A year or so ago, I decided to start using less plastic. I bought an insulated water bottle for my morning coffee, as well as bee’s wax wraps to replace cellophane, and I largely stopped putting my groceries in plastic bags. Instead, I switched to cotton tote bags.

I felt really good about myself, until I saw a report published earlier this year by Denmark’s Ministry of Environment and Food that said that plastic bags are better for the environment than organic cotton tote bags. In fact, of all the shopping bags the study looked at — from paper to recycled plastic — cotton tote bags fared the worst: they need to be reused thousands of times to have the same environmental footprint as a lightweight plastic bag, according to the report. A study published in 2011 by the UK Environment Agency reached similar conclusions. So, was my decision to ditch plastic bags bad for the environment?

The answer is not that easy. First of all, these studies — called “life cycle assessments” — have to be taken with a grain of salt. The research looks at different types of shopping bags through all of their life cycles: from the extraction of the raw material needed to make the bag to the way the bags are used and then discarded. It then determines how “environmentally friendly” each bag is based on several impact categories, such as climate change, toxicity, and water use.

Here’s the rub: it’s basically impossible for one bag to score better than all other bags in each impact category, says David Tyler, a professor in the Department of…