It's the question underneath every other question people ask us. Nobody really wants to know about micron ratings. They want to know whether they're going to spend their first year in Mexico City getting sick.

Usually they're asking because they've been buying garrafones since the week they arrived, they're tired of it, and they want to know whether switching to a filter at the kitchen tap means taking a risk. (On the garrafón side of that trade, we've written about what the bottles themselves cost you.)

The honest answer has two halves, and most water filtration companies only tell you the flattering one.

What actually makes people sick in Mexico City

The CDC's guidance on travelers' diarrhea is blunt about where the risk concentrates: “Poor hygiene practices in local restaurants and underlying hygiene and sanitation infrastructure deficiencies are likely the largest contributors.”

Read that again, because it matters for what you're about to buy. The largest single driver is hygiene in places where food is prepared for you — the restaurants, stalls, and kitchens you'll never see behind the counter, in Mexico City as anywhere else. A water filter under your kitchen sink has no reach over any of it.

What the same guidance also says is that unsafe water is often how food gets contaminated in the first place: through rinsing, ice, wet hands, a cutting board wiped with a dirty cloth. Water and food aren't rival explanations. They're the same problem meeting you at different points.

The pathogen breakdown is worth knowing too:

  • Bacteria cause 75–90% of cases: most often enterotoxigenic E. coli, then Campylobacter jejuni, Shigella, and Salmonella.
  • Viruses cause 10–25%.
  • Protozoa cause about 10%: Giardia and similar, and disproportionately in people who stay long-term rather than visit.

What a water filter actually protects you from

An ultrafiltration membrane rated to 0.01 microns is a physical barrier. Bacteria and protozoan cysts are larger than that opening, so they don't pass. It isn't a chemical process; the pores are simply too small for them to fit.

That maps directly onto the two biggest categories above. The bacteria responsible for the large majority of cases, and the protozoa that disproportionately affect people who actually live here, are both squarely within what these systems are rated to stop.

Where ultrafiltration falls short: viruses

Ultrafiltration is not a reliable barrier against viruses.

Viruses sit at the very bottom of the size range, close enough to the membrane's pore distribution that removal becomes partial and dependent on the membrane being in perfect condition. Reverse osmosis, at roughly 0.0001 microns, does considerably better. Our 6-Stage RO + UV system adds an ultraviolet lamp specifically because UV addresses what size exclusion can't guarantee.

So when we say viruses are 10–25% of cases, we're telling you that our entry-level systems have a real gap. That's not a sales pitch for the expensive one. For most households in this city, ultrafiltration is still the right call, and we say so on the UF versus RO page. It's just not something you should find out afterward.

Ice, produce, and brushing your teeth with CDMX tap water

Ice. Ice made at home from filtered water is filtered water. Freezing doesn't undo filtration. Ice you're handed somewhere else is outside your control and outside your system's reach.

Washing produce. Rinsing in filtered water is a genuine improvement over rinsing in unfiltered tap water, and it's one of the more useful things a kitchen system does that people never think to count. It doesn't sterilize anything. Plenty of CDMX households also soak produce with a few drops of Microdyn or similar, and that habit is worth keeping.

Brushing your teeth. The volume is tiny and most of it doesn't get swallowed. If you have a filtered tap in the bathroom, use it; if you don't, this is genuinely low on the list. We say the same thing on our bathroom sink page.

Pets. Same water, smaller body. If you're filtering for yourself, filling their bowl from the same tap costs nothing.

Cooking. If you're boiling it hard, pathogens aren't the concern: boiling handles those, as we cover in our piece on CDMX tap water. Filtration still matters for taste, sediment, and anything dissolved, which boiling doesn't touch.

Your rooftop tinaco: the part upstream of your filter

Most Mexico City buildings store their water in a rooftop tinaco or a cistern before it comes back down to your kitchen. That tank sits between the municipal supply and everything we install, and it's generally meant to be cleaned about every six months, an interval that, in a Roma Norte or Condesa building where nobody has agreed whose job it is, often quietly never happens.

A point-of-use system filters that water on its way to your glass, which is exactly the point. But a filthy tank upstream shortens your cartridge life and gives the system more work than it should have.

We don't clean tinacos, and we're not trying to sell you one. We're telling you to get it done anyway, roughly twice a year, by whoever your building uses. It's cheap, it's boring, and it's the highest-value water habit most people in CDMX never get around to.

So will a water filtration system stop you getting sick?

It removes one real category of risk and leaves others standing.

Correctly installed and maintained, a drinking water system is a reliable barrier against the bacteria and protozoa that account for the large majority of cases. It is not a barrier against a virus, a restaurant kitchen, or a system whose cartridges nobody has changed in two years. Filters that stop being maintained quietly stop being filters, which is the entire reason we track the schedule rather than leaving it to you.

What we can tell you is what we do ourselves. Our own apartment in Roma Norte sits on the same municipal supply and the same kind of rooftop tinaco as most of the buildings we work in. We drink that water, filtered, every day, including the ice. That isn't a claim about your building. It's just the answer to the question people are usually too polite to ask.

If you want to know what's actually in your building's water before choosing anything, lab testing answers that. Otherwise our side-by-side comparison lays out all four systems, or tell us what's worrying you and we'll say straight whether a filter actually solves it.

Sources

  • Travelers' Diarrhea, CDC Yellow Book, 2026 edition — pathogen distribution and risk attribution.
  • Food and Water Precautions for Travelers, CDC Yellow Book — on unsafe water as a route to food contamination.
  • Membrane pore-size classifications and virus removal performance: Safe Drinking Water Foundation; NESC Tech Brief on Membrane Filtration.