Something just wasn't sitting with me about the document you shared [0] and toxicity categories [1], so I asked my partner, who is an environmental engineer, to explain it to me.
The EPA's toxicity categories [2] are determined by the amount of a substance that will kill you. So if it only takes 50mg/kg or less to kill you it's highly toxic, where as if it takes greater than 5000mg/kg to kill you it's considered non-toxic.
As my partner was explaining to me how to interpret the page, she furrowed her brow because she couldn't figure out how Dipropylene Glycol was Cat 4 and not Cat 3. Per the EPA guidelines the reported lethal levels for Dermal and Inhalation should make it Cat 3.
Further, the document says "Upon reviewing the available toxicity information, the Agency has concluded that there are no endpoints of concern for oral, dermal, or inhalation exposure to propylene glycol and dipropylene glycol." But if you look at the data provided, there is no inhalation exposure data for Propylene Glycol.
So why are we assuming PG is safe to inhale? And why is DG considered Cat 4 and not Cat 3? It's not entirely clear but my partner suspected lobbying which she said is quite common by industry groups like the American Chemistry Council.
I looked around and couldn't find anything specifically related to PG but the American Chemistry Council's Ethylene Glycol Ethers Panel petitions the EPA quite a bit to reclassify and remove reporting requirements. An example is their petition to remove reporting requirements for ethylene glycol monobutyl ether [3] which has similar toxicity categories to PG and DG. When you look at their section on Acute Toxicity, the rationale is not that it's safe but rather that it's unlikely to kill anyone the way it's used and stored in industry. So a lot of the justification that things are safe is based on how it's used. Vaping is a relatively new usage for PG.
When you look at the EPA's executive summary of propylene glycol [4] you can see that there's very little data around inhalation. The one thing I did find was a link to a Military Exposure Guidelines document [5] which indicates that Propylene Glycol exposure at 500 mg/kg (0.5 mg/l) was critically dangerous and resulted in convulsions in Monkeys. Wikipedia's article on propylene glycol [6] however references a study where subjects were exposed to 871 mg/m3 (0.8 mg/l) and only reported mild respiratory irritation. Keep in mind that your original document stated 2.4 m/l would kill you. So I'm inclined to believe that MEGs are closer to the truth than a Wikipedia study locked behind a pay wall.
The more I look into this, the more I see that there's very little research about the long term effects of exposure AND more importantly that much of the categorization, which predates vaping, is based on how it was being used at the time but does not take into account modern use cases.
My conclusion is that the CDC is right and people need to stop breathing this shit until we better understand it.
[0] https://archive.epa.gov/pesticides/reregistration/web/pdf/pr...
[1] https://en.wikipedia.org/wiki/Toxicity_category_rating
[2] https://www.govinfo.gov/content/pkg/CFR-2005-title40-vol23/p...
[3] https://19january2017snapshot.epa.gov/sites/production/files...
[4] https://comptox.epa.gov/dashboard/dsstoxdb/results?search=pr...
[5] https://phc.amedd.army.mil/PHC%20Resource%20Library/TG230-De...
[6] https://en.wikipedia.org/wiki/Propylene_glycol