Paramedics in Vancouver say two people are in critical condition after a carbon monoxide leak at a building in the city's South Granville neighbourhood Wednesday morning.

At least 11 others were treated for poisoning and taken to hospital. Their conditions remain unclear.

Emergency Health Services said in a tweet that paramedics responded to reports of a patient who had collapsed in the 1600 block of West 5th Avenue near Fir Street at what appeared to be a commercial space.

According to paramedic Jodi Butterman, the carbon monoxide detectors they have been required to wear since 2013 immediately alerted the first responders to the leak. Crews then evacuated the building as quickly as possible.

FortisBC said the company was first called to the building at around 10 a.m. and that a technician found a problem with the boiler. But officials have not confirmed where the leak originated.

Fortis said the incident serves as a reminder of how important it is to take precautions.

"It's important to know that carbon monoxide exposure is preventable," spokesperson Scott Neufeld told CTV News. "Having your natural gas appliances regularly inspected and maintained by a licensed natural gas contractor can help prevent carbon monoxide exposure."

Butterman also said it's "critical" to have functioning carbon monoxide monitors in every home.

Carbon monoxide is odourless and colourless and is produced when fuel is burned, including in vehicles, stoves, fireplaces, barbecues, furnaces and portable generators.

Symptoms of mild carbon monoxide poisoning include headaches, nausea and dizziness. If a patient is experiencing breathlessness, hallucination, convulsions or unconsciousness, their life could be at risk.

The province recommends consumers carefully read the instructions for all fuel-burning appliances and never use barbecues, portable stoves or similar devices indoors.

With files from CTV Vancouver's Allison Hurst and The Canadian Press