Air Pollution level on cruise ships is '20 times worse' than in a busy city centers.

Air on a cruise ship is more toxic than the air we breathe on land.

Passengers aboard cruise ships could be exposing themselves to levels of air pollution that are twice as high as busy, congested city centers, according to a TV investigation that looked at the health effects of cruising.

In a Channel 4 episode of "Dispatches: Secrets of Your Cruise" which aired in the UK this week, undercover journalists boarded a P&O Cruise ship to explore growing concerns among scientists about the levels of particulates that cruise ships emit. 

Using a P-Trak ultrafine particulate counter, journalists recorded 84,000 ultra-fine particulates per cubic centimeter on the deck, downwind of the Oceana's funnels -- double the average found in Central London. 

The level of ultra-fine particulates per cubic centimeter in Piccadilly Circus, for instance, clocks in at 38,400. 

"The ultra-fines are a special set of these particles. They're about a thousandth of the width of a human hair," explains Matt Loxham, an expert in air pollution and shipping at Southampton University.   

"Larger particles that we inhale usually get trapped in the airways by the phlegm that's in our airways or by hairs in the nostrils for example. But ultra-fines can get right into the depths of the lung and distribute throughout the body." 

And while the level of larger air particulates are regulated on land, experts point that ultra-fine particulates are unregulated and that cruise ships are subject to fewer environmental regulations outside territorial waters. 

Environmentalists also claim that one cruise ship emits as much particulate matter every day, as one million cars. 

Cruise ships are powered by heavy fuel oil which contain 3,500 times more sulphur than is allowed on the roads, according to Dispatches. 

For their part, Oceana parent company Carnival Corporation responded by pointing out that they've reduced their fuel consumption and air emissions by 28 percent since 2005. 

The company also said they plan to fit the Oceana with the same exhaust gas cleaning system installed across 60 of their ships, which will improve the quality of air emissions and reduce particulate matter from soot and fumes.