Foam around heat vents

So I am in the process of putting up XPS in my basement, cutting small pieces and putting them in the joist cavaty's against my sill plate (I think thats what its called) Anyway I was reading the can of great stuff that I am using to seal up around the foam and it says it is very flammable.

To that I ask. Can I use it to seal up around the heat ducts that are going upstairs?

I can post a few pictures later today.