system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How gas masks' air filters can safely hold back toxic gasses but let air through?
gpt
The simplest filter out using very fine fabrics or composites, more complicated versions have layers of varying materials and activated charcoal, which acts like a magnet to particles and sucks them in, whilst fabric filters stop things like dust or spores getting through.

Air is small enough to be dragged through the gaps with the power of your lungs alone 

Eventually the charcoal is ‘full’ or the fabric filter is clogged and the mask no longer effective. 

There are different types of masks and this type cannot protect you against gasses that would kill you. For that you require an oxygen supply.