Photo/Illutration The quarantined Diamond Princess cruise ship at Yokohama Port on Feb. 20 (Asahi Shimbun file photo)

Ventilation systems in office buildings, hospitals and commercial facilities across Japan are attracting heightened attention as a possible cause for the spread of the new coronavirus.

Some health experts fear that air circulated in confined spaces may help to disperse the virus further and wider.

Interest in the issue was sparked by the large group infection aboard the cruise ship Diamond Princess earlier this year when the vessel was quarantined at Yokohama Port.

It emerged that 70 percent of the air supply that circulated among the cabins was never topped up with fresh air as an energy-saving step.

Of the 3,711 crew members and passengers, 712, or 20 percent, eventually tested positive for COVID-19 even though guests were confined to their cabins for the duration of the crisis.

Details of the ventilation design were gleaned from a report to an academic journal of the Japan Society of Naval Architects and Ocean Engineers about this ship’s air conditioning system submitted in 2008 by an official of Mitsubishi Heavy Industries Ltd., which designed and built the ship.

Fresh air constituted 50 percent of the air that circulated in stairwells and other spaces where passengers gathered, such as the dining rooms and areas for entertainment.

It also constituted 100 percent for hospitals rooms aboard the vessel to maintain sanitary conditions and avoid odors.

Circulating less fresh air results in significant energy-saving benefits.

A senior official of a leading architectural firm noted that the system is commonplace for large buildings across Japan.

The official said the system had been adopted for large commercial buildings in Osaka and public hospitals in Hokkaido as well as office buildings in central Tokyo.

Aside from energy-saving benefits, a key sales point is that management costs remain cheap, the official added.

However, the ventilation system widely used for large buildings in Japan is not endorsed by the Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA), headquartered in Belgium.

It announced in April that air circulated in this way “is not recommended because polluted substances can spread inside buildings.”

An official of the association also noted that the air conditioning filters used by the Diamond Princess did not prove effective against the virus.

The Society of Heating, Air-Conditioning and Sanitary Engineers of Japan said in April that “the amount of air circulated from room to room should be reduced as much as possible to avoid the virus from spreading."

It recommended that fresh air be used as much as possible. The society is comprised of researchers and technicians on air conditioning systems in Japan.

When the National Institute of Infectious Diseases inspected the Diamond Princess, it detected viral genes in exhaust duct openings of hallway ceilings.

As the causal relationship between the ventilation environment and the spread of infection remains unclear, the institute said further study is needed.

It noted that the virus may spread a considerable distance in such an environment.

“Hotels and casinos also use the same air conditioning system," said a representative of Carnival Japan Inc., the Japanese branch in Tokyo of the operator for the Diamond Princess.

At the peak of the group infection aboard the Diamond Princess, a senior official of the U.S. Centers for Disease Control and Prevention told Carnival Japan that there currently was no evidence to indicate the virus spread via air conditioning systems.