Speculation should not be presented as fact.
We don't know what is causing these cases but you have presented a reasonable hypothesis.
Common cutting agents in the licensed market are PEG200, PEG300, and MCT oil. None are those are proven safe from what I know.
In the black market the initial cutting agent was PEG600. Then TrueTerpenes came out with Viscosity ( https://trueterpenes.com/products/diluent/viscosity-extract-... ). Initially they said it was all terpenes (simple alcohols found in cannabis responsible for some effects and flavor) to sell it. It later came out that it was "mineral oil" but the reality is no one knows exactly what it is but it's a petroleum-based product of some kind. There was backlash against TrueTerpenes for this but selling mineral oil at $1500/liter (price point for cutting agents) is profitable. Some reports have described the symptoms as consistent with lipid pneumonia.
Next FloraPlex released their thickening agent. Then HoneyCutt. I started hearing stories of people selling 0% THC epoxies to unsuspecting buyers as distillate starting a few months ago.
Now there are people making their own thickening agents.
Is it a theory? Yes. I'm normally scientifically conservative but I'd place a friendly wager on this one.
My issue was only that you followed a bunch of informative facts with an untested theory that was not differentiated from the facts that preceded it.
In my research, one finding that stuck out [2] was that natural oils (triglycerides, saturated or unsaturated) are more pro-inflammatory than straight-chain mineral oils. MCT oil would absolutely cause acute inflammation and in severe cases, collapse of alveoli.
However, the long chains, longer than C14, have half-lives measured in months to years. The effects of these is harder to know, but excessive exposure causes essentially COPD.
Edit: oh look, here's an article specifically pointing out lipoid pneumonia [2]. I actually disagree with their thesis, that vegetable glycerine is causing the inflammation. I would bet it's more likely to be one of these thickeners, or a carrier agent for a flavor, causing the inflammation. I know people who go through over an ounce per day of VG solution that aren't getting chemical pneumonitis, but inhalation of sub-milliliter amounts of oils has sent friends of mine to the hospital.
Basically, if it's not air, it doesn't belong in your lungs. However, the degree and character of the damage done depends on the chemical composition.
Shorter molecules, molecules with more hydrophilic nature (alcohols, polyols, ketones, acids), biologically active molecules, are more acutely inflammatory, but usually the body can deal with them. Longer molecules, biologically inert ones, are less acutely reactive to the immune system, but can cause disruption of membranes and long term toxicity. Unsaturated fatty acids are especially bad because they can be oxidized directly into inflammatory signal chemicals. [4]
[1] https://en.wikipedia.org/wiki/Chemical_pneumonitis
[2] pdf - https://www.ajronline.org/doi/pdf/10.2214/AJR.09.3040
"Mineral oil (a mixture of inert, long-chain, saturated hydrocarbons obtained from petroleum) and vegetable-based oils tend to cause minimal to mild inflammatory reactions (Fig. 3). The intraalveolar oils can coalesce in the alveoli and become encapsulated by fibrous tissue, resulting in a nodule or mass (paraffinoma) (Fig. 4). Conversely, animal fats are hydrolyzed by lung lipases into free fatty acids that trigger a severe inflammatory reaction that manifests as focal edema and intraalveolar hemorrhage [5]. "
[3] html - https://casereports.bmj.com/content/2018/bcr-2018-224350
[4] https://www.frontiersin.org/articles/10.3389/fimmu.2014.0048...
"unsaturated fatty acids can be oxidized in a tightly regulated and specific manner to generate either potent pro-inflammatory or pro-resolving lipid mediators. "
[1] https://www.frontiersin.org/articles/10.3389/fimmu.2014.0048...
I wonder if you have any words on "MCT" being too broad a term. I made my carts with Bulletproof's Brain Octain, which claims to be 100% caprylic acid (C8).
I'm not naive enough to ask "Is it safe?" But I'm curious if you think an 8-chain MCT would treat your lungs significantly different than a 12-chain MCT?
I could easily imagine someone learning that you can make carts with MCT, and finding something cheap, only to be unknowingly using 100% C12. Cue the lipid pneumonia.
C8 is really fascinating because, I reckon, it's too short to set off the inflammation cascade directly, so it would be up to macrophages to alert the body "hey this doesn't belong". However you still have the usual risk from hydrophobic product in the lung.
Personally, I would use vegetable glycerine and nothing else if you can at all avoid it. PG is a bit more irritating than VG, but any short polyol is going to be diffused away into the mucous much more easily than hydrophobic products.
For marijuana/hemp you can make a water-based tincture using VG as the solvent just using dried plant matter. A magical butter machine can do it, putting it in a bottle and shaking it up every day for a few months, there are likely other methods. It's similar to making a CBD/THC oil but instead using VG as the carrier.
Making your own tincture from decent-quality dried plant matter is the most economical and surefire way to know that it unadulterated and limited in contaminants.
Theatrical fog causes "irritation of mucous membranes such as the eyes and the respiratory tract associated with extended peak exposure to theatrical fog. " Another study "also found long-term exposure to smoke and fog was associated with both short-term and long-term respiratory problems such as chest tightness and wheezing".
Cigarettes are terrible, but vaping ain't vitamins.
https://en.wikipedia.org/wiki/Fog_machine#Adverse_health_eff...
Not all theatrical fog is the same as vaping as far as I am aware. If you have any articles specifically dealing with the components used for vaping I would be interested to read them. Vaping does not use oil.