Putting bacon and sausage on the same sandwich is even more dangerous than you thought.
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Making your sandwiches greener doesn't just mean adding more lettuce. In fact, according to a new study by researchers at the University of Manchester, it actually means adding less. The paper, entitled "Understanding the impact on climate change of convenience food: Carbon footprint of sandwiches," is the first ever study of the environmental impact of both homemade and prepared sandwiches. It turns out that lettuce takes a big environmental toll — and so does pretty much everything else, unless you're making your lunch at home.

The study, which looked at 40 types of sandwiches, delved into every sandwich component imaginable, surveying the energy that goes into ingredient production, packaging, refrigeration, and the ensuing waste.

The worst sandwich for the environment? A ready-made all-day breakfast sandwich that includes egg, bacon, and sausage. It turns out this breakfast staple produces the equivalent of 1441g of carbon dioxide. For reference, that's about how much in CO2 emissions you'd create if you drove a car 12 miles. So for future egg sandwiches, it might be worth trying to pick just one pork product.

In general, sandwiches containing pork, cheese and prawns are the worst, with store-bought ham and cheese, prawn and mayonnaise, and egg and bacon topping the list of CO2 emitters.

The top 10 worst offenders:

All-day breakfast 1,441
Ham and cheese 1,350
Prawn and mayonnaise 1,255
Egg and bacon 1,182
Ham salad 1,119
Cheese ploughman’s 1,112
Sausage and brown sauce 1,087
Double cheese and onion 1,078
Cheese and tomato 1,067
Roast chicken and bacon 1,030

Ham salad aside... The lowest emitter proved to be a homemade ham and cheese sandwich (provided you use the right recipe), which could reach as low as around 400g of CO2 equivalent. So if you needed more inspiration to bring your lunch to work, know that making your own sandwich at home takes up about half as much CO2 as a prepared option, ranging from 399 – 843 grams vs prepared sandwiches' 739–1441.

But, sandwich-purchasers, take heart, for progress is possible. According to the study, "changes in the cultivation of ingredients, recipe changes, reduction of food waste, alternative packaging and different waste management options," could reduce the ready-made sandwich carbon footprint by up to 50 percent. With 11.5 billion sandwiches consumed each year in the UK alone, that's 9.5 million tons of CO2, which equals the amount emitted from 8.6 million cars a year. So there's plenty to cut. For starters, it recommends cutting down or eliminating ingredients with especially higher carbon footprints like meat, cheese, tomato, and yes, even lettuce.