I could just say, "bless you" after the first incident. That acknowledgement, if you will, might be enough for them to start covering their mouth/nose better.

Grossness warning:
If not, the preface of, "bless you," the first time might pave the way for a second, "bless you" followed by, "Can you please cover your mouth better? It's getting on me." That might be a respectable request.

(The trouble I have is coming up with a way to say this that will put the negative attention where it belongs, and not on me.)