I want to offer you a simple and inexpensive method to provide yourself enough fresh air to breathe in the Dark Room. All you need is a basic Breathing Mask attached to a Hose (about 3/4" or 1" Inch in Diameter) leading to an Air Supply outside of the Dark Room. You'll need to drill a hole in the wall to the neighbouring room for the Hose to pass through. The other end of the Hose would be attached to a hole in a small Box, and a small table Fan would be used to blow Air into the Box and through the Hose. This should provide enough fresh air for you to breathe in the Dark Room, and if necessary a Hose with a Diameter larger than 1 Inch could be used. [As for Glue to connect it up, I would suggest a Glue named "Goop" which is the best I've ever seen.]
I also want to quickly address some environmental concerns brought to my attention by another Photographer. The concern regarded used developing chemicals being poured down the drain. As far as the left-over bath Water, this should just be boiled away on a Stove which will leave behind a very small amount of solid residue for disposal -- most of which would be minerals from the water. It would probably take a few years to produce enough residue to necessitate a trip to a depot for disposal. As for the other chemicals, if your Local or Regional Government doesn't have a programme for recycling old chemicals, there are companies that take chemicals for recycling and disposal.