Cruise ships regularly depart major seaports with thousands of passengers onboard, and are now being called out as major sources of pollution.
At full power, the diesel engines of Harmony of the Seas - world's largest cruise liner - burn about 66,000 gallons of diesel fuel a day, producing much higher levels of pollutants than the diesel used in cars.
One cruise ship emits the same amount of air pollutants as 5 million cars traveling the same distance.
That's largely due to the higher-sulfur diesel fuel typically used by cruise ships, according to the group.
In port and close to coasts, Harmony of the Seas must use auxiliary engines running on cleaner fuel, as well as emissions-abatement hardware.
Southampton - UK's 2nd-largest container port and Europe's largest cruise-ship port, is also one of 9 UK port cities cited for breaching air-quality guidelines.
Increased scrutiny of cruise-vessel emissions comes as the industry expands at a rapid pace.
According to CLIA, about 24 million passengers are expected to cruise in 2016, compared to 15 million 10 years ago.