Clean Air Schools for Hong Kong: Air Quality Insights from Nov 2022 to Jan 2023

Clean Air Network

Clean Air Schools for Hong Kong, organized by Clean Air Network (CAN) and the Institute for the Environment of the Hong Kong University of Science and Technology (IENV HKUST), is monitoring both outdoor and indoor air quality in primary schools across Sham Shui Po and Tuen Mun which are located in densely populated areas next to heavily trafficked roads. The monitoring data and insights in the second quarter, from November 2022 to January 2023, have been released, and the key extracted insights are demonstrated below. 


Better outdoor air quality in winter


Average ozone concentration dropped in the second quarter of the monitoring period. Since ozone formation is dependent on solar intensity and duration, the shorter daytime in winter led to a lower level of ozone concentration.

Generally speaking, outdoor air quality improved in the second quarter. For all the 8 schools, the average Air Quality Health Index (AQHI) ranked low and moderate for over 99% of the time and the average AQHI dropped from 4 to 3. As ozone occupies a greater weight in the calculation of the AQHI, a lower ozone concentration also led to a lower AQHI.

Factors affecting ozone concentration and the reason for lower ozone concentration in winter
Graph 1: Factors affecting ozone concentration and the reason for lower ozone concentration in winter


Improved ventilation enhanced indoor air quality


A drop in classroom carbon dioxide (CO2) levels was recorded in all schools. The opening of windows facilitated ventilation and thus reduced the indoor CO2 concentration, improving the overall indoor air quality (IAQ).


Particulate matter (PM) concentration peaks were recorded during non-school hours in one of the participating schools, which was mainly caused by the operation of the air disinfector inside classrooms. But as the purifiers were operated before school time, time was allowed for the indoor PM to drop to a relatively low level, the risk of exposure to PM would thus be low.