Air quality in Hong Kong is monitored by the Environmental Protection Department (EPD) at a number of Air Quality Monitoring Stations (AQMS) located throughout Hong Kong. The sites of the monitoring stations were selected on the basis that they are considered representative of those parts of Hong Kong with the highest population densities as well as potentially elevated air pollutant concentrations. Both gaseous and particulate pollutants, including sulphur dioxide, nitrogen dioxide, total suspended particulates (TSP), respiratory suspended particulates (RSP), carbon monoxide, lead and ozone are measured. EPD publishes an annual summary of air quality monitoring results which includes a review of the trends in the levels of those pollutants covered by the Air Quality Objectives (AQOs).

At present, Toxic Air Pollutants (TAP), with the exception of lead, are not included in the AQOs. These pollutants differ considerably from the criteria air pollutants due to their presence at substantially lower concentrations in the ambient environment and their health effects are typically carcinogenic in nature. They have not been routinely measured in Hong Kong in the past. However, EPD initiated monitoring of TAPs in 1997, including hexavalent chromium, carbonyls, volatile organic compounds (VOCs), polychlorinated dibenzodioxins (PCDD), polychlorinated dibenzofurans (PCDF) and polycyclic aromatic hydrocarbons (PAHs), at two of their existing AQMSs, in Tsuen Wan (TWS), and in the Central/Western area (CWS).

Both of these AQMSs are "elevated" stations located at 18m (CWS) and 17m (TWS) above ground respectively, and hence present an integrated set of data representative of several sources. However, the most important sources of many of the TAPs of concern are vehicular traffic emissions. Thus, a knowledge of the roadside concentrations of those traffic-related TAPs is considered crucial for the SUSDEV21 baseline database. The one-year roadside TAP monitoring program was designed to fill this information gap.