It sounds to me as if the OP is experiencing the lingering aftereffects of a bad head cold and is probably long past the infectious period, making a mask rather pointless unless the OP wants to try to scare away the person in the adjoining seat. Just taking along some meds that lessen the symptoms most likely to annoy fellow passengers (cough medicine and your favorite antihistamine) should suffice. At this time of year it's highly likely that there will be several people on the plane who sound and ARE more ill than the OP.