Incorrect masks used in the movie "Alien"

During my brief break from blogging, I had a chance to watch part of Ridly Scott's "Alien," and noticed an error in the filming that I hadn't been aware of during previous viewings.  After John Hurt is brought back to Nostromo, he is placed in the infirmary; Ash and Captain Dallas enter and ponder how to remove the facehugger.

For this scene they are dressed in surgical gowns and are wearing masks - BUT the masks chosen for the film are not isolation masks; in fact these masks have absolutely zero filtering capacity.  They are simple oxygen masks.  In the screenshot above, the mask Ash is wearing clearly displays the nipple to which in real life one would attach the green oxygen tubing.  He is also wearing the mask improperly; it should fit under his chin, not be lodged against his lower lip.

Captain Dallas wears the same oxygen mask (and obviously without any oxygen).  When Hurt's body is moved into the scanner, they stay in the room but remove the masks (which would not be appropriate in an infectious environment) until his body comes out of the scanner.

Anyone with even the most rudimentary knowledge of hospital procedures and equipment should know that these masks are not isolation masks.  I presume the director opted for being able to visualize more of the actors' faces rather than for scientific accuracy.

This error is not recorded at the Movie Mistakes website - but there are 47 others listed, some of them quite interesting.