moisture have created a steam-bath effect that is likely to last through the weekend.
"This particular heat wave will be remembered years from now because of the higher amounts of humidity," said Carl Erickson, a meteorologist with AccuWeather.com in State College. "There haven't been that many record highs. The problem is the humidity."
Wednesday's high for Pittsburgh, according to the National Weather Service, was 91 degrees, the region's third day of 90-plus temperatures this month. And the forecast calls for similar scorchers today through Sunday.
Western Pennsylvania averages 3.6 days in the 90s in July.
"But as hot and as miserable as it's going to be, it's possible we may not even break a record with this particular heat wave," Mr. Erickson said.
AccuWeather.com has what it calls RealFeel temperatures, which take into account sunshine, winds and other factors as well as heat and humidity. Like wind-chill factors in the winter, they translate into what people are experiencing rather than what the thermometer says.
The National Weather Service, which uses the phrase "heat index," puts the feeling today at 107 degrees, followed by two more days of 100-plus.
"With the humidity, it's going to feel closer to 100 and 105," Mr. Erickson said. "That's the trend for the coming days, and there's no real relief in sight. It'll still be in the upper 80s next week."
The Department of Health on Wednesday issued a cautionary note urging Pennsylvanians -- particularly older and at-risk residents -- to take steps to protect their health.
"The humidity makes the heat worse because people don't perspire as much, therefore their internal temperature increases," said Dr. Glenda Cardillo, a public health physician with the department. "Usually, the body will regulate its temperature in accordance with variables. But when you're not sweating, you run the risk of heat exhaustion and heatstroke."
And the Department of Environmental Protection announced "air quality action days" for most of the state, urging people with respiratory problems and those vulnerable to the effects of air pollution to take precautions and limit outdoor activities.
Health officials' recommendations may be familiar but that doesn't make them any less wise. Stay hydrated with plenty of water and try to stay cool. Wear light, loose-fitting clothing and never leave children and pets in cars. Check on the elderly and high-risk individuals.
Limit your time outdoors and consider exercising indoors or at a gym. Activities that you could handle on normal days might be too much during a heat wave.
Blistering summer heat is an underappreciated killer, claiming by some estimates as many as 1,000 U.S. lives each year -- more than any other type of weather.
One federal study found 40 percent of heat-related deaths were in people 65 and older. Those numbers could be lower if more heeded heat warnings aimed at seniors. Yet research has shown many people over 65 don't think the warnings apply to them -- because they don't think they're "old."
A number of Allegheny County senior community centers are taking steps to accommodate people who might need shelter. All county residents 60 and older are invited to beat the heat, socializing and engaging in activities as well as refreshments. You do not have to live in the municipality or neighborhood of a center to attend.
For a list of Allegheny County senior centers -- some of which will be operating with extended hours -- visit: alleghenycounty.us/dhs/seniorcenters.aspx.Municipalities also are prepared to help. The East McKeesport Community Senior Center on Chicora Street, for example, can accommodate 175 people and has a plan if more turn up.
"We take care of disasters like flooding and extended power outages, but in the summer we're also aware of the need for elderly and children to have an escape from the heat if their homes aren't air-conditioned," said Rose Mary Badstibner, spokeswoman for emergency management in East McKeesport. "Heat can be very dangerous."
Allegheny Valley Hospital will be hosting two cooling centers, in Natrona Heights at 1301 Carlisle St., and in New Kensington, at 651 Fourth Ave.
The culprit for these conditions is a high-pressure system in the upper atmosphere over the Plains states, causing the hot air below it to sink and compress, said Eli Jacks, a meteorologist with the National Weather Service in Silver Spring, Md.
"We're locked in this pattern, and it's expanding into the East," Mr. Erickson said.
Meanwhile, the northwest corner of the country is experiencing cool, cloudy days, where the temperatures in Seattle are stuck in the 60s.
Associated Press contributed.