WATCH: The container fire at the Port of Metro Vancouver is now out and a protective barrier has been placed around it. John Hua has more on what we know about the fire tonight and what happens next.

A chemical fire at Vancouver’s port was declared fully extinguished Thursday night after burning for more than 24 hours, allowing investigators to start their work to determine what ignited it.

The city’s fire department issued a news release late Thursday night announcing the emergency was over.

“Vancouver Fire and Rescue Services has given the all clear designation and will release fire crews when fire protection services are no longer needed,” said the statement.

The fire prompted a Hazmat response, the evacuation of hundreds of people and forced the closure of the port.

Trichloroisocyanuric acid (a bleaching agent and industrial disinfectant) was burning for more than an hour. The fire started in one shipping container, quickly spreading to three containers. Fire crews had a difficult time reaching the container because it was buried in the middle of a pile of containers – six deep on each side, and two high from the ground. Its exact cause is not known.

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“We don’t know how it started,” said Chief Dan Wood of the Vancouver Fire Department.

“It could have been contamination, it could have been a leak in the container from water, or maybe an oily floor board. We’re not sure, but we’ll investigate.”

The fire was contained and the overall damage was relatively minimal. But at least three people were checked into hospital with respiratory concerns, and the effects of hundreds of thousands of people being exposed to the acid remain to be seen.

As of Thursday morning, the fire is about a third of its original size.

Vancouver Fire & Rescue are on site focussing on monitoring air quality in the area for acceptable levels.

Thirty firefighters and three aerial ladder trucks are on scene.

WATCH: Global News 1 captures the Port of Vancouver container that was the source of a massive response from Vancouver firefighters and area evacuations.

DP World, the company who owns the material in the container, said on Thursday they have brought an occupational hygienist to the site to assess and assist with the necessary control measures to minimise the impact of the fire. A specialist environmental contractor has also been engaged to ensure minimal impact on the surrounding area and marine habitat.

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The fire started just before 1:30 p.m. Wednesday and was quickly upgraded to a four-alarm blaze. The call was put out for 21 firefighters to staff multiple fire trucks. At least eight fire trucks and more than 24 firefighters had already responded to the incident.

“They evacuated all the mechanics out of the maintenance yard,” says one witness, Vince De Benedetto. “It’s been getting worse by the second.”

Credit: @JakNew . Credit: @JakNew . Credit: Tower Camera. Credit: @coriennahagen . Credit: @CHELSinVan. Credit: @coriennahagen . Story continues below advertisement Credit: @JakNew. Clayton Little. Clayton Little. Clayton Little. One of our cameramen on the scene of the fire.

WATCH: Aerial footage of the fire:

Two fireboats were dispatched to stage at the north end of Centennial Pier. By 6:30 p.m., Port Metro Vancouver said the fire was considered under control, as crews worked diligently to contain a blaze that could be seen throughout downtown and could be smelled for many miles to the east.

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“In a vapour [Trichloroisocyanuric acid] can be very dangerous,” said Vancouver City Manager Penny Ballem.

“It can be explosive, that has not happened. We’re very gratified about that.”

Stay inside, shelter in place – now lifted

Everyone east of Main, north of 1st Ave and west of Nanaimo, were told to close their windows, stay inside and shelter in place. Vancouver Police asked people that if they had to go out to leave a note on the door to let people know they self-evacuated. People were also advised to cover their mouth and nose with a wet cloth for safety.

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Dr. Patricia Daly from Vancouver Coastal Health says there was not a need to evacuate the public. “If they can remain in place at the moment, doors and windows closed, that’s the best advice,” she says.

To shelter in place:

Go indoors immediately and stay there.

Close all windows and doors.

Turn off the furnace, air conditioners and exhaust systems.

Listen to the radio or TV for more information.

If you’re in the Downtown Eastside & don’t have housing, find a safe place right now at Carnegie Ctr + Evelyn Saller Ctr. #PortMetroFire — City of Vancouver (@CityofVancouver) March 5, 2015

In the zone affected by the fire at the port? @VPL‘s Carnegie, Britannia and Hastings branches are open if you need a place to shelter. — Vancouver Public Lib (@VPL) March 5, 2015

While the smoke is lessening, people are still urged to stay away from the area at 700 Centennial St.

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#PortMetroFire ALL CLEAR: Near the fire and experiencing shortness of breath or difficulty breathing? Please see a physician ASAP. — City of Vancouver (@CityofVancouver) March 5, 2015

Transit affected

The Vancouver Fire Department closed off a portion of the downtown, impacting bus operations as well as cancelling all West Coast Express service departing Waterfront.

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All bus operations slowly resumed service in the hours after receiving the all clear from the City of Vancouver’s Emergency Operations Centre.

All West Coast Express travellers should find alternate transportation or travel via the Millennium or Expo SkyTrain lines from Waterfront Station to Lougheed Station, where a bus bridge will take them to the various WCE stations all the way to Mission. Buses will be clearly marked as either “Special” or “West Coast Express.”

The West Coast Express was the only form of transit disrupted for the entire evening, as debris from the firefighting efforts was still on the track. TransLink believes morning service will not be disrupted.

Passengers are encouraged to use Translink’s online trip planner to get specific bus route information to their destination http://tripplanning.translink.ca/ or contact the customer service desk at 604.953.3333 for detailed trip planning advice.

WATCH: David Rogers from BCHAZMAT tells Global News about the safety protocols in place

What is trichloroisocyanuric acid?

Trichloroisocyanuric acid is a bleaching agent and industrial disinfectant. It is also a possible eye and skin irritant. Karen Bartlett from the UBC School of Population and Public Health says when the substance burns it is going to release a chlorine gas.

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READ MORE: Here’s what you need to know about trichloroisocyanuric acid

Her concern is for residents of the Downtown Eastside, who do not have somewhere to go find shelter. They are advised to go to a mall or somewhere similar to seek protection.

“There’s the possibility there will be inflammatory response and the lungs don’t like that irritancy and they produce fluid,” says Bartlett.

“It’s an acute, but not chronic problem.”

Residents in other areas of the Lower Mainland also reported seeing and smelling the smoke. The whole stretch of Lougheed Highway, between Boundary Road and Commercial Drive, was very hazy while the fire was burning. The smoke seems to have come up and over the hump and was sitting in the Grandview Highway Valley. Everyone, including drivers, were advised to keep their windows closed. Meteorologist Kristi Gordon says the winds are not expected to change much at this time, and although the smoke has drifted far, there is not much dispersion.

– With files from The Canadian Press