The Irish-headquartered airline Ryanair ran a series of ads in the United Kingdom on September 2019, boasting of “low CO2 emissions” and of being the "lowest emissions airline" at 66g CO2 per passenger-kilometres flown. The airline justified this assertion with technological and average load factor arguments. 

On February 5, 2020, the Advertising Standards Authority (ASA) concluded to misleading claims from the company. One ad was in breach of the UK Code of Non-broadcast Advertising, Sales Promotion and Direct Marketing, more specifically rules rules 3.1 (Misleading advertising), 3.7 (Substantiation), 11.1  and 11.7 (Environmental claims). Other adds were in breach of the Code of Non-broadcast Advertising, Sales Promotion and Direct Marketing, specifically rules 3.1 (Misleading advertising), 3.9 (Substantiation), 9.2  and 9.8 (Environmental claims). 

ASA considered the CO2 per passenger distance metric appropriate, and that consumers would understand the relative nature of the claim. It took its decision on the basis that consumers would find insufficient information in the ads on the basis of the claims and that Ryanair did not actually provide the necessary evidence to substantiate that they would reduce their personal CO2 emissions compared to flying with another carrier. It also put in doubt the data references used by Ryanair, and underlined the fact that well-known competitors were absent from the calculation.