A few months ago I happened upon this question, dealing with the safety of using hot tap water for food preparation. Since this primary goal of this site is food preparation and this is a common method, I can definitely see why the general question is on-topic.
However, when I read the accepted answer and other answers, I noted the discussion of bacteria concerns, but no mention of the advice often given by official government organizations -- which is to avoid using hot tap water because of possible lead concerns (especially in very old buildings, but even in newer ones). I myself was somewhat surprised to see the number of warnings about this, but given our general advice here to cite official government organization guidelines for food preparation, I felt like I needed to address what they said.
Anyhow, I hesitated to even write up the answer to that question, because I didn't know whether giving advice to avoid lead poisoning would be on-topic here. I've poked around Meta a bit looking for questions related to this, and it seems like the closest advice I could find was that things dealing with acute medical problems caused by food safety (usually bacteria, but also toxins related directly to bacteria in contaminated food, or perhaps mold, etc.) were on-topic. Essentially, if it could send you to the hospital relatively soon due to food handling, it was on-topic. If the consequences were more likely long-term, it was a "nutrition" question and thus off-topic.
In general, this distinction seems reasonable.
But I'm still not sure how it deals with the question I linked above. Things like lead or mercury poisoning are of course real, but in some cases exposure levels may be influenced by food preparation vessels, techniques, etc. We allow various cookware questions, and if cookware (or food storage containers or whatever) would pose a threat due to harboring bacteria, I assume that would be on-topic. But would a question like "Is it safe to use this pot with some lead in X (paint, glaze, coating, metal alloy)?" be on-topic?
Similar issues came up in some other recent questions, such as here and here. The latter asks about "purifying" water (which brings up issues not only about bacteria or parasites but other dangerous dissolved compounds that could be contained in untreated water); the former involves potential long-term exposure of cookware materials at high temperature to water that may gradually concentrate any dissolved elements, including inorganic toxins.
Obviously we don't want to open the door to medical questions or questions about nutrition. On the other hand, when someone asks "Is this food preparation technique safe?" and we only address the bacteriological side of food poisoning when there is a threat of something like poisoning from other compounds, we arguably do more harm than good if we just answer by saying, "Yeah, the bacteria will be killed if you follow FDA/USDA guidelines."
So, I guess this boils down to a few related questions:
(1) Is the rough guideline I referenced above (which I can't seem to find right now in Meta), an accurate assessment of our general policy -- i.e., food "poisoning" causing acute medical problems from errors in handling or contamination is on-topic, while chronic "poisoning" is generally off-topic?
(2) Are food or cooking-related questions that involve recognized high-risk toxins (e.g., lead, mercury) ever on-topic, even if they may require chronic exposure to create risk rather than our typical bacteria/mold/etc. acute food safety questions? (By "recognized high-risk toxins" I mean things that organizations like the FDA, CDC, EPA, etc. might have long-established official warnings to the public about, rather than questionable chemicals and additives which may or may not be "safe" and are more properly the domain of nutrition/medical advice.)
(3) If a question asks about food "safety" where a risk of such toxin exposure (as in (2) above) is important to consider, are answers that address such risks on-topic? (E.g., "Is it safe to cook in X vessel/container using method Y?")