I've built two darkrooms with ventilation based on filtered inward blowers rather than extractor fans. The blower fan is mounted anywhere in a back darkroom wall or (inward opening) door and the air outlet is a passive vent in the wall at the far side of the sink. Advantages include:
The fan and its noise are at the back of the room away from your ears. That's so you can hear your sound system better.
The darkroom is positively pressurised so dust and smells leak out rather than being sucked in. The darkroom door is being pushed shut by air pressure and seals better.
Air movement is away from your face, across the trays, and out the vent on the far side of the sink.
Importantly, the ability to detect something with your nose does not mandate harm or potential harm. Low or no odour chemistry is available for those still worry. And the dust being suppressed is not chemical powder but rather ordinary house dust. No one serious about a clean workspace mixes powders in a darkroom. A few stray specks of fixer powder won't hurt you but they will instantly ruin expensive film and paper wherever they touch.
Darkroom processing solutions are much safer on a relative basis than household cleaners, gasoline, or whisky but fear of "chemicals" is still rife. I'm not the most adroit searcher of the internet but I can't find a single substantiated, documented instance of toxic harm coming to anyone in the world in a non-commercial photographic darkroom in the last few years. Fear, anxiety, and anecdotes, yes; harm, no.
I used to be a research chemist and toxicologist until I went commercial. Here are a couple of old incidents from my time as the hot-line guy for Eastman Chemicals:
A darkroom worker complains of headaches, dizziness, and general malaise while working in a well ventilated darkroom. He blames the "fumes" from the "chemicals".
When I check his work space I find that the guy is doing 5 hour darkroom sessions while wearing a double cartridge (dust and vapour) face mask. He's got a deep fear of chemicals but what's getting him is ordinary anoxia because he's not breathing adequately.
An urgent call comes through from someone with metol allergy. The stuff is literally eating his hands even though he is wearing surgical gloves. It turns out that the victim has latex allergy from the gloves and the metol is innocent. But the story has spread to every darkroom in town and it takes ages to hose it all down.
Sometimes dramatic behaviour stemming from the fear of chemicals is more dangerous than the chemicals themselves. A darkroom that requires respirators and gloves for worker survival is absolutely not worth going into at any price. Clean air, clean surfaces, and ordinary common sense work practice has routinely delivered darkroom safety for more than a hundred years.
