Hong Kong disappeared under a blanket of filth as the air pollution index shot up on January 19,1999. According to country's Environmental Protection Department, the smog was caused by low winds and a temperature inversion -- in which cool air is trapped by a layer of warmer air above. Ships were put on alert because of low visibility, while pollution readings hit a record in the areas of New Territories. Environmental activists described conditions as "disgusting" and "appalling" and demanded urgent action. Officials, on the other hand, said they were doing all they could. Edwin Lau Che-fong, assistant director of Friends of the Earth, an environmental group, said certain areas of Hong Kong Island were smothered by a "yellowish mist" in the morning. "I really worry about the health of people living in these conditions," he said. Michel Porro, a photographer, said he had heard tourists commenting on the pollution and saying they would never return to Hong kong.