## 3 Results
### Close-Agreement Analysis between DYU Air Box and Commercial Instruments  
In order to double-check the agreement, confidence, and reliability of the proposed DYU Air Box, the acquired IAQ index, PM\({}_{1.0}\) and PM\({}_{2.5}\) concentrations, CO\({}_{2}\) concentration, brightness, temperature, and humidity were simultaneously measured using the corresponding measurement instruments: TES-5322A IAQ meter, TES-5110 particle counter, TES-1370H CO\({}_{2}\) meter, TES-1337B light intensity meter, and TES-1364 temperature and humidity meter, which are commercially available. The readings are depicted in Figure 7a,c-h. In order to easily analyze the agreement between the DYU Air Box and commercial measurement instruments, the differences of IAQ index, PM\({}_{1.0}\) and PM\({}_{2.5}\) concentrations, CO\({}_{2}\) concentration, brightness, temperature, and humidity are depicted in Figure 8.  
Taking the measurement results of the above measurement instruments as references, the relative difference between the proposed DYU Air Box and the corresponding instruments is defined in Equation (1). The corresponding performance indexes of the MAE, MAPE [29], and RMSE for close-agreement analysis are also defined in Equations (2)-(4). As shown in Figure 8a, the reading difference of IAQ level between the DYU BME680 AQI sensor and TES-5322A IAQ meter ranges from \(-\)0.1396 to 0.1002; therefore, the related MAE and RMSE values are 0.06 and 0.07, respectively. Moreover, the corresponding MAPE value is 0.31%, which means the agreement of measurement results between the BME680 sensor in the proposed DYU Air Box and TES-5322A IAQ meter can reach up to 99.29%. Because the readings of IAQ in the H708 classroom at Da-Yeh University are far below the threshold value of 50, the IAQ levels of the BME680 sensor in the DYU Air Box and TES-5322A IAQ meter are the same which makes the differences between them zeros as shown in Figure 8b. Therefore, the values of MAE, MAPE, and RMSE for IAQ level are 0. The MAPE for IAQ level is 0 as well. Figure 8c,d illustrate that the difference ranges of PM\({}_{1.0}\) and PM\({}_{2.5}\) concentrations between the PMS7003T sensor in the DYU Air Box and TES-5110 particle counter are \(-\)0.1157-0.1183 ug/m\({}^{3}\) and \(-\)0.1182-0.11201 ug/m\({}^{3}\), respectively. Their corresponding MAE/RMSE values are, respectively, 0.06/0.07 ug/m\({}^{3}\) and 0.06/0.07 ug/m\({}^{3}\); therefore, the MAPE values of PM\({}_{1.0}\) and PM\({}_{2.5}\) concentrations are 0.37% and 0.50%, respectively. This means the PMS7003T sensor in the DYU Air Box can provide measurements of PM\({}_{1.0}\) and PM\({}_{2.5}\) concentrations that are up to 99.6% and 99.5% similar to those of the commercialized TES-5110 particle counter. As shown in Figure 8e, the measurement difference of CO\({}_{2}\) concentrations between the SCD30 CO\({}_{2}\) sensor in the DYU Air Box and TES-1370H CO\({}_{2}\) meter is \(-\)3-4 ppm with the related values of MAE and RMSE being 1.48 ppm and 1.72 ppm, respectively. The resulting MAPE is 0.35% which reveals the corresponding measurement similarity of up to 99.65% between the SCD30 CO\({}_{2}\) sensor in the DYU Air Box and TES-1370H CO\({}_{2}\) meter. The above close agreement of CO\({}_{2}\) concentrations and IAQ criteria including AQI value/level and PM\({}_{1.0}\)/PM\({}_{2.5}\) concentrations significantly reveals the measurement confidence of the SCD30, BME680, and PMS7003T sensors in the proposed DYU Air Box. On the other hand, the illumination difference is depicted in Figure 8f which illustrates the difference range is between \(-\)15.77 lux and 17.32 lux. Therefore, the corresponding MAE, RMSE, and MAPE are 1.83 lux, 3.70 lux, and 0.25%, respectively. The MAPE of 0.25% for brightness means the BH1750 light sensor in the proposed DYU Air Box can offer measurements that are 99.75% similar to those of the commercialized TES-1337B light intensity meter.