Rather than taking in a big breath of fresh air, cruise lovers might actually be inhaling pollution on par with the air in Beijing when they step out on the ship deck.

That's according to the findings of an undercover study by Ryan David Kennedy, an assistant professor at Johns Hopkins University's Department of Public Health. He analyzed the air quality on the decks of four different ships and routes at varied times of day between October 2017 and November 2018. The ships were Carnival's Liberty (Bahamas) and Freedom (Western Caribbean), Holland America's Amsterdam (California and Vancouver, Canada) and Princess' Emerald Princess (Southern California and Mexico).

Kennedy measured the concentrations of particulate matter, or the amounts of liquid droplets or solids found in the air, at the bow (front), stern (rear) and exercise areas on each of the four ships and found that the levels were significantly higher behind the smokestacks. In fact, they were up to eight times higher.

Those numbers, he said, are comparable to the pollution levels in Beijing and Santiago, Chile.

Kennedy concluded that the source was likely the ships' exhaust systems, which emit waste via the smokestacks. Ship exhaust, he noted, contains "harmful constituents, including metals and polycyclic aromatic hydrocarbons (PAHs), many of which have toxic, mutagenic and/ or carcinogenic properties."

More concerning: Vacationers and crew members could be at prolonged risk of exposure – even out in open water – because the ships Kennedy studied never stopped producing exhaust: "The four cruise ships observed in this study were continuously emitting exhaust through their smokestacks because, in addition to providing propulsion, the ship generates electricity to power the cabins, air conditioning, and general operation of the ship."

Kennedy's study was published by the environmental group Stand.Earth, whose website says its mission is "shifting corporate behavior, breaking the human addiction to fossil fuels, and developing the leadership required to catalyze long-term change."

Carnival disputed Kennedy's findings in a lengthy statement to USA TODAY, provided by spokesperson Roger Frizzell.

"We test the air quality of our ships and they meet or exceed every requirement. The air quality on our ship decks when in port compares favorably with a typical urban or suburban environment," it read. "Independent testing on our funnels – which is the area where the exhaust originates – further validates our claims."

t continued, "It is worth noting that our own investigation into this concludes that their data is faulty. Separately, it is absurd to suggest that some researcher surreptitiously taking random samples while posing ‘undercover’ as a cruise tourist could in any way replicate or compare with the controlled and scientific air quality testing that we do continuously, and which portray a vastly different and positive picture. The onboard comparisons with the air quality of Beijing or Santiago are as ridiculous as they are wrong."

Frizzell noted, "We have installed Advanced Air Quality Systems on nearly 80 percent of our global fleet in close coordination with the EPA, so these systems are environmentally friendly, in addition to rolling out new ships powered by LNG, the cleanest burning fuel available."



He added, "We are tested regularly by authorities around the world, such as the EPA, in order to be approved to sail in key ports."

Carnival's statement also took issue with Stand.Earth itself, saying, "This particular organization, for fundraising purposes, is constantly in search of a problem in our industry even if it has to create fake tests that really have no scientific basis."

USA TODAY has also reached out to Holland America and Princess Cruises for comment on the study.

"Globally, the cruise industry has already invested $1 billion in new technologies and cleaner fuels, to significantly reduce ships’ air emissions," said Megan King, who represents the Cruise Lines International Association trade group, in a statement to USA TODAY. She added that more than $8 billion has been committed to building more advanced ships that run on liquified natural gas in order to achieve lower emissions and higher efficiency.

"Cruise ships are one of the more high-profile and easy-to-target flashpoints when air pollution and emissions are discussed," she said. "The cruise industry is only a small part of this issue, an issue which is facing the wider tourism and shipping sector; but we want to be a large part of the solution."