## 4 Discussion  
The characteristic earthly smell of soil is mainly produced from microbial metabolites, geosmin and 2-methylisoborneol. These odor molecules also play an essential role in the taste and odor of the drinking water obtained from surface water [4]. The taste and odor are highly linked with the suitability and safety of drinking water. In addition, humans can easily detect these molecules at very low concentrations due to extremely low levels of odor threshold. According to the odor type (pleasant or unpleasant), the odor molecules significantly affect brain function through the olfactory system. Furthermore, it was reported that odor molecules influenced stress biomarkers, oxidative stress and dopamine in humans. In a positive approach, odor molecules have been used for decreasing mental stress and enhancing relaxation as well as cognitive function [8,18]. In this context, the present study examined the effect of inhalation of geosmin and 2-methylisoborneol on the human EEG activity in relation to brain functions. In the EEG study, delta (0-4 Hz), theta (4-8 Hz), alpha (8-13 Hz), beta (13-30 Hz), and gamma (30-50 Hz) are the most important brain waves used to understand the role of odors in the psychological and physiological conditions of humans. In the current study, absolute fast alpha, absolute beta, absolute low beta, absolute high beta, and absolute gamma activity significantly decreased, particularly at the Cp6 region, during the  
Figure 4: A schematic representation of inhalation of geosmin and 2-methylisoborneol on human EEG activity. AA, absolute alpha; AFA, absolute fast alpha; AHB, absolute high beta; ALB, absolute low beta; RT, relative theta; RA, relative alpha; RB, relative beta; RG, relative gamma; RFA, relative fast alpha; RSL, relative slow alpha; RLB, relative low beta; RMB, relative mid beta; RHB, relative high beta; RST, ratio of SMR to theta; RMT, ratio of mid beta to theta; RSMT, ratio of SMR mid beta to theta; RAHB, ratio of alpha to high beta; SEF50, spectral edge frequency 50%; SEF90, spectral edge frequency 90%; ASEF, spectral edge frequency 50% of alpha. inhalation of geosmin in both genders (Table 2 and Figure 2). In general, increase in alpha wave activity is mainly associated with relaxation and calmness states in the brain. Hence, a decrease in alpha wave activity is highly linked with stress state [13; 19; 20]. However, absolute waves such as beta, low beta, and high beta significantly decreased at the same region (Cp6) during the inhalation of geosmin. Beta waves play a major role in the enhancement of cognitive performances in humans. An increase in beta activity is associated with alertness and concentration states of brain [21]. In the posterior regions, increase in beta wave activity is highly related to the enhancement of reading speed, reorganization of language, and rhythm-learning task [22; 23; 24]. [PERSON] et al. [25] reported that the beta wave significantly increased at the anterior center and left posterior regions due to the exposure of jasmine oil, and these changes may produce positive psychophysiological effects in humans by enhancing active and fresh feelings. In a recent study, _Angelica gigas_ root essential oil induced significant changes in the absolute low beta activity at the left parietal region [26]. Collectively, beta waves occur when the brain is alert. In the present study, the decrease in beta wave (beta, low beta, and high beta) activity at the Cp6 region due to geosmin exposure might produce the opposite effect--enhancing calmness and relaxation. [PERSON] et al. [27] reported that the inhalation of (R)-(-)-linalool produced a significant decrease in beta waves after work when compared with before work. Furthermore, the results revealed that geosmin affected absolute wave activity only at the Cp6 region in both genders. In addition to absolute waves, ratio values such as RMT, RST, and RSMT significantly decreased at the Cp6 region.