How much PFOS (perfluorooctane sulfonate) does it take to be toxic to people and animals? Scientists and regulators are racing to better understand and measure the toxicity of PFOS, one of the most common chemicals that falls under the larger category of PFAS (per- and polyfluoroalkyl substances). To do this, they are turning to a small, striped fish in the minnow family.

Highly sensitive to pollutants, zebrafish rank first among fish and fourth overall in PFOS susceptibility (USEPA 2022). Understanding their response to PFOS is key to setting safe water limits for PFOS-contaminated ecosystems.

Geosyntec’s Zach Pandelides, Jenn Arblaster, and Jason Conder are the forefront of PFAS risk assessment and ecotoxicology. Their recent work, published as the cover story in the January 2024 edition of the peer-reviewed journal Environmental Toxicology and Chemistry, delves into the adverse outcomes of long-term PFOS exposures to zebrafish through a review of 12 key ecotoxicological studies.

Below, Zach and Jason discuss what zebrafish PFOS toxicology means for regulators and why robust toxicity studies and data quality are crucial for understanding water quality when PFAS is detected.

About the Speakers

Zach Pandelides, PhD, is a scientist who specializes in assessing environmental risk and sediment and water quality. He has coauthored more than 10 peer-reviewed articles on environmental toxicology, chemistry, and molecular biology and has presented technical work at numerous scientific conferences. His recent work has involved ecological risk assessment regarding PFAS and obtaining critical data to assess the toxicity of PFAS to marine organisms.

Jason Conder, PhD, IBERAD, is a principal scientist with more than 15 years of experience addressing contaminated sites and environmental risks. An internationally recognized expert on PFAS, Jason has conducted product- and site-specific PFAS fate and risk assessments and formulated guidance for the United States Department of Defense, including a PFAS “Frequently Asked Questions” document and guidance on PFAS ecological risk assessment.

Q & A

What do you think is the headline, when discussing using zebrafish to understand toxicity?

Zach Pandelides

We published two papers on zebrafish. One is a wide-ranging review of 12 key ecotoxicological studies, covering all the existing research and studies on PFOS’s effect on zebrafish. The other is a 2024 laboratory study we coauthored with the United States Army Corps of Engineers and the USEPA. The second one is also included in the wider study and is published in Environmental Toxicology and Chemistry’s January issue.

In this case, our review indicates that we’re seeing that PFOS has no effect on zebrafish at concentrations of 30 micrograms per liter or lower. Low-level effects start to appear when concentrations are around 100 micrograms per liter.

These two numbers, 30 and 100, are really important for estimating a PFOS criterion that will be applied to protect aquatic ecosystems. However, there’s been a lot of confusion over what numbers to use. A previous study from Europe that suggested effects that occurred at much lower concentrations, and some regulatory agencies used this data in their calculations of safe levels.

Jason Conder
The European study is called the Keiter et al. (2012) study. It suggested that effects could occur in zebrafish at a much lower concentration in water. Using this study would lead to lower, more stringent criteria. However, the study was limited in its design and data analysis and not really sufficient for identifying the thresholds for regulatory uses.

The United States EPA didn’t use it to develop their PFOS criteria. But 8 years ago, environmental regulators in Australia did, likely because there were less data on the toxicity of PFOS to fish back then. However, based on the work we’ve published recently, as well as other zebrafish PFOS studies, we haven’t seen the low effects that the European study claimed.

Why does this difference in toxicological studies matter?

Jason Conder
Toxicological studies are meant to provide regulators with high-quality data they can use to derive criteria for targeted and site-specific ecological risk assessments. These criteria are used to evaluate measurements in water at sites where PFOS is detected so we can protect aquatic life.

Some may say there’s no harm in setting criteria very low so that we can be protective. But if the criteria are too low, you end up crying wolf a lot, suggesting that there’s a problem based on chemistry measurements, when in fact, the fish may be fine.

When every detection of a chemical sets off alarm bells, you’re diverting time, money, and attention from other chemicals or other environmental challenges that could actually be harmful.

Australia has spent millions addressing the perception that PFOS is extremely toxic in water; this perception is based on the PFOS criterion they calculated in 2016. As I mentioned, a big factor in the PFOS criterion was the limited scope using the Keiter et al. (2012) study findings.

But we’ve learned a lot on PFOS ecotoxicology in the last few years. We hope this new take on zebrafish toxicology will lead their agencies to reconsider their PFOS criterion. With a less stringent, but more accurate criterion, society can focus resources on bigger water quality challenges.

What makes your 2024 laboratory study more trustworthy?

Zach Pandelides
Our study covers a wider concentration range of PFOS, including four concentrations below 100 micrograms per liter, to capture the range of effects reported by Keiter et al. (2012) and see if they are reproducible.

The study featured many improvements on the Keiter et al. (2012) paper. It had higher replication, using five replicate tanks instead of the two. It also included more frequent sampling of the water and more evenly spaced exposure concentrations.

These changes are important because they give us greater confidence in the results, which showed that the Keiter et al. results were not repeatable and overly sensitive by roughly 100 times.

And you know, we spent a lot of time making sure that the data were analyzed and interpreted correctly, working with the US Army Corps of Engineers, the USEPA, and others. Plus, all the raw data is available for other folks to independently review.

Why did you publish in Environmental Toxicology and Chemistry?

Jason Conder
Environmental Toxicology and Chemistry is the go-to journal for understanding chemical effects on ecosystems. It’s a resource for government, academia, and industry.

Zach Pandelides

And it’s important for our papers to be seen by the people who can put our findings into action.

Speaking of action, do you have advice for people who manage PFAS contamination at industrial and construction sites?

Jason Conder
We have suggested that fish are affected by PFOS at concentrations of about 100 micrograms per liter, but they seem to be fine at concentrations of about 30 micrograms per liter or less.

If there’s a creek at your site, you may want to check if its water concentration is within this range, and whether fish are safe from PFOS exposure.

More broadly, we toxicologists say that the dose makes the poison. Just because PFOS is detected by a chemist in water does not mean it is at levels that will cause adverse biological effects.

More Information:

Read the paper from Environmental Toxicology and Chemistry: Establishing Chronic Toxicity Effect Levels for Zebrafish (Danio rerio) Exposed to Perfluorooctane Sulfonate – Pandelides – 2024 – Environmental Toxicology and Chemistry – Wiley Online Library

Read the second paper here: Survival, Growth, and Reproduction Responses in a Three-Generation Exposure of the Zebrafish (Danio rerio) to Perfluorooctane Sulfonate – Wiley Online Library
Read an interview with Jason and Zach on their work condensing 16 Purdue University studies into easy-to-reference PFAS screening levels for environmental managers: What Frogs, Toads, and Salamanders Can Teach Us about PFAS Toxicology

For consultation on PFAS, contact Jason at JConder@Geosyntec.com.

Listen to Jason describe human health concerns with PFAS: PFAS: Human Health Concerns | Jason Conder, PhD – YouTube
Learn more about Geosyntec’s PFAS services: Per- and Polyfluoroalkyl Substances (PFAS)