Rook and Bellar each obeyed the first principle of invention. Each brought alien expertise to the problem. Then a bomb fell. Both Rook and Bellar made the same horrifying discovery. More chemical pollutants came out of the chlorinated water than went into it.
Chlorine reacts to create new carcinogens.
The worst of these is chloroform. So we were rid of cholera at the cost of cancer. The government responded by putting a legal limit of 100 parts per billion on chloroform in our drinking water. That's still enough to cause one extra cancer death in 10,000 people who drink the water all their lives. The trouble is, we pay a price for getting even that low.