The level of pollution on some cruise ship is worse than in the world's most polluted cities like Delhi, an investigation on Britain's biggest crew operator P&O Cruises has found.
The investigation found one cruise ship can emit as much particulate matter as 1 million cars in a day.
The air on the deck downwind of the ship's funnels had 84,000 particulates per cu cm, according to data from particle counter.
Directly next to the funnels on the deck, the numbers rocketed to 144,000 with a peak at 226,000.
Passengers are likely to be breathing some of these particulates, which are harmful for health and the environment.
"Short term exposure can cause increasing respiratory symptoms. People who are asthmatic for example, that might give them a wheeze. Similarly for people with cardiovascular disease," an expert said.