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.

“You have to be cautious about drawing big conclusions.”

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 Chemistry and Biochemistry at the University of Oregon. “So you have to decide, when you talk about the impact on the environment, what environmental impacts am I most interested in mitigating?” Tyler says.

I decided to give up plastic bags because I wanted to do something about the scourge of plastic pollution in our oceans. Scientists estimate that around 8 million metric tons of plastic trash enters the oceans every year. That plastic doesn’t degrade, and it poses a threat to wildlife, including corals. Sea turtles that eat plastic bags thinking they’re food can choke. Just last month, a dead whale in Spain was found to have more than 60 pounds of plastic waste in its stomach, including bags. Several cities in the US, like Austin, Los Angeles, and Seattle, banned single-use plastic bags to address the litter problem. Last month, New York governor Andrew Cuomo proposed a bill to nix plastic bags in New York state for that same reason.

The Danish study didn’t include marine litter as an impact factor. (The researchers followed EU guidelines for which categories to include, like climate change and ozone depletion, says study author Anders Damgaard from the Technical University of Denmark.) That’s not right or wrong, says Travis Wagner, a professor in the Department of Environmental Science & Policy at the University of Southern Maine. All of these life assessment studies are done differently, and they assess bags that can be found in one specific country, but not in others. The studies also need to make many assumptions about how people use these bags, Wagner says. Do you line your trash can with a plastic grocery bag? How many items do you put in your tote bag vs. a paper bag when you go shopping?

“You have to be cautious about drawing big conclusions,” Wagner says.

“That’s why it’s tricky.”

Still, in all these life cycles assessment studies, plastic bags do seem to be “more green” than cotton tote bags. That’s because cotton requires lots of land, water, and fertilizers to grow; then, it needs to be harvested, processed, and brought to market. “Cotton is a very thirsty crop,” Tyler says. One study done by the Australian government in 2007 found that plastic bags also have a lower carbon footprint than paper bags. Making paper from trees sends a lot of waste to the landfill, Tyler says. In comparison, “petroleum is generally considered so valuable that there’s very little waste,” he says. Plus, transporting 1,000 paper bags across the country with a truck consumes much more fuel than transporting 1,000 thin plastic bags, Wagner says. “A lot of the carbon production is the function of the weight of the bag,” he says. “That’s why it’s tricky when you start comparing them.”

At the end of the day, it’s all a matter of what you care about the most. If marine litter is your biggest concern, paper bags are better because paper degrades and doesn’t stick around for years. In New York City, the Department of Sanitation spends more than $12 million a year to dispose more than 10 billion single-use plastic bags, so that’s why the city has tried (unsuccessfully) to impose a 5-cent fee on them. In Africa, the concern is that discarded plastic bags also pool water that can breed disease-causing mosquitoes.

So, what should you do if you want to limit your environmental footprint? Many of the experts I talked to say that using reusable plastic bags — whether made of recycled plastic, nylon, or woven polypropylene — is best. “You can use them hundreds of times. The ones I’m using, they’re showing some wear and tear, but I’ve had them for a couple years, and they’re in good shape,” Tyler says. “When you can use a bag so many times, eventually you reach a break-even point.” These bags are durable and can easily be cleaned if, for example, meat juices spill out.

“That’s the goal: how many times can you reuse them?”

Whatever the grocery bag of your choice, try to re-use it as many times as possible, even if it’s a single-use plastic bag. You can line your trash can with it, use it to carry your lunch, or carry it with you next time you go grocery shopping. “All these bags pay off if you use them a significant number of times,” says Wagner. “That’s the goal: how many times can you reuse them?” When it comes to mitigating climate change, however, what grocery bags you use has a relatively small impact, says Jonna Meyhoff Fry, a senior consultant in life cycle management who conducted the UK study in 2011. It matters more what you put inside those bags (Do you eat lots of meat? Do you buy local?) and how you get to the grocery store. (Do you drive your car or walk?)

As for me, my apartment is awash in tote bags — I have more than 20 from my favorite bookstores and shops — so it makes sense for me to keep using those as much as I can. I won’t be buying new ones anymore, and I’ll just say no next time I’m offered a free one.