Some sort of mask that protects against hazardous gaseous materials.

That’s right, a gas mask. I’m sure people have done it before, but I can’t find anything.

I’d prefer it to be a new model( though I don’t care what kind of mask it is, Russian or otherwise) and be able to fit on the L4D characters heads, but if you want to hack it off of this guy you can do it.

Here are some reference pictures:
Russian GP-5

http://img.alibaba.com/photo/11113423/Gas_Mask_GP_5_GP_7.jpg[/media]
[media]http://www.surplusandadventure.com/images/product/main/gp-5-grey.jpg[/media]

Us M40
[media]http://www.jesseshunting.com/images/gasmask-m40-usmc-front.jpg[/media]
[media]http://i7.photobucket.com/albums/y292/SnakeHead_hl2/m40.png

For the first one http://www.garrysmod.org/downloads/?a=view&id=26878

Much obliged good sir.