The bags-as they leave the factory-are light tight. The sharp edges of film and paper can and sometimes do punch a small pinhole in the bag. Film particularly. It is never a good idea to leave film or paper out in room light just in the black inner bag. Kodak, and formerly Agfa/Germany and Forte (both now defunct) packaged their film products in thick sealed foil and paper bags that were hermetically sealed. These bags, before opening are quite safe to leave out.
I have been using a bit of X-ray film lately, and I am quite amused at the flimsy packaging. The outer single layer boxes just have fold-over lids, and you can see the black plastic inner bag through the corners of the end of the box you open. But I haven't yet had any xray film fog. X-ray film has an ISO of about 100. But, of course, one of the reasons for using X-ray film is the cheap price. The film is cheaper than b/w photo paper (RC).