Air pollution is an undeniable environmental hazard that poses a severe threat to human health. Its sources are manifold, ranging from factories, and vehicles, to and burning of fossil fuels. The Ministerio de Medio Ambiente y Recursos Naturales (MARN) is the state institution responsible for monitoring air quality in El Salvador. However, the current capacity of MARN is insufficient, as it only has three stations for the entire country. The Pan American Health Organization (PAHO) has acknowledged this situation, and has categorized El Salvador as a country without the ability to monitor air quality adequately. We propose a methodology for the design and implementation of a mobile IoT system to monitor air quality. The IoT system monitors critical air quality parameters such as particle matter (PM1.0, PM2.5, PM10), Formaldehyde (HCHO), temperature, and relative humidity. Besides, we present a low-cost IoT architecture that can be implemented in developing countries. The IoT system is composed by IoT nodes for the real-time measurement of air quality parameters, and an IoT data platform. The IoT node is based on an ESP32 microcontroller, a SIM800L GSM board, and a PMS5003ST air quality sensor. Google App Services tools are utilized as the IoT web platform and cloud data storage. The outcome of this work is a prototype of a mobile IoT system to monitor four air pollutants through graphical web dashboards. The results obtained in preliminary field tests have been satisfactory and show that the proposed approach is an efficient and low-cost option. As a field test of the proposed IoT system, we deployed three IoT nodes in different cities in El Salvador, namely San Salvador, Santa Ana, and Cojutepeque for one month. Comparing PM data reported with respect to the limits of the Central American Air Quality Index (CAQI), the air quality in the studied cities is mainly in the good to moderate range.