Yes you can, for example people usually use Filter > Blur > Gaussian Blur filter to soften edges of the mask.
A gas mask is used to filter out toxic agents in the air.
No. The gasmask's filter will be destroyed. Because the membranes in the filter is like a sponge. When you use it in a chemical disaster, it sucks the chemical substanses in to itself. The same happens if you use it under water. The filter will suck all the water into itself, and the filter will be destroyed.
unsharp mask is bad name for filter that is used for sharpening image. It is sharpening tool, people use it to sharpen image details.
Protective Mask
The air-filtration intake filter.
The primary filter
It removes harmful chemicals from the air you are breathing.
Use unsharp mask to sharpen picture and bring more details to an image.
The same way you put it on I presume.
An N95 or similar mask will filter viruses (H1N1 included) But its not the micron filtration size that needs to be focused on, its the proper fit of the mask. Leaks from improper fit and deep breaths will severely limit the effectiveness of simple filter masks. The other consideration is behavior while masked. Wiping the eyes, picking your teeth under the mask, etc.
No they don't, Gas Mask filters are designed not to go bad despite if the lid is unscrewed off, unscrewing the lid of a gas mask filter doesn't have any effect on the gas mask or it's filters.