Question: how come chlorine can kill you but its in swimming pools

Alex Haragan answered on 19 Jun 2018:

Chlorine is an interesting element. Like most chemicals it varies immensely in its role depending on its state, what compound (or not) it is part of and concentration.
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Chlorine forms compounds that are perfectly normal – sodium chloride is salt and not only eaten but chloride ions play a hugely important role in our bodies. We can also make hydrochloric acid in our stomachs as this kills bacteria.
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Its the same properties that make it useful in swimming pools. Dissolved in water it kills bacteria and keeps the water hygienic (well, relatively).
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However when it is a gas it is most dangerous – forms of chlorine gas make the basis of mustard gas and chemical warfare. Essentially when its breathed it forms hydrochloric acid in your lungs which can burn its way through lungs and can kill.
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As nasty as that sounds – its actually the basis of chemotherapy used in cancer treatment and where much of the original research came from. Which explains why chemotherapy is not a nice thing to undergo!
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The point is though, that chlorine is hugely versatile and is a good illustration of why understanding chemistry makes these things make sense.