Is it safe to swim in a lake with an algal bloom?

A bloom's harm depends mostly on how much of the water you swallow or swim through. A short, careful dip in a lake with a light bloom usually does little more than irritate a healthy adult's skin or stomach. The blooms behind most warnings are cyanobacteria, better known as blue-green algae, the most common cause of harmful blooms in fresh water. Some of them make toxins, poisons that can make people and animals sick.

You take in those toxins by swallowing water, getting it on your skin, or even breathing the fine droplets that drift off the surface. For most people it's minor and clears up on its own: a rash, itchy eyes, a sore throat and cough, or an upset stomach. Heavier exposure is another matter. The most common toxin, microcystin, attacks the liver, and another, anatoxin, hits the nervous system. In a big enough dose, they can cause serious illness and, in rare cases, death.

Small children and dogs face more risk than adults. A young child reaches a harmful dose from far less water than an adult does. Dogs will drink even foul-smelling water, so a dog can die within hours of swallowing the toxins. Keep kids and pets out of any water you wouldn't enter yourself, and see a veterinarian right away if a pet seems sick afterward.

You can't tell a dangerous bloom by looking, since the toxins are sometimes in the water even when there's no bloom to see, and not every bloom you can spot is toxic. Go by the warning signs instead: stay out if the water looks discolored, carries foam, scum, or paint-like streaks, smells bad, or has dead fish along the shore, and follow any posted advisory. Rinse off with clean tap water as soon as you can if you do get in. When you're not sure, stay out.

If you think the water made you sick, call your doctor or Poison Control at 1-800-222-1222. Get medical help right away if you suspect poisoning.