We estimated how big an explosion would occur if the azide prep was done on various scales, and what mitigation is readily available (blast shields, capabilities of hoods, etc.), and from this we estimated that a prep done on a 5 gram scale (or less) could be handled safely and with minimal risk. That is, an explosion of a 5 gram prep, if properly mitigated by a standard blast shield, would be unlikely to cause harm. Obviously, we’re guessing here, but in a somewhat educated way (we hope).Also, via Twitter, Minnesota professor Ian Tonks responds:
Basic idea is that 5 g could be contained w engineering protocols in our labs (fume hood, safety sash, blast shield)
I think the important point is to emphasize that there are limits for all sorts of rxn risks (tox, explosion, fire, etc) that when reached should trigger discussions w/ PI/peers/safety committee wrt how to (or can you?) safely carry them out.
I don't think a hard cap is necessary in many cases because all labs are differently equipped, but in this case it made sense.
Caps can engender a sense of complacency if you're operating under the limit, so open discussion on hazards is still critical.There's probably nits to pick with this policy, but overall, it seems reasonable.