After both, 20 years as a respiratory therapist, and a also having degree in construction technology, I tend to err on the side of caution. If the directions say use a mask, I don't open it until I have a mask on.
Bottom line, if it's toxic you shouldn't know what it smells like. If you can smell it, you're too close. And there are two ways to find out if you have been too close.
1) You inhale a big breath by accident (or design) and end up in the E.D.
2) You are careful but not careful enough and 20 years from now you have respiratoy problems and can't figure out why.
If you are not now and have never been a smoker, you probably won't have any trouble just being cautious.
This never makes me popular but it is frequently the combination of smoking and any other exposure that causes problems down the line so if you don't smoke you're probably okay.
If the can says, use a mask, there's a reason for the warning.
barbie