AERIAL photographers who help produce annual maps of Hong Kong have been grounded by pollution for the first time this year. Lands Department surveyors packed their cameras and got on a Government Flying Service plane on Tuesday, only to come down to earth with a bump. 'It is the first time this year I have had to abort a flight,' said land surveyor Andy Leung Tsz-ming. 'Because of the thick haze, I could not see the ground features even though I was flying at 4,000 feet, so I made a decision to abort the flight. It is the first time I've had this experience.' He said that yesterday was even worse. 'I didn't need to jump into the plane. I could make the decision from the ground. It is not cloud, it is haze - dust and particles in the air.' Mr Leung, who spends over 100 hours a year taking aerial photographs for the Lands Department, said he had never before encountered impenetrable haze. The Royal Observatory estimated visibility at the airport at 4 pm yesterday was about 3,800 metres. A forecaster said a continental air-stream brought fine and stable weather that kept a lid on dust particles. 'All the fine particles are trapped in the lower part of the atmosphere,' he said. Senior environmental protection officer Pang Sik-wing said the haze was mostly due to very fine particles produced by vehicles and burning processes, such as restaurant kitchens and power stations. Other dust, for instance that from building site work, produces bigger particles that do not float so easily. The air pollution index was 'moderate' yesterday - 83 in the industrial area, 79 in urban centres, and 78 in less developed areas, he said. But he forecast worse today - 95 in the industrial area, 90 in urban centres and 85 elsewhere. Levels above 100 are classified as 'unhealthy', when anyone with respiratory problems is recommended to avoid exercise and stay in.