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Comment by cato_the_elder | Hacker News Reader

This is an exaggeration, of course. The EPA guideline of 70 ppt for PFAS is probably just too low compared to other standards. For example, a Canadian guideline suggests an MAC of 200 ppt for PFOA, and 600 ppt for PFOS. These are the most common PFAS. Also, these are based on a lifetime exposure, and short term exposure to these amounts is probably not particularly harmful.

Additionally, even in the US, the guidelines by the individual states vary between 13 to 1000 ppt. [2]

[1]: https://www.canada.ca/en/services/health/publications/health...

[2]: https://www.nature.com/articles/s41370-018-0099-9

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Since PFAS never really breaks down and we're still creating it, is a 200 ppt inevitable at some point?

The scary thing for me is that it's in the rain, so it'll be in crops, water reservoirs, trickling into the water table and into drinking supply. We have to stop this right now.

> You can't use techniques like distillation to remove PFAS

Can we get a citation on this? I’m not a chemical engineer, but distillation seems like a pretty good way to get rid of virtually everything.

I'm sorry, I had it wrong. I've checked my facts. It works. This is actually a relief for me.

Rain is a form of distillation.

This is a good point. I don't know what to think now. I presume 3M isn't spraying this stuff into the air as an aerosol.

PFAS components do end up in the atmosphere from the manufacturing process, which is how it precipitates (droplets nucleate around the airborne particles).

Same story with any particulate emissions, like soot, dust, tires, etc.

a form of distillation with virtually no isolation, compartmentalization, and unlimited precursor conditions, which pours into a huge mass of possibly dirty air and debris.

Some things absolutely distill over, it’s not magic, but depends on (to simplify) the boiling point of the substance compared to the boiling point of water.

For example it is not possible to completely separate ethanol and water with distillation.

A quick internet search seems to indicate PF* can be separated with distillation.

Most of the PFAS do break down -- a half life of a few years. It's mostly PFOA that's of grave concern.

Citation? I tried googling and couldn't find anything to confirm that.

When I searched on half-life, the results that came up mostly seemed to discuss the half-life in the body, not the half-life in an environmental sink such as the ocean.

> the results that came up mostly seemed to discuss the half-life in the body

Yah, there's a lot of data on "serum" or "elimination" half lives.

What you care about is "atmospheric", "aqueous abiotic", and "aqueous biotic" half lives.

So, e.g. for PFOA, atmospheric half lives are about 130 days... https://pubs.acs.org/doi/pdf/10.1021/jp036343b

I can't come up with a link to share for PFOA in aqueous solutions, but it looks like 90-150 years.

It's an interesting article from 2004; might not be reading it right but seems like this one from 2020 is less optimistic about hydroxyl radicals (OH) breaking down PFOA in the atmosphere. https://pubmed.ncbi.nlm.nih.gov/31978654/

Your same link notes that the majority of PFOA is not broken down in the atmosphere and points to "wet and dry deposition" mostly, which sounds like it's being dropped down to Earth remaining as PFOA.

> It's an interesting article from 2004; might not be reading it right but seems like this one from 2020 is less optimistic about hydroxyl radicals (OH) breaking down PFOA in the atmosphere. https://pubmed.ncbi.nlm.nih.gov/31978654/

That's an interesting paper. If so, multiply the atmospheric time by 2-4.

> Your same link notes that the majority of PFOA is not broken down in the atmosphere and points to "wet and dry deposition" mostly, which sounds like it's being dropped down to Earth remaining as PFOA.

Yes. That doesn't affect the "half life" here.

In the end, not much PFOA ends up in the atmosphere in the first place, and most precipitates out to less convenient places where it lives longer. Of the stuff resident in the atmosphere, it does break down on human timescales, at least...

Guidelines for all of these things individually seem to ignore the likely unstudied possibility that they have synergistic effects. Or even just additive effects because many of them are so similar to each other.

What short term exposure? if it's in the rain in Antarctica then you can bet it's in your water, vegetables: it's likely a lifelong exposure.

Research suggests even the EPA recommendation is too high.

How can we have a guideline for ALL PFAS? They are all completely different molecules with different Hazzard levels - likely many safe.

These generalizations make conversations about them useless.

There is insufficient evidence for "likely many safe". You can't just start throwing that around.

The burden of evidence is demonstrating that they are each unsafe.

Lumping a hundred chemicals under a label and then claiming that any time ANY of them are found that they are harmful, is dishonest.

Which generalization should we change ? The "they aren't safe" or the "likely many safe" one?

The guideline is 0.02 ppt for PFOS and 0.004 ppt for PFOA.

I think you have the units, the number of the zeroes, and and the actual figures wrong. Each µg/L is 1,000 ppt (part per trillion, i.e. ng/L).

No.

1 µg/L = 1,000,000 ppt

1 ng/L = 1,000 ppt

You can find the figures here:

https://www.epa.gov/sdwa/questions-and-answers-drinking-wate...

Rain water and ocean spray already exceed these limits by hundreds of thousands to several million times.

> 1 ng/L = 1,000 ppt

I don't think so. See here (the green box in the upper right corner of page 2): https://www.epa.gov/sites/default/files/2015-09/documents/ep...

> nanograms per liter (ng/L) = parts per trillion (ppt)

Or alternatively: https://pubs.usgs.gov/circ/circ1133/conversion-factors.html

I don’t know what to tell you. Is this one of those billion/trillion differences again? In that case the EPA values are definitely off by a factor of 1000 when compared to SI units, so maybe rainwater is then only some hundreds to some thousands of times over the limit.

https://www.google.com/search?q=1+ng%2FL+in+parts+per+trilli...

I think it's just that Google's instant answer is scraped from endmemo.com, and that website has it wrong.

If you check the second search result (https://www.llojibwe.org/drm/environmental/content/concentra...), it also corroborates what I'm saying:

> 1 nanogram/liter (ng/l) = 1 ppt

Technically speaking, ppt/ppb/ppm should be used for dimensionless quantities, but the convention used here is kinda contrary to that. The argument for the convention is something like this: 1 L ~= 1kg of water, so 10^-12kg of something per 1 L of water can be considered as dimensionless, since the kg and the L cancel out.

It doesn’t have it wrong, it just uses long scale trillions and the US EPA uses short scale trillions.

Oh, interesting! I'd never heard of the short/long scale thing before.

You could argue that it's still wrong though, in the sense that the website is in English, and currently the short scale is used in all main Anglosphere countries. [1]

P.S. I just realized that even the short/long scale thing doesn't explain the endmemo.com claim. It's probably just a mistake. In the short scale, a trillion is 10^12, while in the long scale it's 10^18. So, if we were to use the long scale, 1 ppt would a smaller unit by a a factor of 10^-6 (not 10^-3), and equal to 1 fg/L (femto is the SI prefix for 10^-15, while nano is for 10^-9).

In other words, 1 ng/L = 1,000,000 part per long scale trillion.

[1]: https://en.m.wikipedia.org/wiki/Long_and_short_scales#Short_...

I'd settle for 0 ppt.

I'd settle for what's already here and it's too late, but stop this industrial madness right now without further delay.

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