Electricity usage hit record highs, municipalities tried to protect their elderly residents and tourists sought out air-conditioned attractions as a heat wave that has baked the mid-Atlantic region extended to a fourth day Thursday.

The region was eagerly anticipating a break, however, with temperatures on Friday expected to top out in the low 90s.

"It's hard to call that refreshing, but it is going to be nice having a little cooldown," said James Brotherton, a meteorologist with the National Weather Service in Sterling, Va.

High temperatures Thursday reached a record 101 degrees at Reagan National Airport, a record-tying 100 at Baltimore-Washington International Thurgood Marshall Airport and a record-tying 99 at Dulles International Airport. At the Maryland Science Center in downtown Baltimore, the temperature topped out at 102, according to the National Weather Service.

In Maryland, there have been four heat-related deaths recorded this week, said John Hammond, a spokesman for the state Department of Health and Mental Hygiene. All four victims were elderly, including a 90-year-old man. Three lived in Baltimore city, and the fourth lived in Carroll County.

Heat-related deaths statewide in 2006 totaled 19, which is down from the same point last year, Hammond said. The worst year in recent history was 2002, when 50 deaths were recorded.

In Baltimore city, there have been six deaths this year, compared with 16 at this time last year, said Dr. Joshua Sharfstein, the city's health commissioner, who credited outreach programs that provide treatment for the city's vulnerable senior citizens and an aggressive public information campaign.

"We've seen basically a twofold increase in the number of 911 calls for heat-related conditions," Sharfstein said.

The District of Columbia's Office of the Chief Medical Examiner said Thursday evening that there was one death attributed to the heat, apparently the first such death in Washington this year.

Regional utilities BGE, which serves central Maryland; Pepco, which serves Washington and its suburbs; and Delmarva Power, which serves Delaware and the Eastern Shore, all reported record or near-record hourly electricity usage on Tuesday, Wednesday and Thursday.

Mary-Beth Hutchinson, a Pepco spokeswoman, said Friday's expected drop into the low 90s would be "all the difference" to the strained power supply.

"The difference between 92 and 98, even in terms of walking around, it's great, and in terms of the electric system, it's substantial," Hutchinson said.

The Eastern Shore town of Centreville had a sweatier day than most after a downed power line caused about 1,750 Delmarva Power customers to lose power for about four hours Thursday afternoon. The outage was likely the result of heat, said Matt Likovich, a spokesman for Delmarva Power.

"Lines tend to sag during the hot weather," Likovich said. "That's our suspicion at this point."

Centreville town manager Bob McGrory said generators were used to maintain the air conditioning at the county jail and at a nursing home, but that the town took the heat wave and the blackout in stride.

"We had National Night Out on Tuesday night on what was one of the hottest days of the year," McGrory said. "We had a good turnout, and the dunking booth was very popular."

Despite the strain on the power grid, there were no rolling blackouts or brownouts. PJM Interconnection, which manages power distribution for 51 million people in Maryland, Delaware, Virginia, the District of Columbia and 10 other states, was able to avoid voltage reductions, in part because some businesses participated in market-based demand response, in which they can be paid the market rate for electricity by reducing their usage or shutting down entirely during peak demand hours, said Ray Dotter, a PJM spokesman.

"They have a double benefit," Dotter said. "They're not paying for the electricity they didn't use, and they're being paid not to use it."

Two heat-related deaths had been reported in Delaware as of Thursday afternoon, said Jay Lynch, a spokesman for the state Department of Health and Human Services. Cooling stations were open in all three counties, public libraries extended their hours and some swimming pools waived fees or entrance requirements.

"Barriers have been removed to getting to places that would keep you cool," Lynch said.

The heat contributed to a spike in emergency room visits at some hospitals. Dehydration related to exertion can aggravate chronic medical conditions, including diabetes, high blood pressure, kidney disease, congestive heart failure and sickle cell anemia.

"Whenever there's an extreme heat wave, you see more of these cases," said Dr. Gary Little, chief of emergency services at Prince George's Hospital Center in Cheverly. "We're seeing three or four such cases a day instead of one or two a week."

In Montgomery County, a butterfly exhibit at the Brookside Gardens Conservatory was closed for a third consecutive day because the glass-walled building was too hot for people.

"The butterflies tolerate the heat pretty well. It's the staff and visitors who have problems," said Marion Joyce, a spokeswoman for the Maryland-National Capital Park and Planning Commission.

Parents were warned not to take small children to unshaded playgrounds because some equipment can get hot enough to cause first-degree burns. Joyce recommended that children participate in a winter sport by visiting one of the two indoor ice rinks operated by the county.

"This is a good time to go ice skating," she said.