Measurement and analysis of personal exposure to nitrogen dioxide from indoor and outdoor sources
The study of exposure to nitrogen dioxide (NO2) is important because of its significant health effects. As it is associated with combustion processes, road traffic is one of the main outdoor sources and gas cookers and gas heaters are the main indoor sources. Indoor NO2 is a significant health problem due to people spending most of their time indoors. Activity patterns and lifestyles vary and, consequently, people may be exposed NO2 from several different sources during a typical day. In order to understand and quantify total personal exposure, it is, therefore, important to determine both the indoor and outdoor concentration levels. This thesis reports on two pilot studies, spring and summer 2000 and three full campaigns, autumn, winter 2000 and summer 2001 to investigate the relationship between NO2 personal exposure of office workers in relation to indoor and outdoor sources and activity patterns. The study has been carried out in the area of Hertfordshire, UK. This region is adjacent to London and has a population of just over one million people. It consists of several major commuter routes connecting medium sized towns to London. Volunteers using gas cookers and electric cookers in their kitchens were asked to fill in activity patterns records and questionnaires. At the same time, weekly average personal exposure to N02 and indoor (bedroom, living room, kitchen and office) and front door N02 concentrations were measured by using passive diffusion tubes. Correlation between weekly personal exposures and mean indoor and outdoor concentrations during the same periods were examined. The results show significant differences in indoor and outdoor concentrations of NO2 in autumn and winter. The data indicated that NO2 concentrations in all rooms in houses with gas cookers were significantly higher than those with electric cookers especially in kitchens where levels of NO2 were 3 to 4 times greater. Interpretation of time activity daily diaries showed that the subjects spent on average 80% of their time indoors. Despite the very high concentrations in kitchens with gas cookers, personal exposure did not increase similarly as volunteers only spent a small amount of time cooking over the 7 day period. Good correlation was observed between the average indoor NO2 concentrations, especially in bedrooms and living rooms, and personal exposure. This indicated that indoor levels in areas like the bedroom and living rooms could be used as a proxy for NO2 personal exposure for this group of volunteers. An empirical time weighted average concentration model was developed based on the NO2 concentrations measured in the microenvironments and the data on time spent in each microenvironment. This was tested by comparison between time weighted average calculations and the personal exposure measurements of NO2 concentrations. The comparison yielded good relationships for most of the campaign periods despite the fact that NO2 concentrations were not similar in the different micro environments and the fact that subjects spent varying times in these places. Statistical tests were performed for time weighted average concentrations of N02 and the personal exposure to NO2 concentrations and differences were found to be non-significant.