As temperatures ticked toward triple digits and broke at least one century-old record, Baltimore City Health Department officials declared a Code Red extreme heat alert through Thursday.

“Excessive heat is the leading weather-related killer in the United States,” Acting Health Commissioner Mary Beth Haller said in a statement Monday announcing the alert. “The fact that we’re located in an urban area makes things worse because of population density and the presence of heat absorbent asphalt.”

Haller warned that extreme heat can be especially dangerous to older and younger residents and those with chronic medical conditions. In the statement, she urged residents to “protect yourselves and check in on your family, neighbors, and pets.”

The Labor Day holiday, a day off for many across the state, kicked off a seven-day stretch of predicted hazardous heat and humid weather conditions in Central Maryland, Northern Virginia and the District of Columbia, according to the National Weather Service — and the steady chorus of chirping cicadas.

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The high pressure system delivering the humidity extended from the mid-Atlantic to the Great Lakes.

Temperatures were expected to rise to near 100 degrees Monday in Baltimore. The heat index, or how hot it will feel to the human body, may reach the low 100s.

Weather Service forecasters anticipate highs in the mid-90s to 100 through Monday of next week.

On Sunday, the temperature in Baltimore hit 98 degrees, breaking a record for Sept. 3 set in 1898, when the thermometer peaked at 97, according to historical weather service records.

Should Baltimore temperatures surpass 101, the city will break its own record set in 1930 for the highest temperature recorded on Sept. 4, and also the hottest September day ever recorded, which was in 1881.

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The last heat alert declared by city officials occurred on July 26 for hot weather anticipated over July 27-29, when temps reached 97 degrees.

To keep cool and safe during extreme weather events, the Centers for Disease Control and Prevention lists several tips on its website, including to stay indoors in air-conditioned spaces, reduce or avoid exercising outdoors and stay hydrated.

City residents can call 311 for cooling center information.