What Is a Health-Based Water Concentration?

By SubcontractorHub Editorial Team·Published October 2026

Dealers already have a page for the PFAS contaminants themselves. This page is the denominator those four names use when a mixture is scored. SubcontractorHub is software for the customer record. The definition below is not a treatment design, and it is not medical advice. A quotient written without a measured value is only a definition.

The Printed Meaning of a Health-Based Water Concentration

Footnote 9 on the national primary drinking water regulations says that term is used to calculate the hazard index. A ratio is developed for each PFAS by dividing the measured level of the PFAS in drinking water by the level below which adverse health effects are not likely to occur. That level is the denominator. The national page says these primary standards apply to public water systems.

The sentence has two parts, and they are easy to swap. The measured level is whatever the lab printed on a jug. The printed figure is the denominator. Dividing one by the other makes a hazard quotient. Adding the quotients makes the hazard index. A denominator without a measured level is not a result. A measured level without the matching denominator is not the index.

This page did not measure a sample. If a measured value equaled the printed denominator, and both numbers used the same unit, the quotient for that one name would be 1. That sentence is a definition of the ratio. It is not a well in any town. The contaminant story, including why a dealer might see these names on one sheet, stays on PFAS in well water. That page says it does not calculate a hazard index. This page is the definition of the denominator, not a second contaminant post.

The Four Denominators Cornell Prints

Section 141.61, as printed on the Cornell eCFR page fetched the same day, puts those denominators in paragraph (c)(2). The footnote says the hazard index is the sum of component hazard quotients. Each quotient divides the measured component concentration by the relevant denominator, in the same units. The footnote then prints the nanograms-per-liter figures it calls a simplification: PFHxS 10 ng/L, HFPO-DA 10 ng/L, PFNA 10 ng/L, and PFBS 2,000 ng/L.

The formula on that footnote is the HFPO-DA water concentration divided by 10, plus the PFBS water concentration divided by 2,000, plus the PFNA water concentration divided by 10, plus the PFHxS water concentration divided by 10. The water concentrations in that formula are in nanograms per liter. Using milligrams in the numerator and nanograms in the denominator would not be the ratio the footnote describes.

The same footnote's MCL table prints the milligrams-per-liter pair next to three of those names. HFPO-DA, PFHxS, and PFNA each show an MCL of 0.00001 mg/L and a denominator of 0.00001 mg/L. The footnote's 10 ng/L line is the same concentration written in the smaller unit. That scale is printed in the rule this page fetched. It is not a conversion invented on this site. How parts per trillion sit next to those small units is parts per trillion in drinking water.

What Is Not That Denominator

PFBS has no individual MCL and no individual MCLG on the national table. Its denominator cell prints 0.002. Cornell prints the same 0.002 mg/L and the 2,000 ng/L simplification. A dealer who writes an individual MCL for PFBS is writing a cell the table left blank.

PFOA on the national table has an MCLG of zero and an MCL of 0.0000040 mg/L. The denominator cell says not applicable. PFOS prints the same pattern: goal zero, MCL 0.0000040 mg/L, denominator not applicable. Those two names are on the MCL table. They are not in the four-name hazard-index formula. Do not put 10 ng/L under PFOA because three other names use 10.

The hazard-index row itself is 1, and the unit is unitless, on both the goal and the MCL. The health cell for that mixture says low levels of multiple PFAS that individually would not likely result in increased risk may result in adverse health effects when combined. It names liver, immune, and thyroid effects, and developmental and thyroid effects following repeated exposure during pregnancy and/or childhood. That sentence is the public table's potential-effects cell. It is not a finding about a household. A treatment technique, which is a different kind of requirement, is explained on what is a treatment technique.

What the Note Holds, and What It Does Not

The table above is the note. It is not a calculated index, and it is not a removal design. Paragraph (c) applies the synthetic-organic MCLs, and the denominators that sit with them, to community water systems and non-transient, non-community water systems. A private well is not named in that sentence. The difference between a city system and a private well is what is a public water system.

What the Named Technologies Do Not Promise

Cornell's best-available-technology table for the PFAS in paragraph (c) names anion exchange, granular activated carbon, reverse osmosis, and nanofiltration for the hazard-index group, and the same four names for HFPO-DA, PFHxS, PFNA, PFOA, and PFOS. A name on that table identifies a technology the rule lists for compliance with the MCL. It does not say what percent a device in a basement will remove.

No removal percentage was fetched for these rows, so none is printed here. No installed price was fetched, so none is printed. If you sell one of the named processes, say you sell that process. Do not say the table is a guarantee on a private well. A city plant and a point-of-entry tank are not the same machine.

What Should the Health-Based Water Concentration Note Say?

Five lines keep the next visit from treating a denominator as a sample, or from scoring PFOA inside a formula that does not include it. If a line does not apply, mark it skipped. A blank line looks the same as a step you forgot.

1. The PFAS name as the lab printed it, and whether that name is one of the four in the hazard-index formula.
2. The measured value and its unit, or a mark that this visit has no measured value. Do not invent a sample so the formula has a numerator.
3. The printed denominator for that name: 10 ng/L for HFPO-DA, PFNA, and PFHxS, or 2,000 ng/L for PFBS, which Cornell also prints as 0.00001 mg/L and 0.002 mg/L.
4. Whether the sample is a private well or a public system. Paragraph (c) names community and non-transient, non-community systems.
5. What you did not claim: no hazard index from an unmeasured jug, no removal percent, no installed price, and no denominator for PFOA or PFOS.

Where the Denominator Sits on the Job

The lab sheet belongs with the customer, the address, and the date of the sample. A price for equipment is a later page, after someone has read which names were actually tested. Writing a hazard index because the sheet mentioned PFAS, without a measured value for each name in the formula, skips the ratio footnote 9 describes.

How a dealer reads a full panel, including which tests were actually run, is the well water testing guide. The software that holds the customer file is water treatment contractor software. This article does not size a filter and does not tell a household the water is safe to drink.

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