Clear the Air says:

The SCMP omits the fact that, as well as the first two red minibuses at a stand allowed to idle their engines, if a passenger sits in the third bus , it can also idle its engine and the one behind also ! So if the stand operators have one person sitting in each of the following minibuses , they can all run their engines legally.

i.e. a pointless and flawed piece of legislation in its current state.

The ban on idling engines could improve air quality by at least 30 per cent, according to a test conducted a day before and a day after by the South China Morning Post (SEHK: 0583, announcements, news) .

However non-compliance as a result of lax enforcement and polluting vehicles could also offset these potential benefits.

The Post took measurements at idling hot spots between 7pm and 9pm on December 14 and 16, in Causeway Bay and Mong Kok, to assess the effectiveness of the ban, introduced on December 15.

Using a hand-held device that automatically registered air quality data every two seconds, the Post measured the carbon monoxide reading at each hot spot for about four to five minutes on both days.

Air quality improvements were recorded in parts of Tung Choi Street, Mong Kok, where red minibuses line the left and middle lanes on a 100-metre stretch between Fife Street and Argyle Street.

On the day before the ban was introduced, about eight out of 15 red minibuses parked on the street had their engines idling.

The new law allows idling for a maximum of six buses for the three bus routes operating from the street.

On the second day, only six out of 16 buses were found idling.

Readings taken during a four-minute walk along the street showed carbon monoxide readings had dropped by 30 per cent from 7,400 micrograms on the first testing day to 5,200 micrograms.

There was also a noticeable improvement near a news stand close to the end of the street, where the average reading fell from 7,300 micrograms to 3,100 micrograms.

The stand owner said he felt the street was becoming “a lot more peaceful” than before.

But air pollution readings fluctuated outside an electronic product shop in the middle of the street, rising from 7,200 micrograms to 7,700 micrograms.

There was also an increase in Fa Yuen Street near Tung Choi Street, possibly due to more buses idling.

However, a semi-covered red minibus station next to Langham Place in Mong Kok remained a hot spot despite the ban, with more than 12 minibuses inside the station running their engines on both days the measurements were taken.

Average readings from a five-minute air sampling at one of the passenger queuing areas hit more than 18,000 micrograms on the first day, and a huge 59,000 on the second day.

The Environmental Protection Department (EPD) sets the maximum hourly allowable carbon monoxide levels at 30,000 micrograms.

A spokesman said the Post‘s findings, based on these short periods of measurement, should not be benchmarked against the department’s objectives.

Dr Lau Ngai-ting, who is with the University of Science and Technology’s environment division, said longer measurement periods would be needed to ascertain the benefits of the ban. But he wondered if other emission sources, including moving vehicles, might have overshadowed the improvements.

“No doubt there are incremental benefits if engines are switched off, but these can also be offset by other factors like moving traffic nearby,” Lau said.

This idea was supported by the Post’s findings. At one point on the second day, the maximum reading soared to a maximum of 157,000 micrograms as a diesel minibus overtook a LPG-powered bus and closed in on the air sampling device. “The reading was quite consistent until the diesel bus came closer and pushed the reading to this extremely high level,” said Nancy Fong Siu-pui, a research student who helped the Post with the measurement.

Lau said clamping down on idling engines was just one way of reducing pollution. Other measures, such as tighter vehicle emission standards and monitoring of ageing and polluting vehicles, were also required.

People working in the area said they had not yet seen officers enforcing the ban. One elderly worker said the pollution was better in the cooler weather, but worse in summer. “I am used to it,” he said.

The EPD did not respond directly when asked if officers patrolled the bus station and issued warnings to drivers breaching the ban, but it promised to refer the issue to the Transport Department.

Across the harbour in the section of Lockhart Road between Cannon Street and Percival Street, there was little change in the pollution readings. This was because most drivers had already started switching off their engines before the ban, with no need for air conditioning now the weather has cooled down.

chifai.cheung@scmp.com