## 3 Results and Discussion
As their occurrence was low in the control group, the percentage areas of the spectrum for ethyl 3-cyano-2,3-bis (2,5-dimethyl-3-thienyl)-acrylate and 1-phenantherol were not determined. It was previously reported that the presence of 1-phenantherol is possibly attributed to endocrine disruption or accumulation of acetylcholinesterase at the synapse, and that it further disrupts the function of the nervous system and impairs behavior in a fish model [18; 19]. Hence, this could suggest an association of high levels of 1-phenantherol and the occurrence of AD. In addition, no ethyl 3-cyano-2,3-bis (2,5-dimethyl-3-thienyl)-acrylate was found in the PD group. Although there have been no specific reports on the role of ethyl 3-cyano-2,3-bis (2,5-dimethyl-3-thienyl)-acrylate in the development of neurodegenerative diseases, it could be an indicator to distinguish between the AD and PD groups. Further investigation is necessary to provide better insight into the correlation between exhaled ethyl 3-cyano-2,3-bis (2,5-dimethyl-3-thienyl)-acrylate and the occurrence of AD. Moreover, 1,2-benzenedicarboxylic acid diethyl ester had a higher percentage area of the spectrum in patients with AD than those in the PD and control groups, whereas patients with PD had lower levels than those in the control group, although the differences were not statistically significant. A previous study showed that the use of this compound induced exitotoxicity and neuronal cell death [20]. Furthermore, it may also contribute to the impairment of memory as a consequence of declining choline acetyltransferase function [20]. In contrast, one study also demonstrated the potential use of 1,2-benzenedicarboxylic acid dinonyl ester in reducing A\(\beta\)-induced neurotoxicity [21]. Hence, further studies of this compound in exhaled breath could be useful. Additionally, triphenyl phosphate had a higher percentage area of the spectrum in healthy individuals than in patients with AD. Despite the difference being insignificant, these results may suggest that disease progression reduces the production of triphenyl phosphate in exhaled breath. Triphenyl phosphate has been proposed as being useful in combating the diseases involving the irreversible inhibition of acetylcholinesterase [22]. A previous report also described the potential use of the isomers of triphenyl phosphate, such as dialkyl phenyl phosphate, as inhibitors of cholinesterase to slow down the progression of AD [23]. Furthermore, to distinguish the exhaled breath samples between the patient and control groups, we carried out an exhaled breath clustering analysis using the exhaled breath sensor systems. The normalized responses of the sensitivity characteristics for the gas sensors in system A (Figure 2) and D (Figure 3) were significantly different between the AD, PD, and control groups. System A, with TCS 2602, MICS 5135, MICS 2610, and MICS 2611, had distinctive sensitivity characteristics, whereas TCS 2602 from system D was found to have significantly different sensitivity characteristics in the AD, PD, and control groups. The TCS 2602 sensor from both systems had distinctive differences between the patient and control groups. The normalized response of the sensitivity characteristics of TCS 2602 in the AD group was higher than that in the PD and control groups. The TCS 2602 sensor is designed to detect volatile organic compounds and odorous gases. By comparing the results to the GC-MS analysis, polyaromatic hydrocarbons, such as 1-phenantherol, were also found to have high percentage areas of the GC-MS spectrum in the AD group. This may imply a correlation between the findings from the exhaled breath systems and the GC-MS analysis. Additionally, TCS 2600, MICS 2610, MICS 2611, and MICS 5135 had higher levels in patients with PD than those in the AD and control groups. Some of these gas sensors, such as MICS 5135 and MICS 2610, had high normalized sensitivities in the detection of silicone vapor.