Nitrogen monoxide (NO) and nitrogen dioxide (NO2) are gases produced by hightemperature combustion through reactions between nitrogen and oxygen. Both nitrogen in fuel and nitrogen in the air can participate in the reactions. Formation of NO can be followed by further oxidation into NO2 in the atmosphere. NO2 is the most toxic of the nitrogen oxides (grouped generically as NOx) and it is very important from the health point of view. Emissions of nitrogen oxides have generally increased, a fact which has resulted in a lot of studies dealing with nitrogen oxides exposure, particularly over the last 15 years. Measurements of NO2 were carried out in residential and industrial premises over 2-day periods both in winter and summer. Personal exposures were found to vary substantially among the 43 respondents, ranging from 10.2 87.5 ppb with a mean of 43.7 ± 16 ppb in winter and 23.6 ± 7.8 ppb in summer. The results also showed that the season of the year, house size and outdoor occupation could significantly influence exposure as well as factors such as the type of fuel used and the ambient air quality level. Personal exposure to NO2 was significantly greater in winter than in summer, and respondents living in smaller houses and/or having outdoor occupations were exposed to higher NO2 levels (Kulkarni & Patil, 1998.). Another study was focused on measurements of nitrous acid (HONO) and NO2 concentrations in a total of 119 houses. Indoor samplers were deployed in the living rooms of the houses for 6 days and outdoor samplers were placed near the houses. Average indoor HONO and NO2 concentrations were higher than outdoor levels. The results suggest that both HONO and NO2 are generated by indoor sources (Lee at al., 1999). Subsequent surveys focused on characterisation of the air quality in buildings and the investigation of any seasonal changes in the air quality. There was some indication of much higher concentrations of NO and NO2 on the ground floor of the buildings and there was some evidence of a seasonal gradient when the concentrations of air contaminants obtained during the winter and summer survey were compared (Nayebzadeh et al., 1999). In one study carried out in Hong Kong (Chao & Law, 2000) a group of 60 people was selected for a study of their personal exposure to NO2. All the participants spent most of their time at home and in an air-conditioned office environment. It was found that the average personal exposure to NO2 was 46.0 μg/m3, which was quite close to the average level of 47.3 μg/m3 of the 12 participants involved in the measurement exercises in their home environment. The 12 residential premises were studied for NO2 levels in different locations in both the indoor environment and the surrounding outdoor environment. It was found that the average NO2 level in the indoor environment (living room, bedroom and kitchen) was 55.2