Three firefighters were taken to the hospital with what appeared to be heat exhaustion after a 2-alarm fire it appears was started by careless disposal of cigarettes struck an apartment building at Nesmith and East Merrimack streets.

Crews were called to 356 East Merrimack St., about 6 p.m., for a report of a fire outside on a second-floor porch in the rear of the large yellow home that contains seven apartments.

The blaze quickly spread up the side of the porch and into the eaves and then the attic, according to Deputy Fire Chief Michael Cushing. He said it appears careless disposal of cigarettes was to blame.

On a day when the temperature reached nearly 99 degrees in Lowell, according to the UMass-Lowell Weather Center, Cushing struck a second alarm to get more firefighters to the scene.

Resident Charlotte Ciaraldi, who is battling cancer, said she was talking to her twin sister Suzanne Robertson when the building's fire alarm went off.

The women grabbed their purses and cell phones and fled the building.

"I said grab your purse, and then we got out because that's what you're trained to do," Ciaraldi said. "It's sad."

Ciaraldi said firefighters later let her back inside the building to retrieve some of her medical records.

Cushing said he feared the blaze could spread a lot further and credited firefighters for working through the heat.

"I thought we were going to lose it there for a while, but they keptfighting and kept fighting," he said. "They did outstanding."

Cushing was initially going to cut power to the entire building, which would have left all residents temporarily homeless, but later decided all but two second-floor units could be reoccupied.

It was not immediately clear how many people lived in those units. The Merrimack Valley Chapter of the American Red Cross was helping them last night.

Three firefighters rested on stretchers and took oxygen provided by EMT's before they were taken to city hospitals to be checked out following their efforts to battle the blaze.

Tara Robertson, who's boyfriend Mike Saber owns the building with his father Ed Saber, said she was watching television on the first floor when she hear someone yelling "fire."

She said someone -- she was not sure who -- rang everyone's doorbell a short time later.

Mike Saber declined to comment at the scene.

Police closed Nesmith Street from Andover Street to Stackpole Street, and East Merrimack Street from Willow Street to Park Street while crews battled the blaze.