AIR pollution levels are very high in some road tunnels and operators are to be asked to improve operations and install special equipment to combat the problem. Levels of cancer-linked nitrogen dioxide exceeded health-based pollution targets and carbon monoxide levels were borderline in two of four tunnels surveyed, the Environmental Pollution Advisory Committee (Epcom) said yesterday. The targets are based on five-minutes exposures, which the Environmental Protection Department (EPD) is proposing to replace the more lax current standard of one-hour exposures. The pollutants build up in the enclosed space of the tunnels, yet ventilation fans to improve circulation were operating under capacity 73 per cent to 99 per cent of the time, Epcom said. Chairman Dr Wang Gungwu said the EPD supplied the data and he did not know which tunnels were involved. But he said the findings were worrying and Epcom endorsed plans to make tunnel operators introduce improvements. ''There's concern for public health for people going regularly through these tunnels,'' Dr Wang said. Tunnel operators would be asked to use their ventilation fans to full capacity and to install detection devices that switch the fans on automatically when a certain pollution level was reached. Dr Wang did not know the cost of these measures but said the administration intended to make them part of tunnel licences and would be going to operators shortly. Coral that was killed by dredging off the Ninepins Islands has started to regenerate, according to preliminary findings presented to Epcom. At least 25 per cent of the coral was smothered by sand and mud that spilled over from the dredging operations, according to surveys done early last year. But the latest data showed ''limited but encouraging signs'' that the coral was growing back, Dr Wang said. The Civil Engineering Department was investigating as part of its survey of all areas affected by dredging and dumping, and would report more details in January, he said. Fishermen and environmentalists claim the work, which is necessary for airport-related reclamation projects, is destroying marine life. Dr Wang said the department's survey was the first of its kind in Hong Kong and used remote cameras and other technological devices to map species and the level of sedimentation on the sea floor.