[defencegrid]

When preparing solutions, how do you know which solutions need to be prepared under a fume hood?

Do you refer to the MSDS?....because the MSDS for NaOH for example says "Very hazardous in case of inhalation" so I would of thought you'd prepare it in the fume hood,  but I see people not using a fume hood when preparing NaOH.

[hypervalent_iodine]

I do everything in a fumehood by default, though it's not always necessary. Preparing simple aqueous solutions of something like NaOH is probably fine to do outside of a hood, but I think it's always better to do them in one since containing spills is easier in one than out. Things like making solutions from concentrated acids are better done in a hood, both for containment, and because of fumes.

[Enthalpy]

MSDS list the possible hazards, but the dangers in special conditions, or even abnormal ones like a huge plant fire, don't necessarily exist under other conditions.

For instance burning wood can release carbon monoxide under some conditions, but you won't fall dead because you sit around a campfire. Or flour can explode if suspended in air, and it sometimes makes huge explosions at silos, but you don't wear an explosion-proof suit to bake a cake.

Similarly, NaOH dust or mist would be badly caustic to your lungs, but under the foreseen conditions (including reasonably abnormal ones), do you expect dust or mist may form?