The deadly SARS virus spread through a Hong Kong apartment complex last year by hitching a ride on microscopic airborne water droplets, according to a study that could help authorities in future battles with the respiratory disease.

The new research, published in Thursday’s edition of The New England Journal of Medicine, debunks previous theories about how severe acute respiratory syndrome (SARS) spread through the private Amoy Gardens apartments.

SARS killed 299 of the 1,755 people known to have contracted it in Hong Kong, the epicenter of the epidemic. Worldwide, about 8,000 people were infected and some 800 died.

Sewage, roof rats, direct or indirect person-to-person contact were blamed when more than 300 people in the Amoy Gardens complex fell ill. The source of that outbreak was a person with diarrhea in Building E.

A team led by Ignatius Yu of the Chinese University of Hong Kong said investigators from the World Health Organization were partly right when they concluded that the illness initially spread within the building because an exhaust fan in a bathroom drew the virus up through traps in the floor drain.

The fan then sent the virus - riding on invisible aerosol droplets - to other parts of the 36-story building.

AEROSOLS

After building a mock-up of the drainage system, “we found that huge numbers of aerosols were generated” in the plumbing system when toilets were flushed, the Yu team said.

In the past, the aerosols would have been blocked by standing water in the drain traps, Yu told Reuters.

These days, many residents don’t clean their bathrooms by flushing water down the drains; instead they mop the floor, spilling little water. As a result, the traps dry up and don’t trap what they should.

But because that theory did not explain how the disease jumped to other buildings, or why only those in certain buildings were affected, the Yu team used computer simulations of wind and airflow to see where the virus might have traveled.

They found that people whose windows were downwind from the apartment where the first SARS case was reported were three to five times more likely than others to fall ill.

Powerful bathroom and kitchen exhaust fans in the complex probably drew in contaminated air and helped spread the infection, they said.

That may explain why few tenants in the other 12 buildings became sick.

Yu said work is underway at Amoy Gardens to change the plumbing so the traps will no longer dry up.

“This is a very common type of design,” at least in Hong Kong, he said.

The researchers said SARS would not have spread as rapidly via person-to-person contact. In addition, people who worked on the ground floor of the building and had frequent contact with residents did not fall ill.

Roof rats were an unlikely source because residents of the upper floors were less likely to fall ill than people on the middle floors.

SOURCE: New England Journal of Medicine, April 22, 2004.