Because it’s a matter of dosage.
Drinking straight chlorine, or water with too much chlorine is horribly horribly bad for you and very well may kill you. Drinking incredibly small quantities of it is not harmful.
Chlorine for example, is widely used in municipal water supplies as an antimicrobial. [The EPA considers levels up to 4ppm to be safe for human consumption](https://www.epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations#Disinfectants).
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