You walk into your local coffee shop, hand the barista your reusable coffee mug, and pat yourself on the back for not using one of those “bad for the environment” single-use cups.

Sounds simple. Right?

Perhaps.

Granted, using a reusable cup lowers the waste-management environmental impacts. But you may not have considered other aspects of the cup’s life cycle such as the materials and energy that went into making your sturdier reusable cup, the soap and hot water that will be necessary to wash it, and the energy source behind the heat of this washing water. A recent life-cycle assessment (LCA) by the CIRAIG* tackled those issues. LCAs compile and evaluate the inputs, outputs, and potential environmental impacts of a product or service from material extraction to end of life.

The CIRAIG study compared the potential environmental impacts of a 16-ounce, single-use coffee cup made of a mix of cardboard and polyethylene (with a lid made of polystyrene) to those of a 16-ounce, reusable ceramic cup and to those of a variety of 16-ounce travelers’ mugs made of stainless steel, polypropylene, and polycarbonate. Over a one-year span (using one cup a day), the reusable cups scored well in the climate change arena—that is, they were associated with fewer greenhouse gas (GHG) emissions than their single-use counterparts. Likewise, they scored better in the human-health category for things such as toxic emissions, smog, and ozone depletion. They also tended to use fewer minerals and fossil fuels than disposable cups did.

But here’s the bitter part. Washing the reusable mugs with hot water and soap puts them at a disadvantage when it comes to ecosystem-quality indicators. These indicators cover issues such as ecotoxicological emissions, acidification, eutrophication, and land occupation.

Perhaps the most important result for the caffeinated among us was that the number of times a cup is used is paramount. Indeed, only with frequent use can one decrease the potential impacts of the reusable cup; it would take between 20 (human health category for a polypropylene travel mug) and more than 1,000 (ecosystem-quality category for all travel mugs) uses, depending on the cup/mug type and the environmental indicator, to make up for the impacts of a single-use cup. If a reusable cup is used fewer times than that, the single-use cup is better for the environment.

What should we do then? Can we help the environment? The answer is yes: by reusing your cup for several years and by limiting the quantity of soap and hot water for washing it, the reusable cup should be the way to go. Limiting your coffee intake could also be something to look at, but that is another problem altogether.

* A center of expertise in life-cycle issues, recognized internationally for its solid scientific research work and its 15 years of applied experience. The International Reference Centre for the Life Cycle of Products, Processes, and Services (CIRAIG) supports industry, governments, organizations, and consumers in their path toward a truly sustainable development supported by life-cycle thinking.