Most people know that eating meat, especially red meat—say, hamburgers—is bad for the environment. Raising enough methane-farting, resource-intensive cows to satisfy our cravings for burgers and steaks produces an outsized carbon footprint that plays a significant role in climate change. But what about your breakfast egg-and-cheese? A new study says you should feel guilty about that, too.

Recent findings reported in the journal Sustainable Production and Consumption examine the carbon footprint of 40 different kinds of sandwiches—from the simple ham and cheese to tuna, BLTs, and breakfast sandwiches—both homemade and pre-packaged. Researchers from the University of Manchester calculated the carbon necessary to produce standard recipes, including the agriculture required for the ingredients, the manufacturing of the packaging materials, the refrigeration required to keep the sandwiches cold, and the waste generated. They sourced their estimates from previous studies on the carbon footprint of producing and transporting ingredients like bread, ham, bacon, lettuce, tomato, and canned tuna as well as the energy cost of manufacturing packaging, transporting materials, and taking waste to the landfill.

They found that of all the sandwiches, those that combined pork (or prawns, because prawn and mayonnaise sandwiches are apparently a popular thing) and cheese are the most carbon-intensive. A bacon, egg, and cheese breakfast sandwich clocked in as the most environmentally taxing sandwich of them all, with a carbon footprint of 1441 grams CO2 equivalent—a measurement of the global warming potential—per sandwich. (The diet of the average meat-eater in the UK produces about 7200 grams CO2 equivalent daily. For comparison, if you drive your car four miles, it emits about 1650 grams CO2, roughly.)

Chicken and tuna sandwiches were slightly less carbon-intensive, but vegetarian sandwiches didn't fare as well as you might think—depending on how much and what type of cheese was involved, they could have carbon footprints as high as some of the meat sandwiches.

The researchers suggest that some improvements to the way sandwiches are produced and sold might decrease their carbon footprint by as much as 50 percent. Reducing the amount of meat, eggs, and cheese used, excluding tomato, lettuce, or mayo, reducing packaging, and other changes could all contribute to shrinking a sandwich's carbon footprint. Unfortunately, here's a limit to how much a sandwich's environmental impact can be reduced. You can't really have a BLT without the B, L, or T.

But if you're making it instead of buying it, you're saving a lot of emissions. As you might expect given the environmental cost of packaging, ready-made commercial sandwiches had a much bigger carbon footprint than their homemade counterparts containing the same ingredients—2.2 times larger, in fact.

Just another reason to feel guilty about not bringing your own lunch from home.