Of the government's initiatives to get us to be more environmentally responsible, perhaps the most effective has been the one espousing that office air conditioners be set at 25.5 degrees Celsius. It is a number many people seem to be aware of, as a quick straw poll proved. Whether the knowledge is put into practice is another matter, of course, but the fact that it seems the message has been widely disseminated is impressive. Here the praise must end, though: the information is only partly right.
This is one of those cases where authorities have only done half their job; 25.5 degrees is a summer setting, not applicable to the other nine months of the year. During colder weather, thermostats should be lowered according to the outside conditions. If outdoor temperatures are less than those inside, we are on average doing more environmental harm than good. Every extra degree of cold or heat raises usage - and fuel bills - by 10 per cent, after all.
The government announced in October 2004 that air conditioners in all public buildings should be set at 25.5 degrees during summer. It launched a 'no freezing summer' campaign the following June to encourage the private sector to follow suit. The figure has since been frequently bandied about by officials, most emphatically by Chief Executive Donald Tsang Yam-kuen in 2006 when he removed his jacket and trademark bow tie in front of legislators to promote his 'Action Blue Sky' policy.
His clear message was that dressing more casually meant rooms did not have to be so cold. Less electricity would be needed, meaning lower bills and less pollution from power plants. It was a strategy that cost nothing to implement and would save money and the environment. Brilliant.
The rare image of a dressed-down Mr Tsang could well be the reason so many of us have '25.5' memorised. I am sure it flits subliminally through our minds whenever we drift near an office thermostat. A quick check, an adjustment if necessary, and the working environment is as it should be, we tell ourselves. We get back to our duties, satisfied that we have done our bit for Hong Kong.
Gerry McMahon, the director of the specialist building services company Facilities Analysis and Control, put the problem in a nutshell: attaining a 25.5 degree temperature in winter would, in most cases, mean heating a building to a point where the majority of people would feel uncomfortable. Ambient conditions naturally determined indoor temperatures, so when they changed, air conditioners should be adjusted accordingly. Some heating may still be required to achieve desired temperatures, but the notion of pushing the target up to 25.5 degrees in winter was ridiculous. Why waste energy in winter just because someone had decided the setting would save energy in summer, he rightly asked.
I have been unable to find out exactly where that 25.5 figure comes from. Authorities have attributed it to 'overseas research'; I assume this means work done by the American Association of Heating, Refrigeration and Air-conditioning Engineers, whose standards are widely accepted. To be fair, the government has suggested it for summer. It has not made clear, though, that it is inappropriate for mild or damp conditions - as Hong Kong generally experiences from the end of August to the start of June.
Lantau environmental campaigner Eric Spain has long argued that it is wrong to make firm rules. The only solution for comfort and economy should be indoor and outdoor sensors and controllers, to automatically ensure appropriate settings, he says. Based on extensive research, he suggests that depending on humidity, comfortable summer office temperatures are generally between 24 and 27 degrees and, during winter, 20 and 23 degrees. Public transport settings should be at the upper end of the scale during hotter, more humid months, and between 18 and 20 degrees at colder times because of air flows.
I can see the convenience of settling on the 25.5 figure. But authorities have done no one - or the environment - a favour by failing to make clear what should happen at other times of year.
Peter Kammerer is the Post's foreign editor