Why Are 96,000,000 Black Balls on This Reservoir?

A Los Angeles reservoir used for drinking water contains millions of small black plastic balls floating on its surface. The reason this unusual ball pit exists is thank to science.

Popular Youtuber Veritasium, also known as Derek Miller, took a trip to the reservoir in Los Angeles to find out just why these black plastic balls are there. He says that the first time he heard about shade balls the claim was they reduce evaporation.

Despite the balls reputation for saving water, they are not here to prevent evaporation. The real reason has to do with a certain carcinogen…


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These balls block sunlight from entering the water and triggering a chemical reaction that turns harmless bromide into carcinogenic bromate.

The balls are made of high density polyethylene (HDPE) which is less dense than water so they float on the surface of the reservoir even if they break apart. They are 10cm (4 inches) in diameter and contain about 210ml of water.

Watch the video to learn more: