flatulence ettiquette

On my way home on the subway tonight, someone who elected to remain anonymous had some terribly disturbing smells of the flatulence sort, repeatedly wafting though the air. I tried to hold my breath, however, I have a 30 minute train ride home, and wouldn't have made it without taking a few breaths. I tried turning my head in different directions to alleviate some of the smell and possibly determine which direction it was coming from so that I could move to the opposite direction, but this effort was useless. The gentleman across from me was bluntly holding his hand over his nose and mouth. While the gentleman to my left (whom I believe was the culprit), put his head down into his lap. This all brings me to my question. 
What is the proper etiquette when the uncontrollable happens, you fart in public?
First, lets narrow down the types of farts into 3 categories. First off, the SBD, Silent But Deadly. When you cannot hold it in any longer, and you adjust your seating and your muscles just so, in order to let out air without sound, you have accomplished partial anonymity. Those around you can definitely smell it, but they didn't hear who it came from. In this instance, those in your immediate perimeter probably still know it was you. Those people next to them, have no clue where it came from.
The second type of gaseous expansion, is the LAS, Loud And Smelly. If this occurs, it is the most embarrassing of all. Everyone within hearing or smelling range knows it was you. You will probably receive some death-stares and hear people mumbling under their breath about you. It is too late to go back, you let it happen.
The third type is the best for you, the TSF, Top Secret Fart. This is a silent and not entirely malodorous fart. In this case, you got away with it completely. Thank your digestive system this time, you may not be so lucky next time.
So, what should our reaction be if each of these happen to us? The SBD/LAS: Do you say excuse me? Do you move yourself to another area? Do you get your perfume out of your purse and start spraying the air around you? The TSF: Do you pretend like nothing happened at all, or giggle to yourself quietly thinking about how you just got away with murder? How do you react when someone around you farts?
I am not sure the answer to these questions, so I am putting these out there in hopes of some responses. What do you think? How should you act or what should you say? The types of foods you eat can contribute to what type of fart, or the frequency of farts you have. So beware and be prepared with what your reaction will be in the worst case scenario.
xoxo