Hi Fred,
I agree they produce a kind of 'smell' especially after I brought in my SD1 and PA speakers after 1 or more gigs. (smoke alike)
Electronic equipment does produce another kind of smell but I do not think it is 'unhealthy' (schadelijk voor de gezondheid)

My studio is about 2.5 x 2.5 m and I have no complaints. Just use a small adjustable ventilation hole. (e.g. in the window : rooster boven het thermopane)

The use of air-purifier equipment is useless and on top of that they produce NOISE !

Roel