A seven-year study from Warwick Business School points to Finnair as the greenest airline. Twenty major airlines were studied to ascertain information on carbon footprints and greenhouse gas emissions.

“Finnair performs best due to the age and type of its planes, the routes it flies and the overall number of connections it offers,” said Frederik Dahlmann, assistant professor of global energy, Warwick Business School. “Plus it is probably among the most advanced when it comes to accounting for and managing its emissions over time.”

TAP Portugal and Virgin Australia rounded out the top three least polluting airlines. On the other end of the spectrum, the biggest polluters were American Airlines, United Airlines and Delta Air Lines.

The study also notes most airlines’ greenhouse gas emissions are actually growing. As airlines add more efficient aircraft to their fleets, they also are increasingly adding more flights and routes, counteracting any reductions in C02 emissions.

While the school attempted to poll more than the 20 major airlines it compiled, many of the more than 200 international airline companies declined to provide data, as there is no global obligation for airlines to disclose relevant information.