The thing to look for is isocyanates (typically diisocyanates) which are typical in many hardeners. I am using AU (Acrylic Urethane) and it is in there. I am using a charcoal mask and painting my one car outside with a light fan to direct fumes away from my face (within reason).

I've read some people can have a quick and severe reaction to isocyanate exposure. Usually begins with a scratchy and swolen throat. I breathed it for sure painting the bottom of my 32 earlier and experienced no problems. It's not so much mixing that is the issue, rather, when it becomes a mist during application that should be avoided injesting. From what I have researched long term exposure is much more of a concern. Like if you did this for a living like HRP!

Kitz