I don't know very much about the plastic used to make night guards. However, my mom happens to be a dental assistant. She was able to get the manufacturer's Material Safety Data Sheet (MSDS) for night guards from a dental lab.
Unfortunately, the MSDS sheet isn't all that helpful. Half of the ingredients are ethyl methacrylate (48 per cent), hydroquinone (one per cent) and P-methyoxyphenol (one per cent). The other 50 per cent of the ingredients are called "Confidential Ingredient A", registered as a trade secret.
Ethyl methacrylate is a base material for coatings and adhesives. It's used in a host of things, from resins to paper finishing to dental products. I can't speak to its safety, and the trade secret ingredient is a huge stumper.
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You don't need to be related to a dental assistant to find out about the products used in your mouth. Ask your dentist to find out more about the product and get contact information for the company. A call or letter to the manufacturer from a concerned consumer might reveal more. And your dentist is your advocate.
Another point worth mentioning is that the Canadian Dental Association is one of 12 national organizations representing all areas of the health sector, along with the David Suzuki Foundation, that have produced a joint statement spelling out a commitment to creating an environmentally responsible health sector. A primary action is "that individuals working in the health care sector both model and advocate for environmentally responsible approaches to delivering health care without compromising patient safety and care."