I recall Formlabs recommended 99% back in the Form 1 days, but later reduced their advice to 90% and added this to their website:
Higher concentrations of IPA are available, but are much more hazardous to manage, while 90% solutions are safe for household use.
I tried to probe a little at the user conference to see if anything specific prompted the change, e.g. if they noticed employees getting symptoms or anything. The response I got was that they’re about equally effective and 99% will evaporate down to 90% pretty quickly anyway (not sure how authoritative that last part was).
I’ve been wondering about health effects from long term exposure to IPA vapors, especially after one user reported complications. Not trying to scare anyone, and Wikipedia says IPA is relatively non-toxic compared to alternative solvents. I wish I could find some kind of air quality monitor that measures concentration.
I appreciate all the expertise shared in this thread and will be watching it. For what it’s worth, I’ve been diluting mine to 90% lately with distilled water, and haven’t noticed any appreciable change in effectiveness.