A lot of silicone is mixed with natural rubber as well. Health wise, silicone is probably much safer than plastic (better to rub sand and vinegar on your skin than petroleum right?) but it doesn't break down either, though it is easily recycled.
@Rosa its not plastic. Silicone is made from silica - which is a derivative of sand. Its mixed with acetic acid (derived from vinegar) neither will hurt you. Some are mixed with other things, but those are the basic components.
Silicone, is made from silica (sand) and acetic acid (found in vinegar) its not plastic anymore than glass (also derived from silica) would be, which is also why its heat resistant, but the other chemicals that make it flexible would be where the question would come in. In general, given a choice between silicone and plastic, go with silicone. Of all the things on the beach to worry about, the sand shouldn't be high on the list.
Good point. I often talk about the importance of identifying values in my classes, and it's one of the most under recognized by ultimately confusing things in the cross-cultural setting.