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When installing ventilation in a darkroom, is it more important that air circulates well (in a closed room) or that there's a source of fresh air?

The room I currently use for printing has a window I can open behind the blackout curtains, but I'm afraid it might draw some stray street light if a light breeze pushes the curtain.

For now I keep a fan running in the room (with closed windows) and I go out for air every once in a while.

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    You should have fresh air in the room. You can do that by opening the window and the door for a few hours before you start work there and once inside you close everything. The amount of oxygen you convert to CO2 in the room will be about 550 liters per day. If you stay in a closed 3 by 3 by 2 meters room for 24 hours, the CO2 volume percentage would rise to about 3% which the typical level where CO2 intoxication starts. So, an hour in such a room shouldn't be a problem. – Count Iblis Jun 22 '15 at 16:16
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    @CountIblis thanks ! I was thinking more about the chemistry though – user40720 Jun 22 '15 at 17:00
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Don't use curtains. You need to construct a "light labyrinth".

Try to install a fan extractor that fits the opening, like the ones people use in a bathroom that has no windows. You just need to need to pull out the air, the fresh air will enter from different parts of the room, like the openings around the door.

You can make a curved shaped box made of cardboard painted in matte black. Seal it with duct tape or silicone (painted black).

There is no point in circulating the same used air inside your darkroom. That would probably help dry the prints, but it will also make the chemicals on the tray evaporate faster, which is not good for your health. Of course this depends on the size of the room.

If your window is too big, you can probably make a board that fits the window and holds the fan and the labyrinth.

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Regarding air supply: my brother ran a big computer fan (one for use at mains power) blowing into a small cloth sack in his dark room in order to minimise the amount of dust/lint contaminating equipment and solutions. He powered it up a time before work. Of course, this was not for oxygen supply but for keeping the existing air in good working order. The room itself was basically a lightsealed (not air-sealed) walk-in cupboard. I don't think we ever even thought about oxygen as a problem, with the time spent in there in one go not being long enough to really matter a whole lot.

But in case you set up any supply or circulation, it may be a good idea to combine this with trapping of dust/lint.

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