The salt breeze on the deck of a cruise ship may not be as fresh and clean as the beach air its meant to evoke, a new study reveals.

An associate professor at the Johns Hopkins Bloomberg School of Public Health, Ryan Kennedy, secretly measured the amount of ultrafine particles in the air on four cruise ships. He found measurements similar to prominently polluted places such as Beijing and Taipei, Taiwan, with the worst readings taken in areas designated for exercise or children's activities.

Carnival Corporation said Kennedy's tests were "completely ridiculous, inaccurate and in no way represent reality".

Kennedy's research contradicts the long-held belief that the air at sea is clean, and that the emissions from cruise ships don't affect passengers.