How coronavirus can be spread on a cruise ship

(NEJM) Coronavirus can be spread throughout a cruise ship via the ventilation system, partly because the particles are so small. (Zhu)

Health officials and engineers are studying ways to prevent the spread of coronavirus. It is fairly clear that getting quarantined on a cruise ship is a bad move.

The Diamond Princess cruise ship two-week quarantine in Japan ended Wednesday, and the coronavirus deaths of two of those quarantined passengers were reported Thursday.

The global tally as of mid-day Friday was 76,767 cases with 34 in the U.S. and 2,247 deaths, according to the World Health Organization and other official bodies.

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Cruise ship air conditioning systems are not designed to filter out particles as small as the coronavirus, which allows the disease to circulate to other cabins, according to one Purdue University air quality expert.

“It’s standard practice for the air conditioning systems of cruise ships to mix outside air with inside air to save energy,” said Qingyan Chen, professor of mechanical engineering at the university. His team has studied the airborne nature of SARS and is working on a ventilation system for preventing the spread of pathogens.

Cruise ship systems can’t filter out particles smaller than 5,000 nanometers, Chen said. The coronavirus is about 60 nanometers to 140 nanometers in diameter, according to researchers in a Feb. 20 New England Journal of Medicine report.

That team used electron microscopy to detect the 2019-nCoV particles found in lungs of patients in China. The virus particles were generally spherical with distinctive spikes ranging from 9 to 12 nm, giving the appearance of a solar corona.

Chen studied the spread of SARS particles, each of which is 120 nanometers in diameter. Given their comparable size, “the air conditioning system would be carrying the coronavirus to every cabin,” he said. “Cruise ships could minimize this problem by just using outside air and not recirculating it.”

By comparison, Chen said that air conditioning systems aboard planes are capable of filtering out particles as small as viruses. Even there, a virus could transfer between people sitting in the same or nearby row as an infected passenger. For aircraft, the coronavirus is more likely to spread by touch than through the air. He also said toilets could be a hot spot, since stool contains viruses. He advised closing the lid before you flush to limit how much of the virus goes into the air.

More than a decade ago, Chen and fellow researchers developed models and sensors to show how flu and other pathogens travel through aircraft cabins

Chen’s lab is working on a ventilation system that is designed to prevent the spread of pathogens by letting each person breathe in his or her own air. Called a personal displacement system, it would blow incoming air from under the seat of a person seated in a theater or airplane and it would be sucked out from vents above. 

That system could be used for other spaces with fixed seats, such as classrooms and offices, but it could not be used in a cruise ship except in a theater or similar space, Chen said in an email.

The current standard for inside and outside air in cruise ships requires they have 10 liters per second per passenger of outside air in a cabin, he said. The air inside a cabin that is returned, or carried, to other areas could be up to 90% of the total air in other spaces. In other words, that 90% could contain coronavirus if it came from an area where someone had the virus.

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