Half of children's respiratory illnesses are caused by second-hand cigarette smoke, according to a new study which yesterday prompted calls for greater prevention of passive smoking. United States research showed passive smoking was responsible for between 40 and 60 per cent of childhood asthma, bronchitis and wheezing. Council on Smoking and Health chairman Professor Anthony Hedley said not nearly enough was being done in Hong Kong to prevent such illnesses. 'It's far too easy for people to be exposed to other people's smoke,' he said. Dr Judith Mackay, director of the Asian Consultancy on Tobacco Control, said the Agency for Health Care Policy and Research figures were higher than any previously recorded. Studies carried out in 1991 found 11 per cent of children in Kwai Tsing and southern districts had respiratory ailments indicating they had been exposed to second-hand smoke. 'We have a limited amount of data,' Dr Mackay said. 'This should give cause to the Government to look anew at the problem of passive smoking, for children. 'This is a vulnerable group of people . . . they don't have much choice.' More than 10 per cent of Hong Kong people aged between 10 and 20 suffered from asthma. Dr Christopher Lai Kei-wai, chief of respiratory medicine at Chinese University, said children who already suffered asthma had more frequent attacks if they were exposed to cigarette smoke. He said there should be more focus on passive smoking. 'People have only been talking about it for the past 10 years or so.'