The Hong Kong Observatory's director said it could have released a day earlier test results showing that radioactive iodine had descended on the city from the damaged Japanese nuclear power station since Saturday, but had held back to verify.
Dr Lee Boon-ying, the Observatory chief, said yesterday that caution was needed for better accuracy of data and to avoid unnecessary public speculation about possible health threats, even though the level detected was extremely low.
The results announcement was made on Tuesday, three days after the radioactive dust was detected.
'We considered whether to release the findings to the public [on Monday] but our decision was to hold them until we had finished another measurement to verify the results,' Lee said yesterday.
'We could not tell the public on Monday that we might have detected it as it could have led to more speculation and even some chaos.'
Lee retires next month after working at the Observatory for 31 years.
The latest readings showed that the iodine level had gone up 56 per cent from the level measured in air samples taken between Sunday and Monday, but was still negligible, the Observatory said.
The level of 296 micro-becquerels of iodine-131 per cubic metre of air would take 800 years of exposure to add up to the radiation from one X-ray scan. No other radioactive material, such as caesium, had been detected, the Observatory said.
Lee said the extremely low radiation level meant that their equipment had to be run for up to 22 hours for reliable detection. With the full-day air sampling, the process would last at least two days, meaning data would be 48 hours behind.
Assistant observatory director Ma Wai-man said there were radiation monitoring stations across the city.
Ma said a gamma spectrometry system at Ping Chau in the northeastern New Territories, 20 kilometres from the four nuclear reactors at Daya Bay, was only used for detection of higher-density radiation.
But Ma said that a high purity germanium detector at the Observatory's King's Park laboratory was being used to measure and identify low-level material.
Asked why the mainland seemed to have faster radiation detection, Ma said all the equipment the Observatory was using met international standards and the data reliability level reached at least 90 per cent.
'I don't know what system and protocol the mainland is using, but I am sure that more time is needed to produce more sophisticated and reliable data,' he said.
Professor Zhu Hanmin, researcher at the National Institute for Radiological Protection under the Chinese Centre for Disease Control and Prevention, said that there was practically no difference between the measurement devices used in Hong Kong and on the mainland.
Zhu said the mainland had developed and used radioactivity detectors since the 1960s using its own technology, but that their sensitivity did not exceed similar products imported from overseas.
'Hong Kong's detectors are 100 per cent imported and I have 100 per cent confidence in their sensitivity and reliability,' Zhu said.
Professor Yu Kwan-ngok, of the Nuclear Radiation Unit at City University, also believed the Observatory had tried its best to deliver the most accurate and timely information.
He said the measurement time needed would depend on the concentration of radioactive material, and the lower the concentration the longer the time required.
'It is a matter of trade-off between accuracy and timeliness.'
Habitat for Humanity
HSBC: 502-476591-002 (for HK$)
HSBC: 502-476591-274 (for US$)
Please send bank-in slip with contact information.
Make crossed cheques payable to 'Habitat for Humanity China Limited'. Mark 'For Japan Post-Disaster Response' on the back and send with contact information to 17th floor, Sun House, 181 Des Voeux Road, Central, Sheung Wan. Online donations at www.habitatchina.org 
Make crossed cheques payable to 'Hong Kong Red Cross' and send to Hong Kong Red Cross, 33 Harcourt Road. Write 'Japan Earthquake 2011' on the back. Online donations at www.redcross.org.hk 
Inquiries: 2802 0021
Society for the Prevention of Cruelty to Animals (Hong Kong)
Account name: Society for the Prevention of Cruelty to Animals (HK). Please remark: 'Designated to Japan Earthquake Fund'.
Online donations at www.spca.org . hk/eng/donate/donate2_donation.asp
Post Crisis Counselling Network
Inquiries: 2127 9890
More details at http://www.pccnhk.org 
The Salvation Army
Bank of East Asia: 015-515-40-400333-1
Bank of China: 012-878-1-044486-6
Hang Seng Bank: 024-385-094180-001
Details at www.salvation.org.hk 
Inquiries: 2783 2333
Bank of China: 012-875-1-0810855
Wing Lung Bank: 020-601-003-7634-8
Bank of East Asia: 015-260-81-01210-0
Make cheques payable to 'Hong Kong Committee for Unicef'.
Send cheques, with personal details and 'Japan' written on the back, to Hong Kong Committee for Unicef, 3/F, 60 Blue Pool Road, Happy Valley.
PPS: Use your contact number as bill number. Enter Unicef merchant code: 6012, then select bill type 3 'emergency'.
Online donations at www.unicef.org.hk 
World Vision Hong Kong
Hang Seng Bank: 286-364385-003
Bank of China: 012-883-0-002136-6
Details at www.worldvision.org.hk