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The benzene level is expected to further drop to about 20 per cent of the UK limit when vehicles without catalytic converters are phased out.
The proposed switch of light duty diesel vehicles to unleaded petrol will increase the ambient benzene concentration slightly but Hong Kong will still be able to meet the most stringent international health target.
It should also be noted that diesel engines also emit a proven carcinogen known as Benzo(a)pyrene and there is as yet no effective technical means to reduce its emission. Furthermore, it poses a greater health risk at present level than benzene.
On the point that petrol vehicles emit more carbon monoxide than diesel vehicles, information available shows that the maximum hourly average of carbon monoxide level at the Mong Kok monitoring station is only about one quarter of the AQO limit. The proposed switch will only increase carbon monoxide level to about one third of the AQO limit, still well within the acceptable level.
Regarding the emission of two other pollutants, namely nitrogen oxides and sulphur dioxide which cause to respiratory illnesses and lung impairment, emissions from ULP vehicles are substantially less than diesel vehicles.
On the operating costs of the proposed switch, the Deputy Secretary for Planning, Environment and Lands, Mr Tony Cooper, said the Government used information known to the Transport Department as a basis for the conversion of light diesel vehicles to petrol when listing out the broad operating cost structure in the consultation document.
Mr Cooper said: "Overseas information and manufacturers' data have also been used to arrive at the estimates.
"If the proposed scheme is implemented, the operating cost estimates will be closely monitored and adjusted if necessary.'
However, he added that he would consider now any alternative figures provided by the trade provided these figures could be justified.
As regards the concern of some operators of small school buses and private vehicle fleets that the five-year conversion period might cause undue economic hardship to them because most of their vehicles were well-maintained and could be used for up to 10 years, Mr Cooper said consideration might be given to extend the conversion period for these vehicles.