Indoor Air Quality Investigation on CPA aircrafts was investigated by the Cathay Pacific Airways Limited (CPA) and the Hong Kong Polytechnic University (HKPU). The parameters concerned were carbon dioxide (CO2), relative humidity, temperature, carbon monoxide (CO), ozone (O3), sulphur dioxide (SO2), nitrogen dioxide (NO2), bacteria, fungus, and respirable suspended particulate (RSP). This project covered 16 flights from June 1996 to August 1997. The overall air quality on CPA aircraft was satisfactory. CO2, CO and ozone levels on all flights were below the Federal Aviation Administration (FAA) standard. Peak levels of CO2 and RSP were observed during boarding and de-boarding periods. For smoking flights, the average RSP level (138μg/m3) was much higher when comparing with non-smoking flights (7.6 μg/m3). The minimum relative humidity (4.9%) during cruise was below the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) minimum relative humidity standard (20%). The average temperature was within the range of 20±2°C. The vertical temperature profile on aircraft was uniform and complied with the International Standard Organisation (ISO) 7730 standard.