If you answered incorrectly, there’s no need to be ashamed. The solution to this quandary is not as straightforward as we might like it to be, and the conclusions we draw will have everything to do with our assumptions when comparing the environmental impacts of each option.

For example, if we were to conduct a life-cycle assessment (LCA), as others have done, we would need to calculate the total energy and resources consumed across the life cycle of each product from cradle to grave. Because durable mugs are substantially heavier than disposable cups, they inherently contain more embodied energy, which is a fancy way of saying that they require more energy to produce than flimsy paper cups. The magnitude of this embodied energy, combined with the energy and water required to wash them, may lead one to cast his or her vote in favor of the disposable cup—even when the reusability of the mug is accounted for.

However, when you’re standing in line at the cafe, silently rehearsing your 14-syllable beverage order, it seems most appropriate to consider only those eco-relevant costs that directly follow from your choice of coffee container.

For the disposable paper cup, it's important to consider the costs of waste disposal, transport, and the resources needed to produce a new cup—once you use it, another one has to be made.

For the ceramic or glass mug, only the environmental costs of washing (water and energy) are involved. After all, your choice to drink from a sturdy vessel does not make you responsible for its embodied resources—unless you break it, of course.

Based on an engineer's estimates, the production of a single 16-oz. paper coffee cup requires 0.4 pounds of materials (about one quarter of which is wood pulp), 1.2 gallons of water (for softening the wood pulp to make paper), and 0.15 kWh of energy. The process produces 0.04 pounds of greenhouse gases. Upon disposal, the cup contributes about 0.035 pounds of waste, and an additional 0.035 pounds of greenhouse gases for transportation and landfilling.

The environmental costs of washing a single reusable mug—computed using data for conventional dishwashers and assuming a capacity of 24 mugs per load—involves 0.25 gallons of water, 0.07 kWh of energy (for running the dishwasher and heating the water), and 0.11 pounds of greenhouse gases. Even if the dishwasher is only half full, or if the coffeehouse workers are required to hand-wash each mug before washing it in the dishwasher (as may be the case at some establishments), choosing the mug is still preferable to the disposable paper cup for its resource and waste savings.

Of course, the best option is to bring your own mug. But make sure you use one you already have. If you buy a new one, you’ll have to take responsibility for its embodied energy…but we’ll spare you the calculations on that for now.