Sandwiches are the nation’s go-to lunchtime convenience food, but have you ever considered the amount of energy required to take your BLT from farm to desk?

In a new study, published in the Journal of Sustainable Production and Consumption, researchers have calculated the carbon footprints of 40 sandwich flavors and discovered that the worst offender (the breakfast sandwich containing eggs, bacon, and sausage) produces a whopping 1,441 grams (50 ounces) of carbon dioxide equivalent. To give some perspective, that equals the volume of CO2 emitted by a car driving 19 kilometers (12 miles).

Meanwhile, the most environmentally friendly option is a homemade ham and cheese sandwich, which generates between 399 and 843 grams (14-30 ounces) of CO2 equivalent, depending on the recipe.

“Given that sandwiches are a staple of the British diet as well as their significant market share in the food sector, it is important to understand the contribution from this sector to the emissions of greenhouse gases,” said study author Adisa Azapagic in a statement.

“For example, consuming 11.5 billion sandwiches annually in the UK generates, on average, 9.5 million tonnes [10.5 million tons] of CO2 eq., equivalent to the annual use of 8.6 million cars.”

Britain’s obsession with the sandwich began in 1762 when the historian Edward Gibbon gave a name to the meal people the world over had been eating for centuries. He named it after John Montagu, 4th Earl of Sandwich, who, legend has it, asked his cook to prepare him a meal he could eat without leaving the gambling table. 

Believe it or not, sandwiches didn't become a store-bought commodity until the 1980s, when British store Marks and Spencer started selling pre-packaged sandwiches to busy shoppers. To their surprise, the concept blew up. Brits today spend around £7.85 billion ($11.23 billion) on pre-packaged sandwiches every year and roughly half of all sandwiches consumed are store-bought.

In case you're wondering, the nation's favorite flavor is chicken salad, which produces 962.9 grams (34 ounces) of CO2 equivalent. Other popular choices include prawn mayonnaise (1,254.7 grams, 44 ounces CO2 equivalent), egg and cress (739 grams, 26 ounces CO2 equivalent), BLT (1,007.4 grams, 35.5 ounces CO2 equivalent), and cheese plowman's (1,112.1 grams, 39 ounces CO2 equivalent).

Unsurprisingly, ready-made sandwiches are less environmentally friendly than homemade varieties – 739-1,441 grams (26-50 ounces) CO2 equivalent versus 399-843 grams (14-30 ounces) CO2 equivalent. 

Agricultural production and ingredient processing are the biggest sources of CO2 emissions, accounting for 37-67 percent of CO2 equivalent for pre-packaged sandwiches. Pork, cheese, prawns, and – shockingly – lettuce are among the most carbon-heavy ingredients. Packaging, transporting, and chilling sandwiches also contribute to CO2 emissions. 

The researchers found that changing recipes, minimizing food waste, and switching to alternative packaging and waste management systems could help reduce the carbon footprint of pre-packaged sandwiches by a staggering 50 percent.

Good news considering our appetite for the humble sandwich isn't going away anytime soon.