PITT MEADOWS, B.C. -- Nine people, including a pregnant woman, are recovering Wednesday after a serious carbon monoxide leak inside an office building in Pitt Meadows.

Some workers in the building began feeling unwell after 3 p.m. Tuesday, reporting headaches, dizziness and nausea. They didn't know what was causing their symptoms, but thankfully someone called for help.

When firefighters entered the building, the carbon monoxide monitors they wear on their gear indicated carbon monoxide levels of more than 300 parts per million (PPM). A level of 100 PPM is considered serious.

"Immediately upon entry our gas detectors indicated we had very high levels of CO inside the building. So at that point the decision was made to evacuate the entire building," said Deputy Chief David Biggin with Pitt Meadows Fire Rescue.

John Hopkirk was working in his accounting office on the top floor of the building but felt no symptoms.

He said he was surprised when a firefighter entered his office.

"A guy in uniform comes in to your office and says, 'Get out, get out now' – you follow instructions and get out."

Hopkirk says once he was outside he was told there was a carbon monoxide leak inside the building.

He ended up spending hours in hospital as a precaution. His levels of exposure were among the highest of those sent to hospital.

"I'm feeling lucky the person on the first floor called it in and they came. Otherwise we would have been in there another two hours," Hopkirk told CTV News.

The building was safely evacuated. BC Emergency Health Services says nine people were taken to hospital.

A pregnant woman was one of them.

"She was complaining of a heavy headache and just nausea," Biggin told CTV News.

Everyone exposed, including the pregnant woman, is expected to be OK.

It's believed Tuesday's leak was caused by a malfunctioning HVAC system.

"The combustion chamber had a crack in it, so it was getting incomplete combustion and it was feeding carbon monoxide back into the building," Biggin said.

It's believed the building did not have carbon monoxide detectors. He is encouraging others to let this close call serve as a reminder to make sure their homes and workplaces are protected.

"We always talk about smoke alarms being mandatory, whereas carbon monoxide detectors we don't emphasize as much, but I think it's equally important," Biggin said. "Carbon monoxide is an odourless product, and you won't know. You won't wake up to a smell."

Carbon monoxide incidents often go up at this time of year when furnaces and other appliances like hot water tanks are turned back on as the weather gets colder.

"If they're not being serviced annually you don't know what they're doing internally," Biggin said.

In B.C., carbon monoxide detectors are not required for all buildings.

Starting Dec. 12, the updated B.C. Building Code requires CO alarms be installed in all commercial buildings where there could be a risk from a fuel-burning appliance or an adjoining storage garage, but this does not apply to older buildings.

"The BC Building Code does not apply retroactively but we always encourage safe practice for all building owners and operators to protect those that live and work in these buildings," a spokesperson with the Ministry of Municipal Affairs and Housing write in an e-mail to CTV News.

While most municipalities follow the province's rules, the City of Vancouver is the only jurisdiction that requires all residences with a fuel-burning appliance and/or an attached garage to install CO detectors, according to Technical Safety BC.

Without a detector, it would be nearly impossible to sense the silent killer.

Last December, more than a dozen people had to be hospitalized after a carbon monoxide leak at a Vancouver office building.

A family of five also had to be airlifted to hospital after a leak in their home in Barriere, B.C.

In 2017, a family of four died after a suspected carbon monoxide leak at their home in the Venables Valley, near Ashcroft.

In January, several people, including children, living in a Vancouver home were hospitalized after a carbon monoxide leak likely started by a hot water heater.

In Pitt Meadows, the building where the leak occurred has been re-opened.

WorkSafe BC was on scene Wednesday speaking with employees and checking the building. It confirmed an investigation is underway to prevent similar incidents from happening in the future.

Workers who specialize in heating equipment were also inside.

Hopkirk was among those who returned to work Wednesday, but he has a message to others after his close call.

"Get a CO detector," he said. "Obviously it is a silent killer. It's not like we smelled gas and ignored it."