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Ask Dr. Laundry

Ask Dr. Laundry

Disinfecting Pool Water

By Dr. Laundry February 18, 2010

Q. Knowing the ratio of bleach to cold water that is safe to spray on children’s toys, I thought it would be a good solution for our child’s pool. We have a blow-up pool that holds a little more than 50 gallons. In the summer in Washington, we have two things to worry about: mosquitoes laying eggs in standing water, and a slimy build-up on the plastic.

I figured a cup of Clorox would be just enough to keep the slime and mosquitoes at bay and still be safe for my child to swim in. Well, we added a cup of bleach and it immediately turned yellow. Within 30 minutes the entire pool looked like it was filled with, well, urine. It was truly that yellow.

I have never seen Clorox react in such a way. What could possibly be the reason and is it safe for my child?

A. Wow, that sounds way too strong! The standard disinfecting solution (3/4 cup Clorox® Regular-Bleach/gallon of water) yields a 2700ppm (parts per million) concentration. For pool use, we want no more than 1-2ppm. Your 1 cup Clorox® Regular-Bleach would have been enough to treat a 1,667 gallon pool.

For your 50 gal pool, you probably should have started with only ~2 teaspoons to get the 2ppm level.

So, the yellowing was strong bleach reacting with everything in the pool. I would throw it out and start over. Note the last line above! The smell would have kept me (and hopefully your child) from entering the pool.