No idea where anyone got the idea cellulose paints don't require a mask when spraying. I guess nothing requires one if you don't care about living but in the real world cellulose paints absolutely require a mask, in fact, ANY paint you are spraying should only be done with a good cartridge mask. Even if the atomized paint is not chemically toxic the issues that can arise with inhalation are huge. They have even determined that essential oils, once believed to be a health benefit when in a diffuser are actually causing respiratory issues when used often due to the inhalation and coating of the lungs.
I'm usually in agreement with you on safety issues David but this time I 100% disagree. If you are spraying any paint, water borne or solvent base, or even brushing any solvent based paint you should ALWAYS wear a respirator.
To do otherwise is foolish to say the least, eventually fatal to say the worst.