Ontario's safety watchdog is investigating after four Ottawa housing co-op residents were hospitalized with carbon monoxide poisoning Sunday.

Ottawa fire officials said they were called to 140 Mann Ave. around 12:10 a.m. and detected high levels of the colourless, odourless gas.

The building was evacuated and an OC Transpo bus brought in for shelter until more than 100 residents were allowed back inside around 3:30 a.m.

Ottawa paramedics said they checked seven people and took four of them to hospital after carbon monoxide was found in their blood.

Tests by fire crews showed carbon monoxide was found in four units across four floors, one above the other.

Family had been feeling sick

One of those units belongs to Yordanos Gebre, who moved to Canada from Ethiopia in June and has been living there for two months with her husband and child.

Yordanos Gebre Carbon Monoxide

Yordanos Gebre, her husband and two-year-old son were taken to hospital for treatment. (CBC)

She said she had been feeling sick with flu-like symptoms for two days.

”We called Enbridge, said we heard some unusual noise from the furnace … they said it’s not an emergency and told us to call our contractor,” she said.

“They (also) said it’s not an emergency and to book an appointment to fix something for Thursday.”

Jessica Dizazzo, president of the housing co-op board of directors, said they’ve scheduled an emergency meeting Monday to talk about installing carbon monoxide detectors.

Carbon monoxide detectors are only mandatory in new buildings in Ontario, according to legislation passed in November.

“There are going to be changes to the fire code in the province of Ontario starting in 2014 that will look at requiring mandatory carbon monoxide detectors in all residential units," said Sean Tracey, Ottawa’s assistant deputy fire chief.

Ontario’s Technical Standards and Safety Authority is investigating and could lay charges if it finds any wrongdoing.