A company selling ultraviolet light bulbs that it claims can prevent Sars is being investigated by a government department. Treasure Dream, the Hong Kong distributor of Bright Starlight and Electricity, a mainland firm, has been selling the bulbs for $328. It claims that one 15-watt UV-C bulb can kill 99.9 per cent of bacteria in a 200 sq ft room for a whole day if left on for one hour. Advertisements for the bulbs have appeared in the Chinese-language Sudden Weekly magazine and Oriental Daily newspaper. The bulbs carry a label warning that no one should be present when the light is on, to prevent damage to the eyes and skin. The Electrical and Mechanical Services Department has told Treasure Dream to stop selling the bulbs for domestic use until it has carried out tests to see if they meet safety standards. But Treasure Dream has ignored the department's instructions. A company spokeswoman, Josephine Leung Suk-wai, said 700 bulbs had been sold in the past few weeks. 'We tell our customers to turn it on before leaving for work and switch it off when they get home,' she said. Ken Lo Wai-wah, assistant manager of OSRAM Prosperity, a subsidiary of Siemens which also manufactures UV-C light bulbs, said: 'The biggest problem is that they kill bacteria, but not the Sars virus.'