PRO: Making people pay for their farts might motivate them to fart less and this may lead to less air pollution. Maybe people will at least try to eat less fart inducing foods and this will be beneficial to their health.
CON: If people fart less, the gas will be trapped in their bodies for longer periods, causing gaseous internal damage. People are so strapped for cash already that if they have to pay a fart tax, they will just hold the farts in until they spontaneously combust. You’ll be sitting at your cubicle and the guy next to you will just, all of a sudden, be consumed in a fire and then people will be like, “Well, I guess Stanley had a chili cheese corn dog and an extra-large Coke for lunch and couldn’t afford the fart tax due to his child support payments.”