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Only about 35% of Americans know how to swim, and only 2% to 7% swim well, Osinski says.
CNN: Water safety expert: 'Drowning doesn't just happen'
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In this case, the swimmer would go through the stages of what lifeguards call an "active drowning, " Osinski says.
CNN: Water safety expert: 'Drowning doesn't just happen'
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"You can't tell when they go unconscious, until it's too late, " Osinski says.
CNN: Water safety expert: 'Drowning doesn't just happen'
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Drowning also can be caused by trauma or a medical condition, Osinski says.
CNN: Water safety expert: 'Drowning doesn't just happen'
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Osinski has videos of teens drowning that were recorded on their friends' phones.
CNN: Water safety expert: 'Drowning doesn't just happen'
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"The first thing to remember is that drowning doesn't just happen, " says Alison Osinski, water safety expert and president of Aquatic Consulting Services.
CNN: Water safety expert: 'Drowning doesn't just happen'
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"You're not drowning if you can call for help, " Osinski says.
CNN: Water safety expert: 'Drowning doesn't just happen'
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After 90 seconds, Osinski says, a swimmer will black out.
CNN: Water safety expert: 'Drowning doesn't just happen'
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CNN's Artur Osinski contributed to this report.
CNN: Story highlights