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I make a sandwich at home almost every day. But until today, I never thought about that sandwich’s carbon footprint as I was squirting the mustard or slicing the cheese. A new study out of the U.K. changed all that, and has me patting myself on the back for my relatively low-carbon meal.

Researchers at the University of Manchester analyzed 40 different sandwich varieties—homemade and prepackaged—to get an inside look at the popular lunch option’s climate impact. In considering everything from ingredients to packaging, they found that a triple-decker BLT is the best thing you can eat for the environment.

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Just kidding.

What they actually found was that making your own sandwich at home can reduce carbon emissions by up to half, compared with similar ready-made options. This is in large part due to the greenhouse gases emitted via packaging, transporting, and refrigerating prepared sandwiches in shops and supermarkets. Keeping sandwiches cool can account for up to a quarter of their carbon footprint.

“Once prepared, ready-made sandwiches need to be kept in a cold chain up to the retail stage; this requirement is not necessary for sandwiches prepared at home and consumed on the day,” states the study.

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The authors also found that supply chain food losses result in about 20 percent more ingredients being required for retail sandwiches compared with homemade ones. And conservative use-by dates can add to the wastefulness.

Adisa Azapagic, co-author of the study, said in a statement that “we need to change the labeling of food to increase the use-by date as these are usually quite conservative. Commercial sandwiches undergo rigorous shelf-life testing and are normally safe for consumption beyond the use-by date stated on the label.”

Keep in mind the study looked at sandwiches that are U.K. favorites, which to many of us PB&J-loving Americans sound like things you’d eat on a dare rather than a daily basis. In total, the researchers looked at 24 different ready-made sandwiches with an average of four ingredients each, all but two of which included mayonnaise. The top ten worst carbon offenders include prawn and mayonnaise, ham salad, and sausage and brown sauce.

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Only three prepared sandwiches—egg mayonnaise and cress, tuna and sweetcorn, and chicken and sweetcorn (this was a very British study)—approached the lower-carbon footprint of the homemade option.

That homemade option was actually just 16 variations of ham and cheese sandwiches to help establish a baseline for the various store-bought varieties. The researchers did this because the study would’ve otherwise involved too many assumptions about the variety of homemade sandwiches people create. “Whereas commercial recipes are standardized, consistency in homemade recipes for sandwiches cannot be assumed to be the norm,” they write.

There were a number of other limiting factors to the study—and that’s not even taking into account the differences in sandwich habits and ingredient sourcing between Americans and Brits. The authors assumed that commercial sandwiches are prepared fresh on a daily basis and delivered under refrigerated conditions directly from the manufacturer to the retail outlets on the same day. That seems like a stretch.

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Overall, the study found that the 11.5 billion sandwiches eaten each year in the U.K. produce the same greenhouse gas emissions as 8.6 million cars, with one sandwich producing, on average, the carbon dioxide emissions equivalent of 12 miles of driving.

For those of you really taking this information to heart, there are some personal actions available to you. The study suggests avoiding sandwich ingredients with high carbon footprints, such as lettuce, tomatoes, cheese, and meat. Maybe it’s time to jump on the toast trend after all?

The authors think that the carbon footprint of readymade sandwiches could be reduced by half, with the greatest room for improvement coming from reducing packaging and other waste, “however, these are most difficult to realize as they involve changing consumer behavior.”

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Now I’ll go back to eating my sandwich.