This study focuses on indoor air quality measurements carried out in an apartment in the suburban region of Budapest. The measurements were made by an IQAir AirVisual Pro air quality monitor which is a so-called low-cost sensor capable to monitor PM2.5 and carbon dioxide concentration. In this study we analyze data measured during January 2017 that was characterized by an extreme air pollution episode in Budapest. The aim of the study was to calculate daily indoor PM2.5 concentrations that are

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... omparable with the outdoor concentrations provided by the Hungarian Air Quality Monitoring Network. Given the fact that AirVisual Pro provides data with irregular sampling frequency, data processing is expected to affect the calculated daily mean concentrations. The main purpose of the study was to evaluate the effect of data processing technique selection on the calculated daily data. The results indicated that the uneven sampling frequency characteristic to AirVisual Pro indeed causes problems during data processing and has effect on the calculated means. We propose a 'best method' for data processing for sensors with irregular sampling frequency.