system
Given the following post, respond as if you're commenting on Reddit:
human
Why did the ISS crew wear breathing masks when entering the SpaceX module?

One of the astronauts wore a simple face mask, while the other one wore a proper respirator.
gpt
A precautionary measure in case there was any dust, dirt, or other detritus floating in the air inside Dragon.  No matter how clean you try to keep the interior of a spacecraft, the activity associated with loading it with a bunch of assorted cargo inevitably leads to some foreign matter coming along for a ride.  Then, the shaking of launch and subsequent free-fall provide an environment where that dust/dirt/roach legs/whatever gets disturbed and floats free.  

  The masks were a precaution in case this had occurred so the crew didn't breath in any contaminants. Fortunately, the air was quite clean.

  If you watched any video of the Space Shuttle cargo bay during the first day after a launch, you'd have seen this in action.  After the payload bay doors opened, there was always a lot of visible particles floating out, sometimes for days into the mission.