Now what if there's a bigger issue, like a passenger who is downright stinky or is so large that he or she is physically spilling into your space? This is when sensitivity really comes into play. It might be interesting to point out that most airlines' "contracts of carriage" can refuse passage to people based on certain criteria.
For instance, American Airlines even reserves the right to refuse transport if someone is dressed in such a way as to make others feel uncomfortable. US Airways' contract of carriage can boot anyone over the age of 5 who arrives barefoot or inappropriately clothed.
Delta Air Lines states clearly that anyone who is barefoot, appears to be intoxicated or has a malodorous condition can be refused boarding.
More surprisingly, the major airlines in the United States can remove passengers who are too large to buckle the seat belt. United Airlines says the passenger must purchase another ticket or leave the plane if he or she is not able to fit in a single seat with armrests down and the buckle properly secured with one seat belt extender.
While these rules may seem harsh, many of these airlines only refuse boarding for the greater safety of the passengers and in extreme cases. Also, if boarding is refused, most airlines say they'll refund the trip's cost.
The best thing to do is to patiently and politely ask a flight attendant for assistance if you're confronted with a passenger of size or someone who smells bad. If there are empty seats, the attendant may be able to relocate you.
If that's not possible, the flight attendant will likely be able to help you come to some sort of solution, like a last-minute upgrade of your seat (with more leg room and no screaming kids). Upgrading your seat can also make your trip more comfortable if you're worried about an armrest hog or the person who fully reclines his chair.
If the problem persists even after you've spoken up, you can always report the situation to the airline after you land. Take it up with your carrier's customer service department and have the date of your trip and your flight number handy. Who knows? They might give you flight credits or bonus mileage points. If the service wasn't up to your expectations, it never hurts to bring it up.
Also of interest: What NOT to pack for your next trip.