The university's Institute for Environment and Sustainable Development studied air samples collected from Tai Shing Street market in Wong Tai Sin, Sheung Fung Street market in Diamond Hill and Ngau Chi Wan wet market.

The samples were collected in mid-February when Hong Kong banned all live poultry imports and live chicken sales were limited.

The samples collected at the chicken stalls contained 32,642 bacterial colonies per cubic metre.

The concentration level is 20 times higher than that in the open air, which has a bacteria concentration of 1,271 per cubic metre. In fresh meat stalls the bacteria concentration level was 2,594.

The institute's assistant research scientist, Arthur Lau Pui-sang, said the high concentration of bacteria was caused by the movement of live chickens.

'When the chickens move, the bacteria on their feathers and skin spread in the air. Therefore, the bacteria concentration is particularly high near the chicken stalls,' he said.

The institute's director, Fang Ming, said the high concentration was not a result of poor ventilation in the markets.

'If about 50 per cent of the bacteria come from the chickens when they breathe, the concentration is too high,' he said. 'Having good ventilation is still not enough to dilute the bacteria.'

Dr Fang said the concentration would be even higher after the lifting of the ban on mainland live chicken imports.

He advised children to stay away from the chicken stalls because the cages were of a similar height to children, which put them at risk of inhaling air with a high concentration of bacteria.

The Food and Environmental Hygiene Department said the best way to improve air quality was to lower the number of poultry stalls in wet markets and separate the poultry stalls from other fresh and dry food stalls.

In order to reduce direct contact between humans and chickens, the government has implemented new hygiene and control methods.

The study found that staphylococcus and micrococcus - the two more common bacteria found on human skin and hair - account for almost half of all the bacteria found in the air samples in open air and fresh meat stalls. These do not cause harm in most circumstances.

The university air samples from chicken stalls contained 46 per cent of branhamella gram-negative, a bacterium commonly found in warm-blooded animals that can cause allergies.

Air samples that were collected in other places did not show this bacterium.

Meanwhile, the government said yesterday it would liaise with mainland authorities on the resumption of the importation of day-old chicks, live ducks and geese.

No timetable was announced for the resumption of trade.