The National Weather Service has issued a heat advisory for Lancaster County today and announced an excessive heat watch through Thursday.

A heat advisory means a period of high temperatures and humidity is expected, during which heat-related illnesses are possible, according to the weather service's Web site.

An excessive heat watch means a prolonged period of dangerously hot weather is expected, according to the Web site.

High temperatures for today, Wednesday and Thursday are expected to be at or near the 100-degree mark, the weather services reported Monday.

The heat index -- which is a combination of heat and humidity -- for all three days is projected to range from 110 to 115 degrees.

By Friday, daytime highs are expected to drop back to the mid-80s.

Due to the predicted hot temperatures and high humidity, the state Department of Environmental Protection announced Monday that today will be an "air quality action day."

The declaration was made because today is expected to be a "code orange" day for ground-level ozone.

The U.S. Environmental Protection Agency uses colors to report daily air quality. Green signifies good, yellow means moderate, orange represents unhealthy pollution levels for sensitive people and red warns of unhealthy pollution levels for all.

Air quality action days are declared at orange and red when fine particulate matter reaches unhealthy levels.

Ozone in the upper atmosphere protects the skin by shielding it from harmful ultraviolet rays. But ground-level ozone is a key component of smog.

It forms during warm weather when pollution from vehicles, industry, households and power plants "bakes" in the hot sun, making it hard for some people to breathe.

Young children, the elderly, people with asthma or other lung ailments and those who work or exercise regularly outdoors are most susceptible to the harmful effects of ground-level ozone, which sears lung tissue.

The particulates come from a wide range of sources -- power plants, industry, cars, trucks, buses, wood stoves and forest fires.

Some particles are released when fuels are burned. Others form in the atmosphere from reactions between gases released from power plants and factories.

As a result of the heat warnings, the Lancaster County Office of Aging announced Monday that hours will be extended at several of its senior centers around the county today, Wednesday and Thursday so that they can serve as cooling centers.

Jacqueline Burch, executive director of the office, said senior citizens who do not have air conditioning in their homes are urged to go to the cooling centers for respite.

"This is a typical thing for us during the summer," Burch said. "We want to provide a place for our consumers to stay cool."

According to Burch, the Cocalico and Columbia centers will remain open until 3 p.m.; the Elizabethtown and Solanco centers will remain open until 4 p.m.; the SACA and Lancaster Neighborhood centers will remain open until 5 p.m.; the Lititz center will remain open until 2:30 p.m.; and the Lancaster Recreation Commission center will remain open until 7 p.m.

Most of the senior centers usually close around noon, Burch said.

Lancaster County Emergency Management Agency officials said the county's public safety training center on Champ Boulevard in East Hempfield Township will be opened as a cooling center in the event part of the county loses power over the next three days.

This is the second time this summer that the office of aging has kept its senior centers open longer due to excessive heat.

Burch said that's part of the office's heat emergency plan, which is enacted any time the heat index is expected to be above 90 degrees for several days.

The last time the office put the plan into action was in early July, Burch said.

Besides keeping the senior centers open, Burch said the plan also calls for, among other actions, case managers in her office to make sure seniors considered to be at risk due to their health are regularly checked on. Also, portable fans are distributed temporarily with case-manager approval to at-risk seniors who can't get to air-conditioned facilities.

The county's emergency management agency reported no problems due to the excessive heat Monday.

John Lines, spokesman for Lancaster General Hospital, said two people went to the hospital's emergency department Monday complaining of heat-related problems.