Comment by DannyBee

Depends wildly on the finish. For boiling, i just wouldn't worry.

Most of the toxic fumes/etc come from breaking molecular bonds. There is a minimum temperature, and below that temperature, it just doesn't really occur.

If it starts happening, regardless of whether there is visible smoke/vapor, the finish will quite obviously visibly degrade. Either it will flake off, slough off, or you will just be able to remove it with your fingernail.

Take polyurethanes - they mostly start releasing toxic fumes at 300-400F just about the second they get to that temperature. Below that, nothing.

This is because that's the temperature at which the isocyanate bonds start to break, even if there is no flame. You will not see smoke or vapor. But it will become essentially non-protective and flake off or otherwise visibly degrade.

At a much higher temperature (700-800F) you would break down the polyol, which point it will likely flat out ignite, and burn with a very thick, toxic smoke. People used to actually think polyurethane foam was non-flammable. It's highly flammable. It just has a high ignition temperature. In houses, you are now required to cover it with some form of fire barrier or otherwise meet E-84 criteria through additives, etc.

We don't worry too much about this for wood pieces, because the only time they are exposed to this level of heat is when something is already on fire :)

Also keep in mind that things that are called polyurethanes may or may not actually be polyurethanes.

There is the "colloquial" name that you often find for a finish in marketing literature, and then the actual chemistrsy.

A good example is water-based lacquers, which are usually just acrylic resins.

Most polyurethanes are actually polyurethanes of some sort. Everything else is often a wacky mix.