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Sept2022 – PFAS Inner Iimits

The PFAS Health Advisory Limits – Welcome to the Inner Limits!

On June 15, 2022, the US EPA published the Interim Updated Health Advisory Limits (HALs) for PFOA and PFOS of 0.004 ppt – that’s 4 ppq. While the US EPA acknowledged at a public meeting that these limits cannot be achieved by current analytical technologies and referred to these as “goals,” state agencies and permit writers will likely run with these impractical and unachievable limits as enforceable standards, meaning any positive detection will be considered a non-compliance.

Ironically, recent studies conducted on rainwater have observed PFAS being detected in almost all samples collected in excess of these HALs, many by an order of magnitude (low single-digit ppt). One such study was published on August 2, 2022, by Stockholm University in Zurich whereby the lead author, Ian Cousins, proclaimed “Based on the latest U.S. guidelines for PFOA in drinking  water, rainwater everywhere would be judged unsafe to drink.” (Environmental Science and Technology, August 2022). Notwithstanding the technical practicalities, one needs to wonder how we are to press these challenging inner limits.

I will let others debate the legitimacy of the real or perceived toxicity of PFAS at ppq concentrations given the fact that PFAS rains down on us, apparently every time it rains. As I am a Chemist, I am routinely asked by my clients to assist them in achieving MECA detection limits for their analyte of concern, whether it be in wastewater effluent or groundwater.  Trying to be a good consultant, I always ask the questions – Why do you want to do that? And, what will you be using those data for? (They will have to generate it at that level, forever, by the way). Sometimes there is a very good reason, and other times I talk them off the ledge and suggest pushing back on whomever is requesting/mandating going to those inner limits, taking control of the horizontal and vertical on your TV set (remember?).

Here and now we have amazing liquid chromatography/tandem mass spectrometry (LC/MS/MS) technology that can reliably detect single-digit ppt levels of most currently targeted PFAS in drinking water, albeit I still wonder about the need or concern about those levels after 70 years of all of us having snacked on PFAS. But I digress – Since I have wondered about the technological challenges of achieving these HALs, myself and five of my senior colleagues across other consulting firms and reputable laboratories gathered on a series of calls and asked the question – What if? Specifically, what if we do things to amp the drinking water methods (US EPA Method 537.1 and US EPA Method 533) to potentially achieve those inner limits of 4 ppq.

To begin with, these drinking water methods are not subject to modification by the US EPA so everything stops short there if drinking water data are to be used for compliance purposes. The next opportunity for the US EPA to update these methods is in 2024.  But for the moment, let’s assume my colleagues and I have free rein to modify-away, wave our wands and:

  • Increase sample extraction volume to 2 liters from 250 mL,
  • Decrease extract final volume to 200 µL instead of 1000 µL,
  • Increase the instrument injection volume by 10 fold, and
  • Accept lower signal-to-noise ratio – reducing from 10 times to 3 times for a signal to be considered a real detection.

Doing all those things could theoretically get us a detection limit reaches that inner limit – a double-digit ppq. To the untrained eye, these modifications may look pretty straight-forward, right?

So, let’s review the consequences:

  • US EPA Method 533 specifies elution rate (5 mL/min); run time increases to 7 hours per sample, currently each sample takes about 50 minutes (seven times longer to extract every sample).
  • Extracting a larger volume will change the efficiency of extraction since the extraction is an equilibrium process. Also eluting heavier PFAS analytes from filter cartridges would be a significant challenge. All of which will require the methods to undergo new multi-laboratory validation.
  • The logistics of collecting and shipping large volumes of samples, plus quality control (QC) samples and additional laboratory sample storage space are problematic, resulting in cost increases.
  • The lower the PFAS concentrations we can detect, the more likely we will see “false positive” laboratory results. These can be introduced as a result of sampling techniques and field equipment. Similarly, the sample containers themselves can contain PFAS, even “certified” laboratory reagents can contain PFAS. Finally – let’s not forget that even if the PFAS detections are real, PFAS are everywhere – even in the rain.

So, what does all this theoretically amping of the drinking water methods mean? Basically, although it is theoretically possible to achieve low ppq limits, my merry band of six senior scientists concluded that the technical, logistical, and financial consequences of attaining these levels make it a highly impractical endeavo

Gary Yakub