Investigation of indoor and outdoor air quality at classrooms in Hong Kong

Abstract

An investigation of the indoor and outdoor air qualities at classrooms in Hong Kong was carried out. Classrooms chosen were located in industrial, residential, urban, and rural areas. Pollutions and parameters studied were temperature, relative humidity, carbon dioxide (CO2), sulfur dioxide, nitrogen oxides, volatile organic compounds, particulate matter with diameter less than 10 microns (PM10), particulate matter with diameter less than 2.5 microns (PM2.5), formaldehde, carbon monoxide, and bacteria. The effect of ventilation rate on pollutant concentration was also investigated. Indoor and outdoor PM10 concentrations were high mainly due to vehicle exhaust emissions and construction activities. High PM10 concentrations (200 ug/m3) were observed during classroom cleaning because sweeping of the floor re-suspended particulate matters settled on the ground into the air. Higher CO2 concentrations, with averages over 1000 ppm, were found at classrooms with mechanical ventilation than those classrooms with natural ventilation. All of the maximum CO2 concentrations in the classrooms were higher than the American Society for Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) standard of 1000 ppm due to very high occupancy (over 60 person/100 m2) and inadequate ventilation. Maximum CO2 level of 5900 ppm measured at an air-conditioned classroom because the door and windows were closed and ventilation was shut off. Ventilation rates measured at the naturally ventilated classroom and the air-conditioned classroom were 0.937 air changes per hour (ACH) and 0.217 ACH respectively. The ventilation rates were below the ASHRAE Standard 62-1989 of 7.5 l/s. Results from the control study agreed with those in the previous studies that pollutants with outdoor sources had lower indoor concentrations than the outdoors in an air-conditioned classroom. The air-conditioners, though not providing enough fresh air, could be barriers for outdoor pollutants. Therefore air-conditioning in classrooms can provide a quiet learning environment for children, and also provide a thermally comfortable environment, and lower concentrations of pollutants but not adversely affecting indoor air quality. Improvements in air exchange rates with installation of air cleaning devices can provide a more comfortable learning environment for students.