Health officials say only one person remains in hospital following an alarming carbon monoxide leak at a Vancouver office building that affected more than a dozen people.

The leak was discovered after someone collapsed at a property on West 5th Avenue near Fir Street Wednesday morning. It sent 13 people to hospital with carbon monoxide exposure, including two patients who were in critical condition.

Fortunately, their condition has improved since then – officials confirmed Thursday morning only one person remains in treatment at Vancouver General Hospital, and that the patient is in stable condition.

Most of the affected workers were treated with oxygen and released.

Paramedics were first called to the office building around 10 a.m., and quickly realized they were dealing with carbon monoxide thanks to detectors they've been required to wear since 2013.

"Once the alarms went off from the paramedics, they actually evacuated the building and moved to safety," paramedic Jodi Butterman said. "This could have been quite a terrible incident, to be honest."

Carbon monoxide is colourless, odourless and potentially deadly. Officials said the incident should serve as a reminder to everyone to have working carbon monoxide detectors in their home.

"It's really inexpensive and it can save your family," said Jonathan Gormick, public information officer for Vancouver Fire and Rescue Services.

Firefighters couldn't confirm whether there were CO detectors in the office building.

The source of Wednesday's leak hasn't been confirmed, but FortisBC said a technician found a problem with the office building's boiler.

"We did shut off the appliance until it can be repaired by a certified contractors," FortisBC spokesperson Scott Neufeld said.

On Thursday morning, less than 24 hours after the incident, paramedics responded to another carbon monoxide incident at a home north of Kamloops in Barriere.

Few details have been confirmed, but Emergency Health Services said a family of five had to be airlifted to the Lower Mainland for treatment.

"All five will be treated in Vancouver, likely in a hyperbaric chamber," EHS spokesperson Shannon Miller said.

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 Sheila Scott and Allison Hurst