In 3.0, we see only the gasmask, and biohazard hood as a complete hood. I think in UII, hoods should be part of the rest of the suit rather than a separate piece of headwear. Masks should be separate to the suit, but worn with the rest of the suit.
I think there should be several different levels of suits, as well as masks each offering different levels of protection, or certain levels of protection for different things.
Several types of masks each offering different levels of protection. These could be found in different areas/loot locations of the map.
These would be the most common type of mask, and prevent small amounts of infection from being near zombies, but would offer no protection in deadzones when used without any suit. These would be found in medical locations such as hospitals, medical tents, etc.
Valved Dust Masks
These would be the secondmost common form of protective mask, found in construction sites, factories, and industrial areas. These would offer a bit more protection from zombie infection than the surgical mask, but still not be enough for deadzones.
Half Face Respirators
These would be the minimum and most common form of protection from radiation, allowing you to access deadzones. These would be found in construction sites and industrial areas like the dust mask, but be significantly rarer. They would also offer a significant amount of protection from zombie infection.
You would be able to keep using these masks with the use of filters; the other masks would simply be useless and irrepairable once deteriorated. Respirator filters could be used for both full-face and half-face respirators, found in both industrial and medical locations. These would deteriorate more quickly than military-grade filters.
Full Face Respirators
These would offer a little more protection from radiation and zombie infection, and be slightly rarer than half face respirators. These masks would be found in medical locations like the surgical masks, but be a much rarer spawn.
These would be the rarest and best form of infection and radiation protection, found in military locations. These would require military-grade filters, which would also be found at military locations.
Along with masks, several types of hazmat suits could be found, each offering different levels of protection from infection.
Standard coveralls could be found at industrial locations, factories, and construction sites. These would be about as rare as half-face respirators, and offer some radiation protection in deadzones when combined with masks, as well as a good deal of infection protection from zombies.
These could be found at medical locations with the same rarity as the full-face respirators. These would give a significant amount of radiation protection when combined with a mask, as well as a large amount of infection protection from zombies.
Military NBC Suits
These would be the rarest and best radiation protection when combined with a mask, as well as giving the best protection against infection from zombies. These could be found at military locations, with the same rarity as the military gasmask.