Strong toxic smell coming from heater vents. It seems to get worse at night as it gets colder, just wondering if I have a Freon leak in my radiator.

Just want to know what type of glue is used in furnaces, and at what point do they start to gas off.

FREON??? No, not freon.

Please start at the beginning and describe in detail your furnace or boiler, and the fuel, and do you mean a steam system? You mention vents as well as radiators.
Also please keep questions together. The one about glue applies here.

Nothing is "glued".. Epoxy or a thermoset and they shouldn't gas off. Is this in relation to "toxic" smells?

Do you have a carbon monoxide detector? You should get one and place it in your home.

Unusual smells, especially "toxic" smells, are something which require prompt local professional attention. I strongly urge you to contact a local professional service company quickly.

Have you painted anything lately inside?