Here's an angle you might want to consider.

In my darkroom I chose to install two identical, matched fans units. They are the industial blower type and can move a lot of air. One of them blows to the inside, the other blows to the outside. They are wired through the same speed control so that whatever speed I set, they both move the same amount of air at the same time.

The idea was that I wanted to strike a balance. On the one hand, I didn't want to create a slight vacuum by only removing air from the room. This would have resulted in the sucking in of dust through every crevice in the darkroom - not to mention whenever anyone opened the door, or if I had install a passive intake vent.

On the other hand, I also didn't want a slight positive pressure resulting from only blowing air into the room. While this would have eliminated the dust issue, it would also have forced those wonderful-smelling-to-me darkroom odors (I just love the smell of an acid fixer!) back into the house where my wife and son did not share my enthusiasm for them.

But by installing a matched input/output set I achieve a perfectly neutral pressure, while at the same time changing the air continuously when they are active. And using a speed control let me install overkill capacity with the ability to throttle it back for normal use. But in the event of an emergency I can crank them up and move enormous amount of bad air outside in short order without worrying that any of it will seep into the larger house.

I once spilled a little glacial acetic acid into the sink. No major disaster, but I was very glad I had installed the overkill emergency capacity. Cleared the darkroom completely in under a minute.


P.S. I later decided to filter the incoming air using custom cutout portions of a furnace filter. I made sure to filter both the incoming and outgoing fans to preserve my "matched" configuration. My output draws air from near the ceiling, while my input directs air down to the floor.