One of the major factors that made me feel comfortable building this spray booth is that I’ve switched almost entirely to water-borne finishes, which eliminates flammability concerns. However, after I built the booth and experienced the consistent bulk flow of air, I wondered whether my concerns were justified. Using some high-school chemistry, refreshed by Wikipedia, and with data from the same, I tried to calculate the risk of an explosion if I did spray a solvent-based finish. The question is whether an expensive spark-proof fan is necessary. 

I imagined spraying a full gun-cup of lacquer thinner, which is about a liter, within five minutes. In that time, around 1,000kg of air will flow through the booth. Assuming the solvent is perfectly distributed, is that a flammable mixture? I modeled the lacquer thinner as highly flammable toluene, with a density of 867 grams per kilogram, and a molar mass of 92.1 grams per mole. Air is about 29.0g/mol.  For the same pressure and temperature, the volume ratio is the same as the molar ratio, or 0.03% toluene to air. The lower flammability limit of toluene is 1.2% by volume, or 40 times what I could achieve. And I never spray that fast.

I think the real risk exists not while spraying, but once the booth is turned off and the parts are flashing off. If you don’t provide fresh air then, fumes could build up to a dangerous level. That’s why I make sure to leave the doors open after staining, which frequently is still solvent-borne.

The bottom line is that this is not a commercial booth, so any risk is taken by the builder, but I have been using it with no problems and no real worries, either.

Calculation Reference:
Lacquer thinner is mostly toluene, at 92.1 g/mol
Air is 29.0g/mol
by PV=nRT, volume ratio can be just mol/mol
1L toluene = .867 kg
867g toluene = 9.42 mol
1000kg air = 34,482 mol
vol toluene/ vol air *100% = 0.03%
Toluene LFL = 1.2 % by volume
See Wikipedia “Flammability Limit
Also: http://solvents.americanchemistry.com/Health-Safety/Working-with-Modern-Hydrocarbon-and-oxygenated-solvents.pdf