One State Has a Legal Definition of Microplastics. The Other 49 Don't.

Here's something most people don't realize about microplastics: outside of California, the word doesn't carry a regulatory definition in U.S. drinking water.

Not "it's unregulated." Worse than that. There isn't an agreed-on legal definition of what counts as a microplastic in the water coming out of your building's tap. And you can't regulate a thing you haven't defined.

I sell bottleless water systems in the Madison area. So I have an obvious bias here and I'd rather say it up front than pretend I'm a neutral observer. But this particular gap isn't a sales point. It's a genuinely strange hole in how American drinking water gets governed.

What California actually did

On June 16, 2020, California's State Water Resources Control Board adopted Resolution 2020-0021, defining "microplastics in drinking water" as solid polymeric materials with at least three dimensions greater than 1 nanometer and less than 5,000 micrometers. SB 1422 (2018) had ordered the definition by July 2020, then standardized test methods and four years of testing with public disclosure. The Board adopted Raman and infrared spectroscopy procedures in September 2021 and a policy handbook in September 2022. I can't find another state that has done the same.

Read that again. Not a limit. Not an enforcement threshold. A definition and a test method.

That sounds unglamorous. It's the whole ballgame. Every regulation that has ever restricted a contaminant started here: somebody defined the thing, somebody wrote a lab method that produces comparable numbers across different labs, and then somebody spent years collecting data.

This is exactly how PFAS happened

We already watched this movie.

EPA ran the Fifth Unregulated Contaminant Monitoring Rule, requiring public water systems to monitor for 29 PFAS compounds plus lithium between 2023 and 2025. That nationwide occurrence dataset is intended to inform future federal decisions about drinking water standards.

Then in April 2024, EPA finalized the first national primary drinking water regulation for PFAS — enforceable limits of 4.0 parts per trillion for PFOA and PFOS, 10 ppt for PFHxS, PFNA and GenX, and a Hazard Index approach for mixtures.

Definition, then method, then monitoring, then limits. Years apart at each step.

And even that isn't settled. In May 2026 EPA proposed letting systems request until 2031 to comply and separately proposed rescinding the limits for PFHxS, PFNA, GenX and the mixture rule; the 4.0 ppt limits for PFOA and PFOS stay as written, and final action on both was still pending when I checked in September 2026. So the ground is still moving on a contaminant we've been studying for decades.

Microplastics are considerably earlier in that same process. California is at step one. Nobody else is.

Meanwhile, the measurements keep getting worse

While the rulemaking crawls, the science keeps improving — and it keeps finding more, not less.

A January 2024 study in PNAS using stimulated Raman scattering microscopy counted roughly 240,000 plastic particles per liter across three brands of bottled water, about 90 percent of them nanoplastics. That's dramatically higher than earlier microplastic-only estimates. Not because the water changed. Because the instruments got better.

That's what an undefined contaminant looks like. The number moves every time somebody buys a nicer machine.

The part that's actually reassuring

WHO's 2019 review of microplastics in drinking water put it this way in its summary Q&A: "Conventional treatment, when optimized to produce treated water of low turbidity, can remove particles smaller than a micrometre," and wastewater treatment "can effectively remove more than 90% of microplastics," with the highest removal from tertiary filtration. The same review concluded microplastics in drinking water "don't appear to pose a health risk at current levels," while calling for more research and for less plastic pollution overall.

Filtration works. It worked before anyone defined the problem. It doesn't require a federal limit to exist.

That's the practical takeaway I keep coming back to when a facilities manager asks me whether they should be worried. The regulatory answer is "nobody knows yet, ask again in a decade." The engineering answer has been available the whole time.

What I actually think

We've built a system where the fix is cheaper and faster than the paperwork proving the fix is necessary.

California spent years just establishing what the word means. Most of the country hasn't started. And in the meantime, buildings are making a choice by default — usually the choice to keep drinking from plastic while waiting for a number that hasn't been written.

I don't think you need EPA to tell you what to do here. I think you need to notice how few states have bothered to define the problem, and draw your own conclusion about how long the wait is going to be.