Next time you find yourself on a cruise ship and plan on enjoying a healthy stroll, think again. That walk above the water may slowly kill you.

According to a recent study put on by producers of the French documentary series “Thalassa,” ultrafine particles are concentrated in cruise-ship air at a level that is 20 times worse that what’s found in cities where the boats dock.

As reported by the Daily Mail, the air outside is worse than what’s inside. The German environmentalist organization Naturschutzbund Deutschland (NABU) found 60,000 particles per cubic inch in the air around the top-deck jogging track. Compare that to the 1,000 or 2,000 particles normally found in a cubic inch of fresh sea air.

The Mail reveals that experts in this field are “warning passengers with chronic obstructive pulmonary disease against staying on deck for too long.”

Daniel Rieger, a researcher at NABU, said that the study associates the microscopic in-air particles with fatal lung-diseases, heart attacks, stroke, asthma and diabetes. Rieger attributed the rotten air to cruise ships using cheap fuel and lacking filtration systems.

If it’s any consolation to those who enjoy seafaring vacations, the cruise companies respect ocean waters no more than they respect their passengers. In 2016, Princess Cruise Lines was slapped with a record-setting fine of 32 million pounds for “dumping thousands of gallons of oil and waste off the coast of the UK.”

Presumably, the vessels’ pools are cleaner than the swill that the ships float in.