Family members say the seven who were hurt -- including five children-- were in the hospital Monday after being transported from a building in the 2500-block of North Shore Avenue.
All of the victims are members of the same family that owns the building.
"We're going through a very rough time right now," Shabbir Ahmed, whose mother and niece were the two women killed.
The Chicago Fire Department was first called to five-unit apartment building Sunday morning. When emergency crews arrived, they found the two women sick. Paramedics rushed both of them to a hospital where they died.
Crews checked out the scene and conducted tests for carbon monoxide, but did not find anything initially. However, just hours later, the fire department rushed back to the building only to find another person sick.
Paramedics rushed her to the hospital. Then, firefighters did another check for carbon monoxide. That time, they did find low levels around a water tank in the basement of the building.
Chicago police say Rasheeda Akhter, 77, was pronounced dead at around 11 a.m. Sunday and Zanid Ahmed,18, was pronounced dead at Swedish Covenant Hospital.
"A very big loss for the family, my mom and niece...she was a great student, an honor student actually," said Ahmed.
A 74-year-old woman was listed in critical condition at St. Francis Evanston Hospital Monday. Another adult and five children between the ages of 5 and 12 were also in the hospital.
"The 12-year-old was in pretty bad shape. She couldn't hold herself up. The dad asked that I please help and open the door, and I did," said neighbor Gerardo Millan, who also helped put the girl into a car.
Fire officials say crews ventilated the building where the victims were found. ABC7 is told gas and power have been cut from the building.
"What we find out is that carbon dioxide was leaking through the boiler and that caused the problem," Ahmed said.
The Ahmed family has owned the building for about 20 years. They also own a downtown car wash. They say they will not be staying at the building for at least the next couple of days.
Area residents said they were good neighbors and that the tragedy was frightening.
"When I hear things like this it's like, 'Ok, so is our building ok? Is everything up to speed, you know? Who are my neighbors? Are they going to take care of us?'' neighbor Jessica Ochoa said.
Autopsies were scheduled Monday to determine the official cause and manner of death for the victims.