Health officials have blamed air-conditioning and the use of ineffective cleansers for some of the recent gastroenteritis outbreaks. Another outbreak was also reported yesterday. Thirteen children and a staff member at the Wisely Nursery and Kindergarten on Caine Road, Mid-Levels, developed symptoms including vomiting, abdominal pain, fever and diarrhoea. By Tuesday, the Department of Health had recorded 54 gastroenteritis outbreaks involving more than 1,200 patients this year, compared with 39, involving 892 people, last year. The Noro virus was the cause of most of the recent viral gastroenteritis cases. Speaking at a Legco health services panel meeting yesterday, Acting Deputy Director of Health Regina Ching Cheuk-tuen said although the source of the outbreaks remained unknown, two recent cases were believed to have been caused by cleaning using ordinary detergent which did not contain enough chlorine, instead of bleach. A spokesman for the department warned that people should use bleach containing 5.25 per cent chlorine, at a strength of one part bleach to 49 parts water, to clean and disinfect contaminated items and areas. Dr Ching also said the widespread use of air-conditioning, especially in schools and nursery centres, was another possible cause of the outbreak. Windows were usually closed when air-conditioning was on and a stuffy atmosphere aided the spread of the virus, Dr Ching added. University of Hong Kong microbiologist Samson Wong Sai-yin warned that the highly contagious Noro virus could easily be transmitted through air droplets or contaminated water. People in densely populated areas were more vulnerable, he said. Dr Wong said the virus had strong environmental resistance and could live for a long time in water.