# Development of an Automatic Low-Cost Air Quality Control System: A Radon Application
## 3 Results and Discussion
The system controls the air quality by turning on and off the ventilation system. In these tests, the air quality was measured as the radon concentration due to its too high \begin{table}
\begin{tabular}{c c c c} \hline
**Minimum Radon** & **Mean Radon Concentration** & **Maximum Radon** & **Time Above Limit (h)** \\
**Concentration (Bq/m\({}^{3}\))** & **(Bq/m\({}^{3}\))** & **Concentration (Bq/m\({}^{3}\))** & **Time Above Limit (h)** \\ \hline
278 & 1230 & 3288 & 166.37 \\ \hline \end{tabular}
\end{table} 
Table 1: Statistics for the radon concentration during a week without the air quality control system activated  
Figure 5: Radon concentration evolution and airflow control system activation (red vertical bars) during a week for the tested interpolation time windows: (**a**) 10 min; (**b**) 30 min; (**c**) 1 h; (**d**) 2 h.  
natural values. The system needed to mitigate the radon concentration has to be powerful, so it has a high power consumption and high noise, as the ventilation system technical characteristics show. The system's main objective is to maintain the radon concentration below the threshold. Its secondary objective is to reduce the power consumed by the ventilation system and the noise, thermal and air waves discomfort it may produce, resulting in a worsened indoors environmental quality. The primary objective would keep the people working in the room where the system is deployed safe, and the secondary objective would allow them to work more comfortably and be more productive, while minimizing the cost of used resources (ventilation system). The background of each image in Figure 5 shows, in red, when the prediction algorithm activated the ventilation system. Table 2 shows the mean radon concentration for each interpolation time window tested, the maximum level reached, and the total time the airflow system was turned on during the week that each test lasted. We can see in Table 2 that all the interpolation window values tested can maintain the mean radon concentration below 70% of the threshold, when naturally it is 4.1 times higher than the threshold, as shown in Table 1. The 10 min and 2 h interpolation windows are the ones that provide the lowest mean radon concentration. We can also see in Table 2 that all the interpolation window values tested lowered the Maximum radon concentration with respect to the highest level reached naturally. Using the prediction algorithm with the 2 h interpolation window provides the lowest Maximum radon concentration. In this case, the maximum radon concentration is approximately 15% of the highest value reached naturally, without the ventilation system activated (almost a 86% decrease). The time the algorithm kept the ventilation system on to achieve the previous values are similar, the interpolation window providing the lowest value and the 1-h interpolation window the highest. The algorithm configured with the 10 min interpolation window needed to keep the ventilation system on approximately 28% of the total test time to keep the radon concentration below the threshold. As explained, the regulatory limits are established for annual continuous levels, not temporary ones; thus, the time above limit is the metric we should focus on to select the appropriate interpolation windows for the prediction algorithm. Table 1 shows that the 2 h interpolation window was the value that kept the radon concentration below the threshold the longest. In this case, the radon concentration was below the threshold 90% of the time, when naturally it was below the threshold only 0.01% of the time, as can be calculated with the values shown in Table 1.  
The boxplot shown in Figure 6 compares the natural radon concentration distribution and the distribution of the radon concentration during the tests for the different interpolation time windows. The horizontal red bar shows the radon threshold (300 Bq/m\({}^{3}\)). The figure shows that the system kept the median radon concentration below the threshold for all interpolation windows tested.