How do you know if the tap water is safe to drink abroad?

Tap water safety varies by country and often by region within a country, so the reliable move is checking the CDC's destination pages, which state plainly whether drinking the tap water is advised. Broadly, tap water is safe in the US, Canada, most of Western Europe, Japan, Australia, and New Zealand, and is not reliably safe across much of South and Southeast Asia, Africa, Central and South America, and parts of Eastern Europe and the Middle East, though there are exceptions in both directions and infrastructure changes over time.

Where the water is not safe, the mistake is thinking only about drinking glasses. Ice made from tap water, raw fruits and vegetables washed in it, and rinsing your toothbrush under the faucet are all common infection routes, and travelers who dutifully buy bottled water still get sick from the ice in their drink. Use sealed bottled or properly treated water for drinking and brushing, skip ice unless you know its source, favor cooked hot food and fruit you peel yourself, and be cautious with unpasteurized dairy. Boiling for a minute, or a filter or purification tablets rated for pathogens, works when bottled water is impractical. Traveler's diarrhea is usually self-limiting, but bring oral rehydration salts and know when to seek care, since dehydration is the real danger, especially for children.