General API vs. Roadside API

General API is more relevant as it represents the air pollution that we will be exposed to for most of the time. The general API makes reference to the measurements at our ambient monitoring stations located on 4 to 6-floor buildings. The stations comply with the highest international standards and are designed to be fully accredited and approved by overseas experts to provide accurate and representative air pollution information.

The roadside API provides information on the level of pollution very close to vehicle emission sources in busy streets and roads. The roadside API will, therefore, naturally be higher than the general API on a given day.
For most people, the roadside API will be less relevant as they spend only a short period of time each day in such locations. The roadside APIs will be of more interest to those who spend several hours continuously each day in busy streets.

(Source: EPD, HKSAR Gov't)