J. Clarke says...

That's the insidious thing about isocyanates--not that they're immediately
toxic but that when your system decides that it's had enough, then you
can't even get _close_ to them. And when that point of sensitization
occurs seems to be random--one guy can spray his whole life without any
protection and never have a problem, another guy can get sensitized the
first time he forgets to wear his air mask.

It was a methyl isocyanate leak that killed over 2,000 people in Bhopal
India.