Why do cities put chlorine in the water supply?

In an earlier Just Ask John post, I answered a viewer's question about why he noticed such a strong smell of chlorine in his water.

As I explained then, El Paso Water said the chlorine smell in its water is sometimes more pronounced when the water company switches from groundwater sources to water from the Rio Grande, which happens this time of year. The chlorine smell may also be more noticeable after a water main break.

I then wondered why chlorine is such an important part of our water supply.

In an online search, I learned that a century ago, in 1918, the U.S. Treasury Department called for all municipal water supplies nationwide to be disinfected with chlorine.

That's because chlorine has proven to be one of the most powerful disinfectants around, especially when it comes to killing potentially deadly bacteria like E. coli and other microbes that are commonly found in water.

Experts agree that the practice of chlorinating our drinking water and swimming pools has saved countless lives, and may be the single greatest public health achievement of the past century.

But chlorine is a toxic gas, and while scientists say the levels in our drinking water and pools are safe to consume, some research has shown that the by-products produced by this disinfectant could have health consequences for some, such as DNA damage, an increased risk of certain cancers and asthma.

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