Commercial stock solutions of HNO3 (nitric acid) are usually 70% w/w, and according to several MSDSs (e.g. here), it is dangerous to inhale it:
Symptoms/effects after inhalation : Irritation of the respiratory tract. Dry/sore throat. Corrosion of the upper respiratory tract. Coughing.
FOLLOWING SYMPTOMS MAY APPEAR LATER: Respiratory difficulties. Possible inflammation of the respiratory tract. Risk of lung edema. Blue/grey discolouration of the skin
What I want to know is what is the maximum concentration that 70% HNO3 can be diluted to, which may be safe to work outside of a hood? Are there guidelines/credible sources for such information?
Please note, this is not a medical question, it is a question of safety practice in a chemistry lab.