Asbestos

What it is

Asbestos in water tends to surprise people, but the source is mundane: it is a group of naturally occurring mineral fibers, and in drinking water it most often comes from the corrosion of older asbestos-cement water mains and from natural mineral deposits the water moves through. The fibers are microscopic and do not change the taste, color, or smell - so, as with the other invisible ones, testing is how you would know.

Why it matters

The EPA sets a federal maximum contaminant level (MCL, the legal limit for public water) for asbestos of 7 million fibers per liter, counting fibers longer than 10 microns. That figure is the regulatory reference point for asbestos in public drinking water. What we score is whether a filter is certified to reduce this contaminant, not any health outcome.

What removes it

Asbestos is a mechanical fiber-reduction claim under NSF/ANSI 53, so a filter has to be tested and certified specifically for asbestos to earn credit, and reverse osmosis reduces it too. The trap to avoid: a carbon filter is not automatically an asbestos filter - an ordinary carbon pitcher built for taste and chlorine does not carry the asbestos claim. Certification for asbestos by name is what separates a filter that reduces these fibers from one that just improves taste.

Reference: EPA National Primary Drinking Water Regulations (asbestos MCL); NSF/ANSI 53 (asbestos fiber reduction).