Mainly air (oxygen), I think.  It's definitely not light as only certain pieces of the SNES are affected, such as the top half but not the plastic around the cartridge slot, or the bottom half.  I don't think I've ever seen around the cart slot turn yellow.  They should have made the SNES out of that.  I like how my NES doesn't turn yellow.  They really got cheap with the SNES.