With winter just around the corner, it’s time to start thinking about how you’re going to keep yourself warm, indoors or out. Synthetics and wool are the most common options for a base layer.

Oil and natural gas are the raw materials for most synthetic fibers, and fossil fuels are among the least sustainable resources out there. Producing polyester, the most common synthetic in long underwear, uses about twice the energy as wool production, and generates about four times the amount of CO2.

In addition, wool comes from a renewable resource. But raising sheep does require a lot of water, and sheep produce large amounts of methane, a potent greenhouse gas.

Wool becomes the clear winner when you add performance to the equation. It can absorb a lot more moisture than synthetics without feeling wet or cold. Wool also doesn’t smell as bad after you’ve worn it, it’s fire resistant, and it usually lasts longer than synthetic fabric.

Wool isn’t cost-free, but it is the better environmental choice—and remember that you can cancel out some of its impacts by turning down the heater at home.

Noah Grant wrote this article for YES! Magazine's "YES! But How?" column. To read more YES! But How?, click here.