PFBS (Perfluorobutane Sulfonate)

What is PFBS (Perfluorobutane Sulfonate)?

PFBS is a four-carbon PFAS that replaced PFOS in stain-resistant products after the early-2000s phase-out.

PFBS is the four-carbon member of the sulfonate PFAS, and it exists largely because of its eight-carbon cousin. On May 16, 2000, EPA and 3M announced that 3M would phase out PFOS chemistry, which went into Scotchgard, firefighting foams, and treatments for fabric, leather, and paper. The industry's answer was shorter chains. The Interstate Technology and Regulatory Council (ITRC) reports that 3M kept using a four-carbon route, with PFBS in place of the six-, eight-, and ten-carbon compounds, and finished most of the phase-out by the end of 2002. EPA's own PFBS page gives the reason: PFBS had the properties the class is valued for, with faster elimination from the body than PFOA and PFOS. Shorter chain doesn't mean no concern, and it doesn't mean no data. EPA finished its human health toxicity assessment for PFBS in April 2021. It reported adverse effects in animals on development, the thyroid, and the kidneys, and it derived a chronic oral reference dose of 0.0003 mg/kg per day from decreased serum total thyroxine, a thyroid hormone. A reference dose is an estimate for risk assessors. It's a daily intake, not a blood level, and it says nothing about any one person. The human evidence is thinner. A May 2026 systematic review of 28 mother-child studies on PFAS and thyroid function described the effects of alternatives such as PFBS as weaker and less stable than those of PFOA and PFOS, and tied that to lower exposure levels and limited epidemiological evidence. Thin evidence is a gap in what's been studied. It isn't a finding of no effect. The half-life is what most separates PFBS from its neighbors in this glossary. Xu and colleagues followed airport employees in Ronneby, Sweden, whose drinking water had been contaminated by firefighting foam, after the exposure ended. They measured an average serum half-life of 44 days (95% confidence interval 37 to 55 days). A related Swedish study from the same research group reported a PFOS half-life of 3.4 years. These are different cohorts, so read the comparison as order of magnitude only. Because PFBS clears in weeks, a blood concentration can't serve as a record of how much PFBS someone has taken in over the years. It's a snapshot of recent exposure, and two results from different dates can differ simply because of what happened in the weeks before each draw. PFBS also sits inside a drinking-water rule. EPA's April 2024 National Primary Drinking Water Regulation set a Hazard Index of 1 for mixtures of four PFAS. You calculate it by dividing each concentration, in ng/L, by a health-based water concentration and adding the results: PFHxS over 10, PFNA over 10, HFPO-DA (GenX) over 10, and PFBS over 2,000. EPA deferred a stand-alone limit for PFBS while it looked further at whether PFBS occurs individually in public water systems at levels of health concern. So PFBS counts only as part of the mixture. The rule took effect on June 25, 2024, with initial monitoring due April 26, 2027 and compliance due April 26, 2029. As of October 2026, that mixture rule is unsettled. On May 18, 2026, EPA proposed rescinding the Hazard Index and the individual limits for PFHxS, PFNA, and HFPO-DA, arguing that the 2024 determinations skipped procedure the Safe Drinking Water Act requires. The comment period closed July 20, 2026, and a policy tracker last updated September 18, 2026 lists no final rule. On March 19, 2026, the D.C. Circuit declined EPA's request to sever and pause the legal challenges to those four standards. If the proposal were finalized as written, the Hazard Index, the only federal drinking-water provision that names PFBS, would be removed. The separate PFOA and PFOS limits are covered on PFOA (Perfluorooctanoic Acid). Whatever happens, a drinking-water limit applies to water systems. It isn't a blood threshold. Blood data on PFBS are sparse. CDC's NHANES measured PFBS in serum in its 2009-2010, 2011-2012, and 2013-2014 cycles with a lower limit of detection of 0.10 ng/mL, and almost no one reached it: 19 of 2,233 participants in 2009-2010 (0.85%), 23 of 1,904 in 2011-2012 (1.2%), and 15 of 2,168 in 2013-2014 (0.7%, aged 12 and older). In a separate 2013-2014 sample of children aged 3 to 11, 21 of 525 (4.0%) were at or above the limit. After that cycle PFBS left the serum panel: the 2015-2016, 2017-2018, and 2021-2023 PFAS files list nine or ten other analytes and no PFBS. NHANES blood data therefore can't describe how widespread PFBS is in the population. PFBS is also outside the seven-PFAS sum in the July 2022 NASEM guidance (see PFOS (Perfluorooctane Sulfonate) for that framework), and Quest's PFAS 9 panel, dated June 2024, leaves it out too.

Typical reference context

These numbers describe a laboratory's detection limit, how rarely a national survey found PFBS, and values EPA built for water and for risk assessment. None of them is a target or a cutoff for a person's blood result. Labs report blood in ng/mL and water rules use ng/L, a thousandfold difference in unit size, so the 2,000 in the table and a 0.10 on a lab report are not comparable.

What changed recently

The newest developments concern regulation and research, not new blood data. In May 2026, EPA proposed two drinking-water actions: rescinding the Hazard Index and three individual limits, and offering PFOA and PFOS systems a two-year extension, from April 2029 to April 2031. Comments on both closed July 20, 2026, and no final rule had been listed by September 18, 2026. On March 19, 2026, the D.C. Circuit denied EPA's request to hold the challenges to the four standards in abeyance. In May 2026, a systematic review in the Journal of Toxicology (28 mother-child studies) concluded that thyroid associations for alternatives such as PFBS are weaker and less stable than for PFOA and PFOS, citing lower exposure and limited human evidence. NHANES released its August 2021 to August 2023 PFAS file with nine analytes and no PFBS, so there are still no new national blood data. We found no EPA toxicity assessment for PFBS newer than April 2021; the 2024 rule still cites it.

Tests often checked alongside this

PFBS usually appears, if at all, in a larger PFAS report. The compounds that standard panels list are covered on PFOS (Perfluorooctane Sulfonate), PFOA (Perfluorooctanoic Acid), and PFHxS (Perfluorohexane Sulfonate). EPA's reference dose rests on total thyroxine (T4), a thyroid hormone, and its assessment also reports kidney effects in animals, so thyroid and kidney tests are the ones the PFBS literature touches. Whether any of them is relevant for you is your doctor's call.

Where PFBS turns up

EPA's description of the 2021 assessment, as reported by Water Tech Online, says PFBS has been detected in surface water, wastewater, drinking water, dust, carpeting, carpet cleaners, and floor wax. Because the half-life is short, the timing of a blood draw relative to any contact matters more for PFBS than for long-chain PFAS. Longetic doesn't assess anyone's exposure; your doctor can ask about the history.

What a single result can show

A PFBS value on a report is one number from one date, measured by one lab with its own reporting limit. The unit is usually ng/mL, and results at or under the limit print as "<" followed by the limit. Because some PFAS panels, such as Quest's, leave PFBS out, a missing line doesn't mean it was measured and not found. Keeping each report with its date and the lab's printed limit lets you see your own values side by side, without reading more into any single one.

What to ask your doctor

• Was PFBS part of the panel I had, and if not, is there a reason it was left out in my case?
• What was the lab's reporting limit for this result, and what does a result under that limit tell us?
• Does the timing of my blood draw, relative to any contact I've had, change how you'd read it?
• Which of my other results, such as thyroid, kidney, or lipid tests, do you want to look at alongside it?

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