How heavy is ammonia gas versus the air in an atmospere exposed environment?

I am a draftsman and I have to go to plants ocasionally and one of my co-workers brought up the point of having to get away from large concentrations of ammonia gas if things went wrong in one of those plants. I kind of want to know whether I need to go up or down in that kind of situation. 99.9999999% chance that I will never be exposed to that situation, but it's kind of nice to know and to have my backside covered.