Commuters' exposure to carbon monoxide and carbon dioxide in air-conditioned buses in Hong Kong

The summer in Hong Kong is hot and humid with temperatures over 30 degrees C and relative humidity over 70%. Air-conditioned buses were first introduced in the early 1990s and have become more and more popular. Everyday, millions of passenger-trips are taken in such buses and the air quality and thermal environment in them has become a public health issue. One major source of pollution is the passengers themselves but re-entry of engine emission is also a concern. People have always complained about the thermal environment of the buses especially during rush hours saying they are stuffy, smelly and have inadequate fresh air. We measured conditions in air-conditioned buses and non air-conditioned buses, running on 12 different bus routes in the Hong Kong Special Administrative Region (HKSAR), in both urban and rural areas. Two trips were made for each route. Portable gas analysers measured carbon monoxide (CO), carbon dioxide (CO2), temperature and relative humidity. The measurements showed there was no evidence that CO and CO2 levels in the air-conditioned buses were statistically higher than in non air-conditioned buses (P>0.05). For the rural commutes, the in-vehicle levels of CO and CO2 were not significantly higher than for urban commutes (p>0.05). The Environmental Protection Department (EPD), HKSAR had published a guidance note for air-conditioned public transport facilities and most of the exposures found, including the thermal environment, were acceptable with regard to the recommendations in the note. Momentary high levels of CO2 were found, but this lasted for a short period only.