Just a small doodle dump I did lately – probably to check if I am able to come up with more than one gas mask design ^^' I was considering putting this in the scraps section due to low quality, but then again I am pretty happy with the overall effect.
A short info on the following:

1) A full head two-parter mask with double hose. I guess the whole contraption consist of a rubber hood covering the head save the muzzle and a breathing apparatus placed over the mouth (hence the strap to keep it on). Classic lenses plus a posture collar. The air tanks / filters would be located on the back

2) A similar full-head design. The difference is that the filters are located on the sides of the head, slightly under the hardened rubber “plate” located over the head. The eye-lenses are replaces with much more narrow, horizontal “slits”. The air is delivered either thru the side filters (if the air isn't polluted too much) or via the mouth-hose.

3) Another full head mask, almost the same as #1. The difference is that the breathing apparatus is inserted to the mouth like a gag – the dragons mouth is actually semi-opened, biting the breathing tube, while the rubber hood is wrapped tightly around his head and holds everything in place.

4) Face only mask. The ears and top of the head are uncovered, while the lower part and the neck are coated in rubber. Lenses are held in place via straps. The overall design is more stylized to the canine anatomy, having a lot of molded parts. The air again is delivered via a system of filters and removed thru vents.

5) Face only sculpted and stylized mask. The working mechanics is the same, but the design follows a anatomically correct rubber sculpt of a canine head. Strapped to the face by a series of rubber belts.

That's mostly it. I did have a lot of fun doing those, and will do something similar again in the future. Again sorry for the low-quality.
Hope You all like it!