i myself an curious about the respirator thing too. i know what is recommended and whats required by osha but i have been told that they do stop iso and some say they dont. i have heard of people claim they have been painting with one every day for 25 years so i would think that they do remove the iso. i know pos pressure systems are really the way to go but i think the cartridge type are ok too. does anyone know for sure??

The canister respirator is approved by NIOSH for isocyanate's.
But the respirators makers all say on their product not for use with isocyanate's.
They do work but here is why the respirators say what they say.
If the respirator was handled the right way when you use it, it goes right back into an airtight package. This does not always happen, you see painters wearing them when not painting or setting on the bench and the filters will wear themselves out being exposed to air.
Another problem, iso's have no smell, so there is no way to know
if the filter is good or bad.
Bottom line, the manufacturers want no liability for iso related problems.
As soon as your done painting put the respirator in a freezer zip lock bag and burp the air out when sealing. If you paint daily replace every month if your doing this. One day unsealed replace the next day.

However, you cannot detect when the cartridge is saturated and no longer working. Therefore, it is recommended to change cartridges everyday when painting with isocyanates.
When painting without isocyanates, cartridges should last for a week. Always store your cartridges in a air-tight ziplock bag at the end of the day. Cartridges will continue to saturate with contaminates if left out in the open air.

The 3M product info sheet for respirator cartridges states that OV cartridges work for the 3 different sources of isocyanates but the footnote says "hard to detect".
I will find the source and post a link to this info if anyone is interested.