Even unionist lawmakers who had mobilised the transport trade to strongly oppose the bill are now content with it.

'We will support the law as the government has listened and adopted our suggestions,' said Wong Kwok-hing, of the Federation of Trade Unions, referring to exemptions granted to taxis and minibuses.

No lawmakers have sought to file any amendments to the bill, other than those promised by the government to satisfy various demands from the transport industry.

Even the Democratic Alliance for the Betterment and Progress of Hong Kong, which wanted to exempt all vehicles on rainy days, said it accepted the bill, after officials promised it discretionary enforcement on those days.

But Li Fung-ying, another unionist legislator, said she would abstain from the vote as she still believed there were not enough exemptions.

'The driver's seat is their workplace and there is clearly not enough protection for their health,' she said.

If the law is endorsed, the ban will be introduced in early September after a grace period of six months. From then on, parked drivers will only be allowed to leave their engines running for three minutes over the course of an hour.

Idling engines will only be allowed throughout the day when the very hot weather or rainstorm warnings are issued by the Observatory. In 2009, there were a total of 57 days with such warnings.

The disciplined forces and emergency or medical vehicles on duty will not be affected. The exemptions also cover taxi stands, the first two minibuses at stands, coaches or buses with at least one passenger, franchised buses available for boarding, such as those serving housing estates, and private school buses.

The ban will cover PLA garrison vehicles not on duty and consulate personnel cars - except those that enjoy immunity and privileges.

The ban will be enforced by traffic wardens or inspectors of the Environmental Protection Department on all roads, including private roads and car parks.

Audrey Eu Yuet-mee, a barrister and Civic Party lawmaker who chaired the subcommittee that scrutinised the bill, said disputes were still possible after the ban took effect. 'All legislation is bound to have grey areas subject to arguments. But this is not a reason for us not to legislate.'

Eu said legislation was an effective way to change people's behaviour and the ban was expected to bring more cultural changes than substantial improvements to air quality.

Indeed, the Hong Kong version of anti-idling law is much looser than in Singapore and Britain, where there were almost no exemptions. Toronto has most exemptions - 15.

The legislation required the subcommittee to convene 13 meetings between May last year and January to scrutinise the bill, with the first eight meetings spent on cultivating a consensus among the lawmakers on what should be exempted.

Some 117 organisations, mostly transport trade bodies, and individuals attended hearings about the proposals. The environment minister Edward Yau Tang-wah even agreed to ride in a taxi without air conditioning on a hot day as he tried to convince the trade about the need for a ban.

After all the negotiations, the end result represents a much toned-down version of the government's first proposal in 2007.

The original ban offered no three-minute grace period, no exemption on bad-weather days, and only the first two taxis and minibuses at stands were excluded from the ban.

Changes were introduced in 2009 to exempt more taxis and minibuses, and to cover buses with one passenger on board. The three-minute grace period was also introduced.

But officials did manage to resist calls for a blanket exemption in hot months or for specific trades. They also rejected exemptions for drivers carrying people with special medical needs or rehabilitation buses.

An environment official who wanted to remain anonymous said that while the political process to reach a consensus had taken longer than expected, it had been necessary to find common ground.

The official said the law, in spite of the concessions, would bring a real improvement in air quality to people like shop owners and hawkers.

Eu admitted the final law draft was loose compared with the original one, but she acknowledged it was practical to consider the legitimate demands of the transport trade.

She also agreed that a tragic event in which a driver died in his seat in a minibus with its engine switched off last summer was a 'contributing factor' that forced the government to offer more concessions.

Professor Frank Lee Shun-cheng, a pollution specialist at Polytechnic University, did not think the ban would bring much change to pollution levels. 'The government should step up air quality monitoring at street level to see if more drivers choose to circulate on the road to avoid the ban,' he said.

Keep on running

Where the ban applies

All roads, including private roads and car parks

Exemptions

Drivers can switch on their engines for three minutes in any hour when their vehicle is stationary

During a very hot weather warning or rainstorm warning

All taxis at taxi stands

The first two minibuses for each route at a stand

Non-franchised buses with at least one passenger on board

Franchised buses available for boarding

Private school buses

Disciplined forces' vehicles as well as emergency and medical services vehicles on duty