Acrylic is basically "naturally" flameproof but it will melt, which isn';t a walk in the park.
I don't know about it being carcinogenic, at least not any more so than polyester. Let's face it, it is a petroleum product, like all plastics. Most of the info I can find about it is how the industry is making a shift away from carcinogenic dyes, flameproofing and processing methods.
Also, a lot of the dyes used on yarns both natural and synthetic aren't exactly good for you.
The stuff that makes wool superwash is a resin that I don't know as I would drink or anything, but I have not found it to be a huge issue. It is also a plastic product.
Mamid, if you want to play with natural fibers, I might have a source for you in BC that mainly sells alpaca and llama wools. Prices will probably be fairly reasonable (they are for me) but I will have to find the snail mail address. This could take a couple of weeks, as we are mid-move. If I track it down, i will PM it to you.
ETA: most of the cancer causing stuff would be washed out long before the product ever reaches the consumer, so it is not the time for general alarm. Mostly the processes are worse from an environmental point of view than a personal health point of view.