Neighbours were originally told the chemical cleaning would be conducted from last Tuesday until December 15, between 7am and 7pm on working days. Following complaints, Techoy has agreed to shorten the process to 20 days.

Sent on November 10, the notice insisted that adequate protection would be used to prevent any harm. But it also warned: 'In the interest of safety, residents are advised during this period to: close all windows and put all clothing indoors; move all plants indoors; cover up your cars; avoid coming near our construction site and places without [overhead] covers; use an umbrella to enhance protection if you are near the site or otherwise exposed'.

The developer is a little-known company called Master King Holdings. The new 30-storey building is expected to be ready in February or March, but managers at Centaline and Midland Realty property agencies say they have not been given any information about the new development. It is called 'Jar Din Ho Ting' in Chinese, but appears not to have an official English name.

The building's architects, Simon Kwan and Associates, declined to disclose any details about the developer.

An assistant to Wan Chai District Council member Bonson Lee Hing-wai inspected the site last week and was told that operators would apply an acidic solution to the walls and then spray the walls with water to wash them. This would generate vapours from a considerable height. However, the public notice said it would be an alkaline solution, which also could be corrosive.

'We have received complaints from concerned residents in surrounding buildings,' Mr Lee said. 'My advice for them is to collect evidence if anything is damaged so they can claim compensation later.'

The Environmental Protection Department said there was no regulation to control the washing of external walls on private land, but said it was monitoring the operation.

True Light Middle School principal Leung Yin-ting said she had expressed health concerns to the contractor. 'I asked whether students should have umbrellas when approaching the building and [the project manager] said was it was just a precautionary measure,' she said.

She said classroom windows would be closed and air-conditioners turned on during the cleaning.

A resident of a building next door was outraged that the operation was allowed to proceed next to the True Light complex, which includes a kindergarten and primary school.

'They have warned that it is corrosive to metals, plants, animals and human health and since they are spraying at such a height, the chemical vapours could spread within a one-kilometre area,' he said.

'Chemical fallout could cover the adjacent school area where kids play. There are some very old trees just in front, with beautiful birds nesting on them, and this chemical spraying may kill the birds and destroy old trees. There are two white parrots living in one of the trees.'

A site foreman, who declined to give his name, insisted the cleaning operation was entirely safe.