## 2 Methods
### Accuracy of IAQ Measurement Devices  
An inter-comparison, comprising measurements in the laboratory, was made to ensure the relevance and to estimate the accuracy of the intermediate and low-cost devices (Low1, Int3). The process consists of placing simultaneously an intermediate and low-cost device and a laboratory device in rooms and in a test chamber where the monitored parameters are varied. The different devices are placed near each other. The TVOC and formaldehyde inter-comparison was not performed because it was technically not possible. One laboratory device (Lab2-1) and one low-cost device (Low5) were chosen as reference devices. Indeed, Low5 devices have an auto-calibration procedure for CO\({}_{2}\) concentrations, and the measurements provided by the eight devices over 1 week were compared, yielding the following results: CO\({}_{2}\) (R\({}^{2}>\) 0.99 and 1 \(>\) slope \(>\) 0.94), RH (R\({}^{2}>\) 0.94 and 1 \(>\) slope \(>\) 0.95) and T (R\({}^{2}>\) 0.88 and 1 \(>\) slope \(>\) 0.94). The next test compared Lab2s with Low1s for PM\({}_{2.5}\) and PM\({}_{10}\) (mass) and Lab2s with Int3s for size distribution in number. The last test compared Low5s, Low1s, and Int4s for CO\({}_{2}\), T, and RH. Table 4 summarizes the linear regression results from comparison of the reference and the low-cost devices.  
The general findings are a high correlation for T and RH whatever the device, with an RMSE less than 2% for RH; a lower accuracy for the CO\({}_{2}\) measurements using the Int4s with a coefficient correlation of 0.62 and an RMSE of approximately 100 ppm.  
For the particle number, the Lab2 to Int3 comparison shows a significant overestimation of Int3, with slopes between 1.65 and 2.9, but the overall R\({}^{2}\) value as shown in Table 4 was between 0.86 and 0.94 for 0.3 \(\upmu\)m and between 0.83 and 0.95 for 0.5 \(\upmu\)m. The behaviors of the two Int3 devices similarly show an overestimation compared to the reference device.  
For PM\({}_{2.5}\), slopes equal to 0.97 are identified for the Lab2-2 and for Low1-1 device. For the Low1-2, the slope is equal to 0.82. However, for the two Low1 devices, the R\({}^{2}\) values are lower than that of the Lab2 devices. The RMSE values of the Low1-1 and Low1-2 devices are equal to 0.75 \(\upmu\)g\(\cdot\)m\({}^{-3}\) and 0.85 \(\upmu\)g\(\cdot\)m\({}^{-3}\), respectively, which is approximately equal to 10% of the measured range. Therefore, the Low1 devices are adopted for the PM\({}_{2.5}\) measurement campaign.  
On the contrary, for the PM\({}_{10}\), whatever the device, the slopes are quite far from 1, indicating differences in measurements. First, it is noted in the test sample that the number of particles with a diameter larger than 2.5 \(\upmu\)m is low and equal to 0.07% on average. This observation can explain uncertainties and thereby the observed difference between the two Lab2s and the relatively poor R\({}^{2}\). For the two Low1 devices, the low values of the slope indicate that these devices are less sensitive in detecting large particles. In fact, according to the Low1 devices, the weight percentages of particles with a diameter between 2.5 and 10 \(\upmu\)m are only equal to 2.0 and 2.3%, respectively, on average. According to the Lab2 devices, these values are equal on average to 29% and 30%, respectively. In addition, the two linear regressions of low1 devices are associated with two negative R\({}^{2}\) values, which indicates there is no linear correlation between nodes and reference Lab2 values. As a result, Low1 devices cannot be used to measure PM\({}_{10}\). So, PM\({}_{10}\) was not measured in this study.