HONG Kong residents temporarily lost sight of the sun yesterday as calm conditions saw a buildup of smog more akin to Los Angeles. But while environmentalists feared that heavy pollution might prove a health hazard, Tropical Storm Winona moved into the area threatening to blow all their worries away. The Royal Observatory raised the standby No 1 signal at 10.30 pm and said they might have to run up the No 3 strong wind signal before noon today if Winona maintained her track towards the territory. But while an ill wind looked likely to do Hong Kong some good, Environmental Protection Department (EPD) Assistant Director Fred Tromp said he could not recollect smog over Hong Kong ever being so bad. The Royal Observatory said visibility in the harbour dropped to about 3.5 kilometres yesterday afternoon. ''In the past two days there has been very little wind and this has certainly caused air pollution to accumulate,'' said Mr Tromp. ''The readings we have taken have shown the levels of air pollution are double what they normally are at this time of year.'' With levels at Kwun Tong just below required health standards, Mr Tromp conceded the situation was close to posing a health hazard, but that was before Winona blew into the South China Sea. Information gathered by the EPD indicates the significant reduction in visibility was not related to local air pollutants. ''It appears the particulates are from a regional source which may be from over the border, but could be further away than China,'' Mr Tromp said.