Young children 'need exposure' to germs to develop their immune systems A germ-free environment may increase the chances of a baby developing asthma later in life, warns a Chinese University study. So does a damp house, cooking by gas and not feeding children enough fruits and vegetables. A survey of more than 10,000 children in Hong Kong, Guangzhou and Beijing found that dietary and environmental factors may explain why asthma is up to 10 times more prevalent in Hong Kong than on the mainland. More than 10 per cent of Hong Kong children, or 200,000, are afflicted by asthma, a chronic inflammatory disease of the airways. About 250,000 adults also suffer. This compares with 6.3 per cent prevalence in mainland urban centres such as Beijing and Guangzhou, and only 1.1 per cent in rural areas around Beijing, according to the multi-centre study conducted by the paediatrics and the university's therapeutic medicine in collaboration with two mainland centres - the Guangzhou Institute of Respiratory Disease and the clinical education centre for asthma at the Capital Institute of Paediatrics in Beijing. Professor Fok Tai-fai, dean of the university's medicine faculty, said genetic factors alone could not explain the increasing prevalence of asthma in Hong Kong. Exposure to small amounts of germs, especially until the age of two, may strengthen the immune system. 'There's a period in the first two years that is crucial for determining how a person will react to environmental factors,' said Leung Ting-fan, associate professor of the university's paediatrics department. Parents could encourage this by not isolating their children and letting them be around other people, said Professor Leung. Other environmental factors that can increase the risk of asthma include dampness in the house, using gas as the cooking fuel and sleeping on synthetic bedding. These make the odds of developing asthma 1.5 to 2.6 times higher. By contrast, regular fruit intake and eating raw vegetables can protect against asthma, reducing the risk by up to 30 per cent. However, genetics still plays a significant role, increasing the risk by two to three times. According to Gary Wong Wing-kin, synthetic bedding and foam pillows could be linked to asthma attacks because they contain allergy-triggering hydrocarbons, and can trap allergens such as dust particles. Likewise, gas cooking releases chemicals that may irritate the lungs. 'When you are cooking, close the kitchen door and ensure that there is a good ventilation system, so that the harmful chemicals get pulled out of the house. I would also recommend that people use more traditional Chinese materials, such as cotton, for their pillows,' said Professor Wong.