The rapid spread of atypical pneumonia in a Hong Kong housing estate has caused concern among international experts that the virus could be airborne. Hong Kong health officials and microbiologists are investigating the alarming spread of the disease at Amoy Gardens in Ngau Tau Kok, where 121 residents have been struck down in the past week, with 36 residents admitted to hospital yesterday. Air and water samples were collected from Amoy Gardens yesterday for analysis. Government investigators are checking out several leads in an effort to establish why the disease has spread so rapidly there, a source said. One is that an infected restaurant worker might have brought takeaway food to several flats. Another is whether an infected plumber spread the virus as he serviced several flats. Leading the investigation are Department of Health epidemiologists, who pieced together the first wave of Sars at Prince of Wales Hospital and linked the cases to a mainland doctor who visited the Metropole Hotel. The Guangdong doctor infected six other guests and a Hong Kong man who visited the hotel on February 21-22, spreading the outbreak to the rest of the world. If the virus was airborne, it could spread at a much faster rate and perhaps be passed on through air-conditioning systems. While the possibility has not been ruled out, Hong Kong officials have said the chances of the virus being airborne are slim. But medical experts overseas are concerned. Julie Gerberding, director of the US Centres for Disease Control and Prevention said yesterday that her team was 'carefully monitoring' the possibility of airborne transmission. 'We believe, based on what the investigations have shown us so far, that the major mode of transmission still is through droplet spread when an infected person coughs or sneezes and droplets are spread to a nearby contact,' Dr Gerberding said. 'But we are concerned about the possibility of airborne transmission across broader areas and also the possibility that objects that become contaminated in the environment could serve as modes of spread.' Regarding the outbreak in Amoy Gardens, Dr Gerberding said: 'It's obviously something that we're concerned about. There are clues that face-to-face contact is not always the only means of transmission.' Hong Kong Director of Health Margaret Chan Fung Fu-chun said at the weekend health officials have never ruled out the possibility of airborne transmission. Yuen Kwok-yung, head of microbiology at the University of Hong Kong (HKU), said airborne transmission was unlikely. 'If the virus is airborne, we would expect 10,000 people to have been infected. The data we have so far shows that the virus is transmitted through droplets,' Professor Yuen said. But Microbiologist Desmond O'Toole said it was 'misleading' for University of Hong Kong scientists and health authorities to insist that the coronavirus was being spread by droplets. 'All respiratory diseases are distributed by droplets generated by coughing, sneezing and talking. The droplets are airborne. Droplets remain airborne for varying amounts of time depending on a number of factors,' said Dr O'Toole of the City University. So far, the disease has killed 13 people and sent 530 others to hospitals in Hong Kong. Lo Wing-lok, the legislator representing the medical sector, said the disease could have been spread through some contaminated public facilities at Amoy Gardens. 'I do not think the virus is airborne, it is just being transmitted very efficiently through droplets or contact,' he said. There are three airborne diseases known to the world - chicken pox, measles and tuberculosis - for which effective vaccines and treatment are available. Meanwhile, Tom Buckley, a consultant at the Prince of Wales' intensive care unit, said people could not be too careful as he had seen how health care workers had become seriously ill. Dr Buckley said: 'I think there needs to be much more heightened awareness about personal hygiene in the community. We think we are clean but we really need to look at our own personal hygiene in terms of sharing utensils, washing teeth, having a shower, should we use liquid soap or bars of soap, how much contact should we have with our family members?'