Tianna Hollinside, 13, and Juan Cornelio, 23, may have never known what hit them before they joined the ranks of Chicagoans who died too young.

But it wasn’t a bullet that killed them.

It was cold water.

Hollinside and Cornelio both drowned Tuesday in separate swimming accidents on Chicago’s north lakefront, lured to their deaths by balmy 80-degree-plus air temperatures that belied a fatally frigid Lake Michigan.

Swimming may not even be the right word for what happened.

As described by experts, the shock of jumping into 52-degree water could have prevented either of them from swimming a single stroke before succumbing.

They call it hyperventilation gasp reflex.

OPINION

If someone’s head is underwater when it occurs, as in the case of someone who dives head-first, the mouth reflexively opens and the lungs fill with water.

“It’s almost paralyzing. Some dive in head-first and they don’t even resurface,” said Dave Benjamin, executive director of Great Lakes Surf Rescue Project.

Under such circumstances, drowning can be almost instantaneous, the National Weather Service warns.

“That’s one of the hazards of this time of year,” said Benjamin, whose group tries to save lives by educating people about the dangers of the Great Lakes.

Am I scaring anybody yet?

I’m trying.

After a long winter and a generally unpleasant spring, the beaches look so inviting this week.

But people need to understand that the cold water can be lethal.

“Fifty-two degrees is just bad on the body. Wait for the beaches to open,” advised Fire Department spokesman Larry Langford after the department’s divers tried unsuccessfully to rescue Hollinside and Cornelio.

Hollinside drowned after jumping into the water near Rogers Beach Park on the city’s Far North Side on Tuesday morning, an unauthorized swimming spot popular with kids in the summer.

Hours later, Cornelio’s body was recovered near Montrose Point out beyond the beach and the bird sanctuary.

I know the location well. In the summer, groups of kids often jump from the seawall there to swim, although I was surprised to find some of them there last weekend when my wife and I took a walk.

Information about the circumstances of Cornelio’s death were spotty, but Langford said it is believed he also jumped into the water.

Interestingly, a second man who went into the water with him made it to shore safely on his own.

Both Langford and Benjamin emphasized that being a strong swimmer is no guarantee of surviving the effects of hypothermia, which after only a few minutes can cause of loss of dexterity that makes it difficult to even float.

Benjamin approaches the subject of Lake Michigan water safety with the zeal of a born-again convert, which he almost literally is.

The self-employed painter from Homewood says he was just another overconfident water sports enthusiast before being involved in a nearly fatal drowning accident while winter surfing on Lake Michigan near Portage, Indiana, in 2010.

Now he preaches about the need to treat drowning as a major public health danger, while teaching water safety classes on the side.

Benjamin said another major danger this time of year involves people going out into the lake with kayaks, stand-up paddle boards and inflatable rafts.

With prevailing winds from the south blowing offshore, those individuals often find themselves unable to paddle back against the wind. Then the farther offshore they are blown, the water gets more choppy.

Some panic and ditch their device or boat and try to swim for it, Benjamin said, but the cold water gets them and they can’t make it back to shore.

Chicago beaches don’t officially open until May 26, which only means lifeguards will then be on duty. Kayak and paddleboard rentals should be available soon as well.

The water will still be dangerously cold.

You have been warned.