I just crafted my first gas mask and only had enough rubber on hand for one filter. I decided to use this one filter to go and grab the yellow slug at the nearest gas hive, in the grassy plains, however, as soon as I entered the gas zone I started taking damage. The bar next to the filters was full, but the number of filters left was 0, and I just took damage until I died.
I would expect that while the bar is full (even if the number is 0), I am still protected from the gas. This just wasted a filter and caused me to die