Why not use both good ventilation and a respirator. I have exhaust fans in my darkroom. But at times, depending on the chemicals, I'll still use a respirator. I always order extra cartridges and keep them on hand and write the date on the ones when you install them. This will give you a good idea of when they are near expiration. I'm working with chemicals like ether, pyro, formaldehyde and potassium cyanide and have never had any problems. (knock on wood). Any chemical can be worked with safely if proper care is taken such as good ventilation, skin protection, eye protection and the use of a respirator. The proper size and fit can be obtained when ordering by looking at the sizing chart. After you have the respirator on just cover the cartridges with both hands and inhale, you'll know if it is fitting properly. Just no facial hair along the edge that seals around the face. When we were doing retro-fits on some of the old bridges I had guys that I had to let go because they refused to cut their beards. There is a lot of lead paint on those old bridges. So these guys were quiting 40 dollar an hour jobs.