While trying to change the philosophy of the prediction of adverse health effects due to the inhalation of polluted air, it should be noted again that in a typical urban environment, the population is exposed to about 200 air pollutants or classes of air pollutants. Therefore, instead of investigating the unique effects of specifi c pollutants, it has been suggested that it might be more reasonable to assume that it is a mixture of pollutants that might be considered harmful to health (Dominici and Butnett 2003; Moolgavkar 2003; Stieb et al. 2002; Roberts and Martin 2006; Dionisio et al. 2013). Potential interaction among pollutants seems to be a fundamental problem indispensable for explaining the relations between the exposure of people to air pollutants and their

health condition. However, before any analysis of this problem, some key terms should be precisely defi ned. According to U.S. EPA Guidance (2000), and supplemented by Mauderly and Samet (2009), the following terms may be introduced:

Additivity: effect of the combination equals the sum of individual effects.