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Toronto police are cautioning residents against walking across a partially-frozen harbour after Global News cameras caught multiple people making the trek between the islands and downtown, and at least one person was out for a skate.

Const. Ricardo Gomez, who has been with the police service’s marine unit for more than six years, has been involved in an ice training program on the city’s inner-harbour over the past week and said officers have seen unstable conditions.

“There are times when we’re out on the ice geared up and we’re going through the ice unexpectedly, so I can’t imagine members of the public that are not suited up in dry suits and life jackets without the training to be out there — it’s extremely dangerous,” Gomez told Global News on Tuesday, noting tracks of broken ice are regularly being made to allow boats to pass.

READ MORE: ‘Ice toppers’ are risking safety for the perfect photo

“You’ll walk across [the tracks] not knowing the [ice breakers] actually broke through the ice a few hours earlier and the moment you step on it, you’ll actually crash straight through.”

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Gomez said the most immediate health concern isn’t hypothermia, but something called cold shock response and it can happen almost immediately after falling into the water and last a few minutes. He said the consequences can be deadly.

“If your head goes below the water, you’ll have an involuntary inhalation that causes water to go in your lungs. Even a capful of water going into your lungs will cause your neck muscles to spasm and your trachea to seal,” Gomez said.

WATCH: Staying safe on the ice in Toronto. Tom Hayes reports.

1:52 Staying safe on the ice in Toronto Staying safe on the ice in Toronto

“Technically, they could actually black out in the very first second they inhale that water. And even if they don’t black out in that first second, what will happen is for the first minute, they have a high probability of blacking out with the trachea sealed, so that kills a lot of people.”

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Although Gomez said the marine unit’s typical response time for the inner-harbour is less than five minutes after receiving the call, officers may not be able to get to rescue the victim in time as symptoms set in.

“They could potentially die in the first three minutes from cardiac arrest, the inhalation of water causing them to black out or them hyperventilating and blacking out,” he said.

After the first few minutes in the cold shock response stage, Gomez said the body can be incapacitated between four and 10 minutes and the drowning risk becomes elevated.

“If they’re fortunate enough to last that first minute, two minutes, three minutes until their body goes numb, they can no longer swim.” Tweet This

Toronto fire was called to the Lower Jarvis Street and Queens Quay East area Tuesday afternoon after receiving reports that two people were walking on the ice in the harbour.

In a written statement to Global News Tuesday evening, the harbour master for PortsToronto reiterated the concern about traffic breaking up parts of the ice and highlighted water currents from area rivers that can impact ice.

“The Toronto Harbour continues to be a working harbour in the winter months with ice breaking vessels creating access to/from all three Toronto Island docks and for the Great Lakes freighters that continue to arrive in the Port of Toronto,” Angus Armstrong said.

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READ MORE: Ice breaker clears a path to Toronto Island

“In addition, there is a constant movement of water beneath the ice due to the Don and Humber Rivers as well as the circulation currents in Lake Ontario. All this can compromise ice that might look safe to walk on.”

According to data released by the National Oceanic and Atmospheric Administration, it shows anywhere from 0.1 to 12 centimetres of ice thickness off the shore of Lake Ontario in the Toronto area.

In its ice safety guidelines, the Canadian Red Cross said ice thickness should be 15 centimetres for walking or skating alone. For skating parties or games, officials said the ice thickness should be 20 centimetres.

READ MORE: Video shows man skating on frozen Toronto harbour against iconic skyline

The Canadian Red Cross said clear, blue ice is the clearest indication of strength. The organization said grey-coloured ice should be considered unsafe as it indicates the presence of free-moving water.

However, with conditions that can unexpectedly change and without a clear indication of ice thickness, he strongly cautioned against walking across the harbour.

“The way we’ve been trained here is no ice is safe,” Gomez said. Tweet This

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“Don’t go out on the ice, unless it’s an ice rink, because you just never know.”

— With files from Ross Hull

A map by the National Oceanic and Atmospheric Administration shows ice coverage on Lake Ontario as of Tuesday afternoon. National Oceanic and Atmospheric Administration