I'm going to counsel you to stop stressing about these nonexistent or trivial risks. Hundreds of products you expose yourself to every day have trace chemicals in them, and exhibit off-gassing in almost immeasurable amounts. If I gave you a list, you would either: 1) appreciate that out of all the things you can worry about the ones you mention above are so far down the list that they are comparative non-issues, or 2) move to a desert island in the middle of the ocean to try to get away from all these exposures. You have acid produced in various parts of your body, you already have detectable amounts of formaldehyde in your body as a result of natural biochemical reactions, your kidneys produce ammonia, and when you use the bathroom you produce and inhale naturally produced methane. But a toilet seat, even in the unlikely event that it was off-gassing a tiny amount of formaldehyde is not going to give you cancer.

As for furniture, MDF can off-gas formaldehyde, but this tapers off fairly quickly, and it is typically covered with veneer, laminate, or a hard melamine coating that inhibits its release. Even if you get a piece of solid wood furniture, you don't know what chemicals went into the finish and the glues and adhesives. I've bought many types of furniture over the years, including some very expensive pieces from Europe, that all contained some amount of MDF. It's widely used for stability and to resist warping. I can't remember the last time I opened up a container of furniture and got a chemical whiff, and it's not something I worry about.

It's good to be environmentally aware, but it's not good to worry about every single product that possibly has a worrisome chemical compound in it in almost undetectable amounts. I'm not trying to make light of your concern, but give it some proportion.

I worked for decades at an environmental regulatory agency and I have a healthy respect for toxic chemicals when they are at levels of concern, or when the exposure risks are statistically significant. I also used to deal with the public on a daily basis and often had to talk people down because they had just enough knowledge to scare themselves about various materials, but not any toxicological knowledge to be able to assess that what they were worried about was far less risky than, say, walking across the street. That doesn't mean that I found people never had valid concerns; sometimes they did and sometimes they were significant. But what you described above is not a situation about which you should continue to stress.