We've been taken to task by a reader for our piece yesterday for which we stand accused of "repeating this unsubstantiated and unproven statement that only a small number of old diesel-engined vehicles are causing a relatively high amount of the roadside pollution".
The piece was based on calculations by the organisation Clean Air Network (CAN) on the basis of the recent release by the Environmental Protection Department of Hong Kong's 2010 Air Pollutant Emission Inventory.
CAN drew on information from the Transport and Housing Bureau which showed that 10 per cent of Hong Kong's road transport vehicles have old diesel engines in the range of pre-Euro to Euro II.
According to the EPD's data these vehicles account for 73 per cent of total roadside respirable suspended particulates (PM10), and 34 per cent of total roadside nitrogen oxides (NOx).
This may not be rocket science but it is not wholly "unsubstantiated".
Admittedly the figures do not indicate the breakdown of engine types that contribute to emissions on any one day. But they do provide a useful indication as to where the problems lie.