Air Quality Investigation at Schools in Hong Kong

Occupancy of classrooms in Hong Kong often exceeded the maximum occupancy of the ASHRAE 62-1989
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Indoor and outdoor air quality investigation at schools in Hong Kong 2000

S.C. Lee and M. Chang
Environmental Engineering Unit, Department of Civil and Structural Engineering,
The Hong Kong Polytechnic University, Hong Kong SAR, People’s Republic of China


Five classrooms in Hong Kong (HK), air-conditioned or ceiling fans ventilated, were chosen for investigation of indoor and outdoor air quality. Parameters such as temperature, relative humidity (RH), carbon dioxide (CO2), sulphur dioxide (SO2), nitric oxide (NO), nitrogen dioxide (NO2), respirable particulate matter (PM10), formaldehyde (HCHO),and total bacteria counts were monitored indoors and outdoors simultaneously.

The average respirable particulate matter concentrations were higher than the Hong Kong recommended IAQ Objectives, and the maximum indoor PM10 level exceeded 1000 ug/m3. Indoor CO2 concentrations often exceeded 1000 ppm in air-conditioning and ceiling fan classrooms, indicating inadequate ventilation. Maximum indoor CO2 level reached 5900 ppm during class at the classroom with cooling tower ventilation. Increasing the rate of ventilation or implementation of breaks between classes is recommended to alleviate the high CO2 level. Other pollution parameters measured in this study complied with the standards. The two most important classroom air quality problems in Hong Kong were PM10 and CO2 levels

Carbon Dioxide Levels

Average indoor CO2 levels at the two air-conditioned classrooms (TC and SF) exceeded the ASHRAE CO2 standard, but other average indoor and outdoor concentrations complied with the standard (Fig. 2). All maximum indoor CO2 levels at the five classrooms were higher than 1000 ppm and the highest CO2 concentration was recorded at SF reached 5900 ppm. Closing of windows, doors, and the air-conditioning system during school hours caused the high level of CO2 in SF. Overcrowded classrooms could also be the reason for CO2 levels close to 1000 ppm. The maximum occupancy in classroom environment that was recommended by the ASHRAE Standard 62-1989 is 50 person/100 m2. The occupancy at the fivve classrooms was 65, 85, 83, 45, and 74 for TC, SF, MFS, MFC and SJ respectively. The classrooms exceeded the Standard except for MFC. The eff€ect of metabolic emission accumulation was further demonstrated in the air-conditioned classrooms. TC and SF had higher indoor CO2 levels than classrooms without (MFS, MFC and SJ).


Indoor CO2 concentrations exceeded the ASHRAE standard due to overcrowded classrooms and inadequate ventilation. Implementation of more breaks and recesses between classes might alleviate the high level of CO2.

The occupancy of classrooms in Hong Kong often exceeded the maximum occupancy of the ASHRAE 62-1989 standard. Decreasing the number of students in each classroom can also lower the CO2 level. Increase the rate of ventilation could remove the accumulated CO2; for example, the use of ceiling fans, exhaust fans could increase the exchange of indoor air with the outdoor.

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