It's ethylene. It could be turned into "plastic" bags, but since environmeddlists are busily banning them, the gas is just being burned off. It's interesting: the gas can be captured from natural gas and oil well-heads, turned into "plastic" bags, and then recycled. Evironmeddlists pushed for them as a way to Save The Trees™. And it's true: the ethylene bags are more durable, easily recyclable, less expensive to make, and are manufactured from a waste product. Paper bags cost a lot more to produce, cost a lot more to transport, and aren't as durable (nor as "environmentally friendly").
But the environmeddlists decided to demonize the ethylene bags, because they hate plastics, and so many folks referred to them as plastics. From a PR standpoint, they're a hassle - even though environmeddlists claim to want to "educate" the great unwashed, it's just easier to villify "plastic" bags.
So now we're back to flaring off the ethylene - burning it for nothing, rather than capturing it and using it. The stuff just ain't politically-correct. And as the environmeddlists know full well - the planet isn't going to suffer whether the gas is burned or used. If the gas is used, though, then the environmeddlists see their bottom lines suffer. They can't be seen as supportive of "plastic" because donations will fall off. And that's really the only reason they're environmeddlists in the first place.