BALTIMORE (WJZ) -- Baltimore's health department has extended a previously-issued Code Red Extreme Heat Alert through Monday.
Maryland is under a dome of heat that is expected to last through the weekend, and health officials are urging caution.
"I urge everyone to take this heat wave seriously, and if you don't have to go outside, stay inside," said Baltimore City Mayor Jack Young.
The alert was originally set to expire Sunday.
Four people have died due to extreme heat so far this year in Maryland, and last week alone doctors treated almost 400 heat-related complaints at emergency rooms—most of those in Baltimore.
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Baltimore Fire Department officials said teams would provide temperature checks at nursing homes.
Public pools are also open extended hours.
Baltimore's health commissioner spoke Thursday about the city being an "urban heat island" with higher temperatures because of buildings emitting extra heat and asphalt absorbing it.
"Heat is one of the leading weather-related killers in the United States," said Dr. Letitia Dzirasa. "Heat waves are becoming more common, lasting longer and becoming more severe throughout the U.S."
Dwayne Anderson pushed his wife Debbie's wheelchair in the heat outside City Hall and told WJZ Investigator Mike Hellgren he was concerned about the heat wave.
"She has bad diabetes. I get out of breath pushing her and she gets out of breath just being out here," he said. "We're just going to take it easy."
Authorities are concerned about vulnerable people: the elderly, the very young and those without homes. In Baltimore, workers are doing welfare checks on the homeless Friday starting at 6 a.m.
Extreme heat is in the forecast for the rest of the week and into the weekend, with temperatures expected to climb into triple digits and heat indexes, a measure of how warm it feels outside, reaching up to 110 degrees.
The National Weather Service has issued a heat advisory for much of Maryland from 11 a.m. to 9 p.m. Friday and excessive heat watches from 11 a.m. to 9 p.m. Saturday and Sunday.
Chris Wise works as a waterfront ambassador in downtown Baltimore. He said that it's critical to take regular breaks from the sun.
"Basically, it's always important to step in once in a while, every 30 minutes, step inside for 10, 13 minutes," he said.
Penny Johnson suffers from asthma and relies on a cool apartment for her health.
"I can't deal with this heat, I just can't," Johnson said. "It's too much for me."
Experts say that if you have any heat-related issues, don't hesitate to call.
Cooling centers will be open in a number of communities through the weekend.
People are reminded to drink plenty of water, check up on the elderly and those without air conditioning and never leave children or pets in vehicles.
To find a list of cooling centers, click here.
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