Why nobody drinks the tap water

Mexican tap water is treated and chlorinated at source, and the reason it is still not drunk is what happens between the treatment plant and the tap. Water reaches most buildings through long distribution networks, then sits in a ground cistern and a rooftop tank before it comes out of the tap, and each of those stages is maintained by the building rather than by the utility. The result is a national habit: you wash, shower and cook with tap water, and you drink something else.

That habit is worth copying rather than testing. The Secretaría de Salud's own preventive guidance for acute diarrhoeal disease centres on disinfection, handling and storage of water, washing hands before preparing and eating food, washing fruit and vegetables with potable water, and cooking food thoroughly. Travel health guidance goes further for short visits, recommending bottled or disinfected water even for brushing teeth in places where tap water quality is uncertain.

Garrafón water, and what a restaurant is actually required to do

The garrafón is the visible half of Mexico's drinking water system: a 20-litre returnable bottle, delivered to homes or swapped at a shop, sitting on a dispenser in almost every kitchen. Restaurants and hotels use the same supply chain at larger scale, which is why the water in a jug on a restaurant table and the ice in your glass usually come from a purified source rather than the tap.

Behind that is a rule rather than a courtesy. NOM-251-SSA1-2009, the Mexican hygiene norm for food processing, is mandatory for anyone processing food or drink for consumers in Mexico and requires that water in direct contact with food, or used to make ice, be potable and meet limits on residual chlorine and coliform organisms, with a daily record kept. Where a business cannot document that, the norm requires the water to be boiled, disinfected or purified before it is used to make ice or to prepare uncooked food and drinks.

Ice: what the rule covers, and what it does not promise

Commercial ice in Mexico is a regulated product, not an improvisation. NOM-201-SSA1-2015 sets the sanitary specifications for water and ice for human consumption sold prepackaged or in bulk, and states that it is mandatory throughout the country for anyone processing or importing those products. That is the mechanism behind the common advice that ice in Mexico is generally fine: most ice served in bars, restaurants and juice stands is bought in from a producer covered by that norm, in sealed bags, rather than frozen in a back-room tray.

What the norm does not do is guarantee any individual glass. It regulates producers, not the handling that happens afterwards, and it says nothing about a bag of ice left open on the floor of a stall. Uniform machine-made cylinders or cubes with a hole through them, arriving from a sealed bag and handled with a scoop, are the visible signs of the regulated supply chain. Irregular hand-cut chunks in an open bucket are not. International travel health guidance takes the more cautious line and advises avoiding ice wherever water quality is in doubt, so the honest position is that ice in Mexico is usually low risk and never zero risk.

Reading a busy stall

A street stall is not inherently riskier than a restaurant, and a good one is often safer, because the whole process happens in front of you. What you are reading for is turnover, heat and separation.

  • Turnover. A queue of local customers means the food has not been sitting. An empty stall with a full tray at 3pm means it has.
  • Heat. Food should go from the grill or comal to your plate. Travel health guidance is explicit that lukewarm is the danger zone and that hot food should be steaming.
  • Separation of money and food. Watch whether one person handles cash and another handles tortillas. Where a single person does both, look for whether they wash or sanitise between the two.
  • The water source. A visible garrafón, a covered container with a tap, a bucket of clean water for rinsing utensils. NOM-251 requires potable water and ice to be kept in smooth, washable, covered containers.
  • Cooked to order versus pre-assembled. Tacos al pastor carved off the trompo, quesadillas made when you order, elotes coming out of a hot pot, all involve heat at the last moment. Pre-dressed items sitting under a cloth do not.

Aguas frescas, salsas and the raw category

Raw ingredients are where the actual risk concentrates, and the categories that matter are narrower than "street food." Aguas frescas, the jamaica, horchata and tamarindo sold from glass barrels, are made with water and are only as reliable as that water; a stall using a garrafón is a different proposition from one filling from a hose. Freshly squeezed juice made and handled by someone else, cut fruit, shredded salad and ceviche all appear on travel health authorities' avoid lists, and the CDC's food and water guidance specifically names fresh salsas and sauces made from raw fruit or vegetables, and dishes "cooked" in citrus juice.

The salsa bar deserves its own sentence, because it is the item most visitors never think about. Uncooked salsas made from raw tomato, chilli and coriander, sitting at ambient temperature in open containers, shared with a spoon by everyone who passed before you, combine every risk factor at once. Cooked salsas, the darker ones made with roasted or boiled ingredients, are a different matter. If you want the salsa bar, take the cooked ones and take them early in the day.

Altitude and the first two days

Mexico City sits at high altitude, and it changes how your first days feel regardless of what you eat. INEGI's geographic compendium for Mexico City records altitudes for the city's boroughs in the range just above 2,200 metres above mean sea level. That is below the threshold at which serious altitude illness is usually discussed, but high enough that many arrivals report breathlessness on stairs, disturbed sleep, a headache and a much lower tolerance for alcohol on the first night.

The confounding effect is that altitude symptoms and mild stomach upset feel similar, and travellers routinely blame the tacos for the altitude. Give yourself an easy first day, drink more water than usual, go light on alcohol the first evening, and do not draw conclusions about what made you feel odd until day three.

What to carry, and when to stop self-treating

Pack oral rehydration salts, hand sanitiser, and enough small notes that you never have to choose a stall because it can break a 500. Rehydration is not a fallback but the primary treatment: the Secretaría de Salud's guidance centres on oral rehydration solution and on continuing to eat normally to avoid the deficit that comes with a few days of illness. Pharmacies in Mexico are plentiful and sell rehydration salts, suero oral, without prescription.

Stop self-treating and get medical advice if there is blood in the stool, a high fever, illness that persists beyond a couple of days, signs of dehydration, or if a child or an older traveller is affected. This guide covers food and water risk and altitude adjustment only. It does not cover vaccinations, insect-borne illness, travel insurance, or what a specific medical condition requires, all of which are questions for a travel health clinic before you fly.