In a hot climate and at high altitudes, it’s essential to increase water intake to prevent dehydration. Most travellers, and most Mexicans if they can, stay off the tap water. A lot of the time it is in fact drinkable, and in practice may be impossible to avoid completely: ice made with it, unasked for, may appear in drinks, utensils are washed in it, and so on.
Most restaurants and licuaderías use purified water (agua purificada), but always check; most hotels have a supply and will often provide bottles of water in your room. Bottled water (generally purified with ozone or ultraviolet) is widely available, but stick with known brands, and always check that the seal on the bottle is intact since refilling empties with tap water for resale is common (carbonated water is generally a safer bet in that respect).
There are various methods of treating water while you are travelling, whether your source is from a tap or a river or stream. Boiling it for a minimum of five minutes is the time-honoured method, but it is not always practical, will not remove unpleasant tastes and is a lot less effective at higher altitudes – including much of central Mexico – where you have to boil it for much longer.
Sterilization and purification
Chemical sterilization, using either chlorine or iodine tablets or a tincture of iodine liquid, is more convenient, but leaves a nasty aftertaste (which can to some extent be masked with lime juice). Chlorine kills bacteria but, unlike iodine, is not effective against amoebic dysentery and giardiasis. Pregnant women or people with thyroid problems should consult their doctor before using iodine sterilizing tablets or iodine-based purifiers. Too many iodine tablets can cause gastrointestinal discomfort. Inexpensive iodine removal filters are available and are recommended if treated water is being used continuously for more than a month or is being given to babies.
Purification, involving both filtration and sterilization, gives the most complete treatment. Portable water purifiers range in size from units weighing as little as 60g, which can be slipped into a pocket, up to 800g for carrying in a backpack.