PPM, PPB, PPT: What Those Numbers on Your Water Report Actually Mean

You open your water quality report and see numbers like "6.2 ppt PFOA" or "12 ppb lead." You know those sound bad, but what do they actually mean? Is 6 ppt a little? A lot? How do you even picture something that small?

This guide breaks it down in plain English — with real-world examples that make these microscopic measurements click. No chemistry degree required.

The three units: PPM, PPB, and PPT

All three measure concentration — how much of something is dissolved in your water. Think of it like drops of food coloring in a swimming pool. The units just tell you how many drops, and how big the pool is.

Real examples that make it click

🚰 Lead: 15 ppb — the EPA action level

The EPA requires public water systems to take action if lead in tap water exceeds 15 ppb. Here's the accurate way to picture it:

Some cities have tested far above 15 ppb. Flint, Michigan peaked at over 100 ppb during its crisis — that's 100 drops across 100 backyard pools. Still sounds abstract, but the damage it caused to thousands of children was anything but.

☣️ PFAS: 4 ppt — the EPA limit for PFOA and PFOS

The EPA's 2024 limit for PFOA and PFOS is 4 parts per trillion. Here's the accurate picture:

To put contamination numbers in perspective: Parkersburg, West Virginia tested at 179 ppt PFOA — 44 times the EPA limit. That's like 44 drops per Olympic pool instead of the allowed fraction of a drop. The Cape Fear system in North Carolina hit 490 ppt PFOS — 122 times the limit. These are real communities with real people drinking this water every day.

🥤 Nitrates: 10 ppm — the EPA limit

Nitrates from agricultural fertilizer runoff are regulated at 10 ppm — a much larger unit because nitrates are less potent per molecule. At 10 ppm, you're talking about 10 drops in a fish tank. Still small, but the "pool" is much smaller than for ppb or ppt measurements — which is why 10 ppm of nitrates is dangerous while 10 ppm of calcium (common in hard water) is completely harmless.

Why such tiny amounts matter

The reason these trace levels matter comes down to two things: bioaccumulation and lifetime exposure.

Your body doesn't flush lead or PFAS like it does water. Lead binds to bone. PFAS bind to proteins in your blood and accumulate in organs. Every glass you drink adds to the total. A child who drinks water with 15 ppb lead every day for 10 years has been accumulating lead in their developing brain for a decade. The individual daily dose sounds harmless — the lifetime accumulation is not.

This is also why scientists often argue for limits far below what's legally allowed. The EPA sets MCLs based on a combination of health science and what's technically feasible for public water systems to achieve. The Environmental Working Group's health guideline for PFOA is 0.1 ppt — 40 times stricter than the EPA's limit of 4 ppt. Both are based on health research. The difference is what regulators believe public water systems can practically achieve.

How to read your own water report

When you look at a water quality report — whether from your public water system's Consumer Confidence Report or from WaterCheckup — here's what to focus on:

What actually removes these contaminants

Understanding the units helps you understand why filters matter — and why not all filters are equal:

If your water report shows any PFAS above 4 ppt, lead above 5 ppb, or nitrates above 5 ppm — a certified reverse osmosis system is the right call. Use WaterCheckup to see your specific city's report, then take the 3-question filter quiz to find the right system for your home.