Understanding Water Quality Units
Water quality reports use specialized measurement units that can be confusing. Here's what they mean in plain English.
What Do These Units Mean?
Also known as: Parts per million (ppm)
The largest unit commonly used in water testing. Used for minerals, chlorine, and nutrients like nitrate. If your water report shows a value in mg/L, that's parts per million.
Also known as: Parts per billion (ppb)
1,000 times smaller than mg/L. Used for heavy metals (lead, arsenic, copper), many regulated chemicals, and some industrial pollutants. Most EPA limits are set in this unit.
Also known as: Parts per trillion (ppt)
1,000 times smaller than ug/L. Used almost exclusively for PFAS (forever chemicals) because they cause health effects at incredibly tiny concentrations. EPA limits for PFOA and PFOS are just 4 ppt.
Also known as: A measure of radioactivity
Used for radioactive contaminants like radium, uranium, and radon in water. Unlike other units, this measures radioactivity rather than weight. Higher numbers mean more radiation.
Quick Conversion Table
| From | To | Multiply by |
|---|---|---|
| 1 mg/L (ppm) | ug/L (ppb) | x 1,000 |
| 1 ug/L (ppb) | ng/L (ppt) | x 1,000 |
| 1 mg/L (ppm) | ng/L (ppt) | x 1,000,000 |
| 1 ug/L (ppb) | mg/L (ppm) | ÷ 1,000 |
Why Do Different Contaminants Use Different Units?
Different contaminants are harmful at very different concentrations. Lead is concerning at 15 parts per billion, while PFAS chemicals are concerning at just 4 parts per trillion — that's 3,750 times more sensitive.
Using the right scale makes the numbers easier to read. Writing the PFAS limit as 0.000004 mg/L would be confusing, so scientists use ng/L (ppt) instead, making the limit a clean "4 ng/L."
Understanding Limits vs. Health Guidelines
EPA Maximum Contaminant Level (MCL)
The legally enforceable limit set by the EPA. Water utilities must keep contaminant levels below the MCL. These limits balance health protection with treatment feasibility and cost.
Health Guidelines (EWG, State PHGs)
Recommendations from health organizations based purely on health research, without considering cost or feasibility. These are typically stricter than EPA limits. For example, the EWG guideline for PFOA is 0.09 ppt — over 44 times stricter than the EPA limit of 4 ppt.
EPA Health Advisory
Non-binding guidance from the EPA for contaminants that don't yet have a legal limit. Often used for emerging contaminants while research continues.
What Should I Do?
- 1.Check your city's water quality page to see what's been detected.
- 2.Look at how detected levels compare to both EPA limits and health guidelines.
- 3.If contaminants are above guidelines, consider a water filter rated for those specific contaminants.
- 4.Sign up for water quality alerts to be notified when conditions change.