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.

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    $\begingroup$ What he said! If possible keep it in a hood at all times. $\endgroup$ – Waylander Jan 10 at 13:57
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    $\begingroup$ And if it isn't possible to keep it in a hood you almost certainly need to rethink what you are trying to do. (Yes, my lab regularly uses small amounts of IPA outside of a hood to clean optics and vacuum equipment, but we sure would not use similarly small quantities of nitric acid.) $\endgroup$ – Jon Custer Jan 10 at 14:13
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    $\begingroup$ This is a good question! Probably everyone agrees that 0.0000001 mol / L of nitric acid in water is not particularly hazardous to handle outside the fume hood, and that concentrated nitric acid is very dangerous and must be handled in a fume hood. Where is the in-between point? $\endgroup$ – Curt F. Jan 10 at 17:26
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    $\begingroup$ I feel like the commenters are missing that the OP is not asking about the the amount of concentrated acid that can be handled outside the hood, he is asking about the concentration of acid that can be handled outside the hood. $\endgroup$ – Curt F. Jan 10 at 17:27
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    $\begingroup$ Sounds like you are referring to inhalation risk and damage. Safety may depend on ventilation in the room, how close you approach the solution, and both the volume (or surface area), temperature and concentration, how you handle the solution, to what purpose it is put. If you are working in a lab ask someone associated with the lab in charge of safety. $\endgroup$ – Night Writer Jan 10 at 19:17

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