How can I keep my co-worker's coughing from driving me insane?

I have a co-worker who coughs so loud it can be heard halfway down the hall, and I am in the office next door. It is not a coughing fit associated with some type of illness, but more of a one-cough clearing-the-throat type of thing. At first I thought it was associated with an illness, but it has been going on for well over a year now, so that’s not it. I am resisting the urge to track the frequency and conditions (i.e., worse in the morning or afternoon) of the coughing because that would mean that I really have gone insane, but I can safely say that the frequency is several times per hour. Have I exceeded the statute of limitations on dealing with this? Is it too much to ask that he at least cover his mouth? I don’t want to shut my door all the time either.