FAQ: PFAS Test Results - What Do the Numbers and Acronyms Mean?

PFAS test results may contain terminology that can make interpreting the results challenging.  In addition to your test results, labs are often required to include additional information as part of their accreditation that supports the accuracy and validity of the testing.  The following article explains some common terminology that you may see on your test report.

Units of Measure

When contaminants are measured in water they are typically expressed in concentrations. For water samples, some concentrations are expressed in mass of contaminants, e.g., gram (g), milligram (mg), microgram (µg), or nanogram detected per unit volume of water (e.g. liters (L) or milliliters (mL)).

ng/L – indicates measurements in nanograms per liter and is typically how concentrations of PFAS are measured in water. One ng/L would signify 1 ng of PFAS per liter of water, and this concentration is equivalent to one drop in twenty Olympic-sized swimming pools. Currently, the federal drinking water standards for regulated PFAS range from 4.0 – 10 ng/L.

ppt – is the abbreviation for "parts per trillion." This unit of measure is another way of expressing ng/L and results expressed in ng/L are equivalent to results expressed in ppt.

µg/L -  indicates measurements in micrograms per liter. One µg/L would signify 1 µg of PFAS per liter of water, and this concentration is equivalent to one drop in an Olympic-sized swimming pool.

ppb – is the abbreviation for "parts per billion." This unit of measure is another way of expressing µg/L and results expressed in µg/L are equivalent to results expressed in ppb.

mg/L - indicates measurements in milligrams per liter. This measure is not typically used for PFAS just because PFAS tend to be present at very low levels.  

ppm – is the abbreviation for "parts per million." This unit of measure is another way of expressing mg/L and results expressed in mg/L are equivalent to results expressed in ppm.

These units can be converted from one to another using simple multiplication or division factors, as shown in Table 1 below. 

Concentrations measured in solid materials such as soil, biosolids, plant tissue, or fish tissue samples are expressed in mass of contaminants detected per unit of mass of material (e.g. grams (g) or kilograms(kg)).

ng/kg - indicates measurements in nanograms per kilogram. It is very tiny and is equivalent to one trillionth of a gram of a chemical present in a kilogram of the sample. Thus ng/kg is the same as part per trillion (ppt) by mass.

ng/g – indicates measurements in nanograms per gram. This is typically how PFAS are measured or reported in soil, tissue, biosolids, or other solid materials. It is a thousand times bigger than ng/kg but still tiny. This is equivalent to one billionth of a gram of a chemical present per gram of sample. Thus ng/g is the same as part per billion (ppb) by mass. For example, when measuring PFAS in soil, 1 ng/g would mean that there is one nanogram of PFAS per gram of soil. It is almost like one tiny grain of sand in a huge sandbox.

µg/g – indicates measurements in microgram per gram. This is a thousand times bigger than ng/g and one million times bigger than ng/kg and is equivalent to one millionth of a gram of a chemical present per gram of sample. Thus µg/g is the same as part per million (ppm) by mass.

Common Abbreviations in Laboratory Reports

When labs report results, you may see some additional abbreviations or phrases that describe results in place of or in addition to numerical concentrations.

ND – abbreviation for "non-detect" or "not detected" which means that the compound being tested for was not detected in the sample that was submitted.  Most testing equipment and methods only reliably detect chemicals when there is at least a certain amount present, so anything less than that concentration would be reported as ND rather than 0, since it is unknown if there is an undetectable amount of a chemical or actually none present.

RL – the "reporting limit" is the lowest level of a chemical that a laboratory can accurately measure the amount using the instruments and methods available.  Typically, concentrations detected above the RL will be listed on a test report as a value. If a chemical is detected but is present at a level below this limit, it will be reported as less than the RL (< RL). 

MDL – the "method detection limit" is the lowest concentration of a compound that can reliably be detected by the equipment or methods being used.  The MDL is typically less than the RL, so concentrations above the MDL are usually reported as a value if they are also above the RL.  Results less than the reporting limit may be expressed as < RL, while any concentration less than the MDL will typically be reported as ND.

While the above abbreviations are common, laboratories may use different ones to explain their methods and results. It is important to go over your report for specific explanations or reach out to an Extension Educator to assist with interpretation.

How do I interpret my water test results?

You can compare your test results with current guidelines and standards. US EPA finalized primary drinking water regulations for 6 PFAS as part of its duties to protect human health under the National Safe Drinking Water Act. The regulations established maximum contamination levels (MCLs) shown in the table below.

 Pennsylvania set an MCL of 14 parts per trillion (ppt) for PFOA and 18 ppt for PFOS. Because the US EPA standards are still not enforced, public utilities in Pennsylvania are required to meet the PA DEP MCLs. When the federal rules roll in 2031, public water providers will ultimately need to comply with the federal MCLs to avoid rule violations. Private water systems and farm water resources are not included in the state or federal standards. It is the responsibility of the owner or user to ensure that the water meets existing safety guidelines.

A tool that can help with water test result interpretation is Penn State's Drinking Water Interpretation Tool. This online tool can be used to compare drinking water test results with established drinking water standards. To use Penn State's Drinking Water Interpretation Tool, enter your water test results for the applicable parameters.  Currently, this tool can be used to interpret water test results for the PFAS compounds with established primary drinking water standards.  Once results are entered, the tool will tell you if they meet or exceed drinking water standards, as well as provide a brief explanation of what the contaminant is.

How do I interpret my soil and tissue test results?

There are no federal or state regulations for PFAS in soil, plant tissue, or animal products. As more research and monitoring studies are conducted, states may develop varying guidelines and recommendations. For example, the state of Maine has developed Action levels for perfluorooctane sulfonic acid (PFOS) in beef, milk, and chicken eggs. The action levels are used as guidance to help farmers and the public determine if an animal product contains potentially unsafe levels of PFOS.

Source: Derivation of Action Levels for PFOS in Cows Milk (PDF) and PFOS Action Levels for Beef Derivation Memo (PDF)

Because of the PFAS transfer from soil to planted crops, to dairy cow feed, and eventually to milk, the state of Maine also developed agronomic soil screening levels for PFOS to help meet the above action levels in milk. The screening levels are crop-specific because of varying PFAS absorption rates between crops.

Source:  Derivation of PFOS soil screening levels for a soil-to-fodder-to-cow's milk agronomic pathway (PDF)

For help interpreting water test results, reach out to Faith Kibuye at ffk5024@psu.edu or 814-863-4622, and for help interpreting soil or plant tissue test results, reach out to Daniella Carrijo at daniela.carrijo@psu.edu or 814-863-2535