The other reason of increasing indoor exposure level is environmental tobacco smoke. Compared with sulfur dioxide, nitrogen dioxide has a much lower water solubility, therefore it penetrates deep into the air passages more easily. Finally concerning the indoor environment, indoor air can be contaminated by using gas for heating and cooking and this probably involves a certain health hazard.
The research results show that seasonal differences of indoor exposure level exist there. As continual measurements were done and average monthly values were established for indoor and outdoor environment, these dependencies are shown in figure 1. The average indoor concentration of nitrogen oxides has been found to be higher by approximately 24 µg.m-3 than that outdoor, which has been measured at about 45 µg.m-3 .
The measurements have been done in apartments with gas ranges. Indoor NOx exposure level is much higher in winter time. There are considerable differences between the measured concentrations in various apartments owing to different ventilation levels and gas burning activities, which make the estimation of exposure and the corresponding health risk assessment very difficult.
Average long time values of indoor level exposure are mostly lower than allowed long time standard concentrations. The problem becomes serious in short time NOx exposure. A lot of measurements were done in order to describe situation of indoor nitrogen oxides concentration level after cooking. The values increased so rapidly that it was necessary to open the windows several times. The comprehensive analysis of indoor and outdoor air nitrogen oxides pollution in Slovakia shows that extent of indoor air level exposure varies with the different meteorological conditions, seasonal changes, ventilation level and indoor burning activities. On the whole, in winter months the indoor air quality is worse. In summer the indoor exposure concentration of nitrogen oxides were basically at the same pollution level. This indicated that indoor and outdoor air qualities are similar in summer. It also demonstrated that indoor and outdoor air qualities are usually worse in wintertime than in summer. Both burning and smoking in the rooms have similar risk as major pollutant factors including respiratory illnesses, allergic symptoms and lung cancer. Therefore prevention and research of gaseous pollutants and their impact on health must be put on the same level as prevention and control of other types of pollutants.