The Green Lantern (the cute name for Slate’s enviro-advice column) answered the question of which kind of cup to use in the office. The answer isn’t as clear as you think, as always, there are many issues that need to inform your decision. In sum, use an old mug (don’t buy new ones) and wash using environmentally friendly soap.

The Lantern uses a mug for office beverages, but he’s chosen to go the scavenger route—using an old one someone left in his office. Your colleagues’ instincts are right to avoid polystyrene, but they shouldn’t buy brand-new mugs as a replacement (even the kind that come with cheeky green messages). Unless you absolutely need to drink your coffee on the go, ceramic is better than stainless steel. And when you wash, do it by hand, using phosphate-free soap and cold water. (If you want to use hot water, see if you can share washing duties throughout the office, so the water doesn’t need to be heated separately for each mug.)

What if you get your coffee at the local Starbucks on your way to work? The nationwide chain deserves credit for including 10 percent recycled content in its cups, and paper—unlike polystyrene—has the advantage of being a renewable resource. But in other ways, the wood-based venti cups are even worse than office polystyrene: They’re heavier, which means more energy used to create the cup and more waste once the cups have been crushed. Other coffee retailers are experimenting with cups made out of plant-based material, which can then be composted—a positive step, although one that raises a question of where all that extra corn will come from.