It is widely viewed - more widely, of course, when the temperature hits 99 degrees - as one of the greatest inventions of the 20th century.
Without air conditioning, Houston would still be a cow town, glass skyscrapers wouldn't grace city skylines, and people like Marlene Kells and family would have no refuge from the oppressive heat and humidity that blanketed Baltimore this week.
"We take turns going in that room," Kells said as she sat outside her rowhouse on South Hanover Street, drinking Pepsi on ice and pointing up to the droning window unit in her second-floor bedroom. Every minute or so, it sent a drop of water splashing onto her head.
"Feels good," she said.
Whether you view it as a luxury or a necessity, air conditioning has yet to become something everybody can take for granted.
About one in five families in Baltimore, like Kells', get by with a single window air-conditioning unit, according to the U.S. Census.
Only about one of every three homes in the city has central air conditioning, compared with three of every four in Baltimore County, according to 1998 census data, the last year for which air-conditioning information is available.
And one in five residences in the city - like Linda McCrae's in Pigtown - has no air conditioning at all.
"No, I don't have it. That's why I'm sitting here all miserable on the steps with this jug of water, and with another jug as a backup, and I ain't doing no cooking today," McCrae said at noon yesterday.
In 2 1/2 hours, she noted, her shaded stoop would be under full sun. When that happens - and the $250-a-month, two-bedroom house she and her 8-year-old son live in gets too hot - she often seeks shade by crawling under a 3-foot-high, black metal stairway across the street.
"The only time it's cool is the nighttime," said McCrae, 39, who works during the school year at an elementary school cafeteria.
Last summer, she tried putting in a window unit given to her by a friend. But when she plugged it in, it overloaded her home's circuits and all her power went out.
"It blew everything," she said. "So I took it and pushed it out the window."
Although McCrae shoved it out of hers, air conditioning is in more American homes than ever.
Nationally, between 1978 and 1998, the percentage of homes with some form of air conditioning grew from 56 to 72. In 1978, about one in every five homes in the country had central air conditioning. Now nearly half do.
This week - as is the case with heat waves - the number of air-conditioned residences climbed another notch, with most installers and retailers reporting a busy few days.
"Completely sold out," said a manager at the Home Depot on Eastern Avenue.
"We've had a lot of phone calls," said Johanna Bloom, appliance supervisor at the Best Buy on Belmont Avenue. "I'm anticipating a lot more people coming in."
If anything is able to slow the growth of air conditioning, an industry that took off during the Depression, it might be spiraling electricity rates. Many who go without air conditioning say it's not the price of a window unit, but the effect it would have on their already high electric bills that keeps them from joining the cooled-off ranks.
"It's outrageous," said James Bradds, 34, sitting shirtless on the front steps of his home in Pigtown. It is un-air-conditioned and will stay that way, he said.
"Electric prices are way too high. I was paying $200 a month where I used to live, and that was with no air conditioning."
"I love air conditioning," he added. "But if you can't afford it, you can't afford it."
To cope, the un-air-conditioned run fans. They leave their windows open "even when it rains," like McCrae, or keep their houses closed and the windows covered, like Bradds. They complain. They sweat. They take cold showers. They persevere.
"You get sweaty and then you take a rag and you wipe yourself down," said Elizabeth Adams, as she stood outside in the 200 block of E. Barney St.
In Adams' home, like Kells' - and the 20 percent of others who get by on one window unit - one room serves as an oasis; the rest feel like a desert.
More homes - about one of every four in the city - are cooled by two or more window units, according to the census figures.
A Baltimore Gas and Electric Co. spokesman said that, although energy consumption was setting records during the heat wave, the utility had no information on how many of the 1.1 million households and businesses it serves use air conditioning. Nor could it provide a breakdown on how much of the electricity used in Baltimore goes to powering air conditioners.
According to the U.S. Department of Energy, the average American household spends slightly more to air condition its home (12 percent of total consumption) than to heat it (11 percent).
Air conditioning arrived in Baltimore in the 1930s - first in department stores such as Hutzler Bros., then at hotels, movie houses and restaurants. The Baltimore & Ohio Railroad installed the first air-conditioning system for trains in 1931.
