<title>Abstract</title> The COVID-19 pandemic brought significant consequences on healthcare systems, economy and politics. Nowadays, we know that the pathogen responsible for COVID-19 is transmitted mainly by aerosol droplets exhaled by infected individuals, which remain suspended in indoor air. There has been widespread interest in monitoring the CO2 levels in indoor spaces since an infected patient exhales CO2 and infectious aerosols when breathing. So, we designed and built an Air Quality Monitoring Device (AQMD) that measures and analyzes the levels of CO2 and particulate matter in the classrooms of a university with the aim of mitigating the spread of COVID-19. We divided the AQMD design into 2 phases: i) Data measurement and ii) Estimation of infection risk. Specifically, we measured the air quality in 3 classrooms of a university during different types of activities. Using these data, we calculated the recommended CO2 threshold for our classroom setting and estimated the probability of COVID-19 infection of a susceptible person. Our research shows that indoor CO2 concentrations and the probability of COVID-19 infection are influenced mainly by the type of activity and the number of windows open; besides, the number of students does not significantly impact the indoor CO2 concentrations levels because the range of students in the test scenario (18 to 31) was relatively small.