# Development of an Automatic Low-Cost Air Quality Control System: A Radon Application
## 3 Results and Discussion  
In this section, we provide the results of testing the automatic air quality control system as a whole and the prediction algorithm, in particular, using different values for the interpolation time window that modifies the prediction algorithm's behavior, and thus the behavior of the system. The tests were carried out in a radon research room located inside the Center for Technological Innovation in Construction and Civil Engineering (CITEEC in Spanish) of the University of A Coruna, Spain [48]. The room is in a location where the radon concentration is naturally high. It is equipped with usual office furniture to emulate the heat and air dynamics of a regular workplace. The room has the same ventilation system as the rest of the building rooms, a Soler & Palau TD-350/100-125 ventilation system [49]. This ventilation system can provide an airflow of 350 m\({}^{3}\)/h at free discharge at a theoretical sound pressure level around 40 dB, measured in free field condition at 3m working at 2235 rpm. As previously explained, radon gas can enter a residence through pipes (electrical, water, and similar), cracks in the construction, or other openings in the room walls. The best way to avoid radon gas entering a room is to increase the room's air pressure. This is achieved in the radon research room by using the TD-350/100-125 fan to force air from the outside of the room to the inside. The fan is coupled with a Soler & Palau MBE-125/12B heater [50], which heats the air forced inside the room, increasing the air pressure and avoiding the room's temperature to fall drastically. The ventilation system's total power consumption is around 1220 W, and the total noise is around 70 dB. We attached a control device to the ventilation system so that the air quality control system can activate it remotely. We placed a monitoring device in the room and configured the association between the monitoring device and the control device in the server. The experiments consisted of testing 10 min, 30 min, 1 h, and 2 h for the prediction algorithm's interpolation window. Each test was carried out, in the same condition, for a week (168 h), where the devices gathered data in a continuous mode, sending data to the server every 10 min. All the radon concentrations presented in this section have the error of measurement that the radon sensor has: less than 10% at 370 Bq/m\({}^{3}\). To compare the results with the room's natural radon concentrations, we collected data for a week without the ventilation system activated. Figure 4 shows the radon concentration evolution during that week, where the red horizontal line depicts the radon threshold the system uses (indicated in the WHO handbook). Figure 4: Radon concentration during a week without the air quality control system activated. We chose a week for each test because a week is a time long enough to avoid temporary weather changes that could affect the radon concentrations, and it is short enough to avoid seasonal influences because the total test time is five weeks. Table 1 shows the minimum, mean and maximum radon concentration for that room and the total time the radon concentration was above the threshold during one week without the ventilation system activated. The time above limit is a crucial measure because regulatory limits are established for annual continuous levels, not temporary ones; thus, this is the value we should focus on to select the appropriate interpolation windows for the prediction algorithm. As we can see in Table 1, the natural radon concentration is almost 11 times higher than the recommended concentration, posing a clear health hazard, and the radon concentration was higher than the threshold 99% of the week that the test lasted. As explained, the experiments consisted of testing 10 min, 30 min, 1 h, and 2 h for the interpolation window of the prediction algorithm. Figure 5 shows the radon concentration evolution (blue line) for each interpolation time window tested for a week each.