Q. I’ve been saving gallons of water in unopened plastic jugs since 9/11. Does the water go bad for drinking and cooking?

A. If the water was clean when it went into the containers, if the jugs were perfectly clean and sanitized to begin with, and if you are sure the containers were never opened, it will not have become unsafe to consume.

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If you have doubts about any of these factors, the water can still be used if it is purified. The purification methods usually suggested are filtering followed by boiling or adding a few drops (about eight drops per two-liter bottle) of unscented liquid household bleach.

Authorities like the Red Cross and the Federal Emergency Management Agency recommend keeping emergency water supplies on hand in case of a disaster: a gallon a day per household member, enough for at least three days. It should be either commercially bottled water or chlorinated tap water put into clean, food-grade, soft-drink-type plastic containers, not cardboard juice or milk containers.

The authorities also recommend renewing the supply every six months and keeping it away from heat and sunlight, which encourage the growth of any microbes that may be present. Boiled water can be aerated to make it more palatable by pouring water back and forth between two clean containers. C. CLAIBORNE RAY