it boils down to ( IF ) you are sensitive to it.
IF you are sensitive to it. I hear even smelling the fumes from an hour after painting can put you in the hospital.
if you are not sensitive to it.. then you can paint with it outside with no (noticeable ) ill effects.
FWIW.. I use to use the valspar hardner.. but have switched to the nason quik cure. same $ and it dries faster...
I paint open air, outside. While I'd like to get supplied air.. I have not yet. if I keep painting as much as I have been over the last 15 ys.. i SHOULD.
I also imagine age and overall pulminary function plays a role. IE.. a young guy with good lungs and is not sensitive.. he may get away with it for decades.
take an older guy or a smoker.. and who may or may not be sensitive to it.. and you might find bad / dangerous results.
You may not be sensitive to it now and paint a dozen tractors outside. take a break for a few years and come back and have it knock you out.. etc.
if you THINK you might be sensitive or have existing pulminary issues.. I'd be on the safe side.