I looked at the formula and that Oxygen double bond attached to a carbon with another oxygen bond... yikes!
I have no idea why this is stable. It appears stable, but you wonder about things like UV light, exposure to some cation, etc...
Well, seems like I have reading to do. (Chem engineer here, I feel like its my responsibility, similar to how programmers read open source code)
If you look into molecular orbital theory, the double bond causes overlap, which makes the bonds unstable. It can more easily jump off and create a new bond. If you want to see an even worse situation, look at Nitrogen triple bonds.
Further, the extra oxygen attached to the carbon makes the bond a bit polar since you have such a heavy electron clump on one side of the carbon.
That said, yeah, PLA is known to not be very stable, and that's part of the reason its biodegradable. It degrades into lactic-acid based large molecules that living beings have no problem on destroying. (But are toxic when you get several grams of it, so beware if you are doing strange chemical experiments with PLA.)
PLA is just going to break down into lactic acid. Big deal.
Breathing smoke and dust from PLA is bad, because breathing any type of smoke or dust is bad. Wildfire smoke is bad, but wood is not a harmful chemical.
We actually do understand the chemistry of plastics. And to a very large extent they are very safe, primarily because they are incredibly stable and non-degradable.
Even the studies of e.g. BPA health issues point to very weak effects. You need to do meta-analysis on tens of thousands of individuals to have statistically significant effects, and even then they are weak and it's not easy to say whether you have successfully eliminated confounding factors.
And "BPA-free" plastics just use BPF instead, with very similar chemistry. The consumer-facing BPA fuss is primarily driven by companies that want to sell products; health agencies are primarily concerned with occupational exposure, which is 4-5 orders of magnitude larger in terms of dose per body weight. BPA is metabolized in the kidneys to the benign BPA-G and excreted in urine; a dose 250 000 times the EU dose limit is cleared by the body in 24 hours.
PFAS is a similar story. Over long timescales with stupid disregard for basic PPE, like cross country ski waxing trailers where people were breathing C8-dust without masks for a thousand hours per year, there has been clear health concerns identified. So people are saying "there is no safe lower dose limit" and companies are switching to chemicals that haven't been used in the same context so there is little to no data on exposure safety, and saying "this will be safer".
I'm not asking to play 'gotcha' with your comment, but because when I've attempted to do my own research on this, I find it easy to get information on the polymer itself, but the polymer is never used by itself in a product. As far as I can tell, most polymers are not actually useful without a cocktail of additional chemicals added to them to change their physical characteristics. From my research, raw polymers tend to be extremely brittle and frangible, completely unrecognizable when compared to the products we use made from them.
And those additional chemicals are extremely difficult to find information on. Like when they're used, what they are, etc.
I guess my mind is instinctively reaching for the ways FDA attempts to ensure food safety by compelling manufacturers to publish ingredient lists, and by maintaining registries [0] of hopefully-well-understood additives (and banning the rest, in historically dangerous categories like colorants).
Sure from time to time agitators take that information out of context to whip up a scare about one ingredient or another, and sure some of the food that comes in from overseas has after-the-fact labeling that’s a little sus. But at least there’s an expectation that the information must exist and be legible.
[0] https://www.fda.gov/food/food-ingredients-packaging/determin...