Evaluations of Low-cost Air Quality Sensors for Particulate Matter (PM2.5) under Indoor and Outdoor Conditions

Air quality (AQ) monitoring is crucial for maintaining human health and well-being, whether outdoors or indoors. Particulate matter (PM) is among the most critical parameters that must be routinely monitored. Traditional reference particulate analyzers are expensive and difficult to deploy on a large scale, leading to poor spatial and temporal AQ information. However, the reliability and accuracy of these sensors are yet to be established. This study is aimed at assessing the performance of five low-cost sensors by comparing them with a particulate reference analyzer for AQ monitoring in accordance with the United States Environmental Protection Agency (US EPA)-recommended guidelines. The sensors were tested for indoor and outdoor environments using simple linear regression (SLR) models. The results indicate that low-cost sensors are unreliable for accurately measuring AQ in indoor environments. The correlation between the sensors and the reference analyzer was poor, with coefficient of determination (R-2) values ranging from 0.2 to 0.58 during the three-week analysis period for a 1-h average. However, after increasing the average time interval, the sensor (HPMA115) satisfied the US EPA-recommended guideline with an R-2 value of 0.72. Root-mean-square error (RMSE) values for some sensors exceeded the US EPA guideline of less than 7 mu gm(-3) for PM sensors. The concentration of PM2.5, indoor relative humidity (RH), and temperature were identified as potential factors contributing to sensor behavior. The air conditioning system also affected the sensor performance, with variations in RH and temperature observed between tests with and without occupants. The results showed that low-cost sensors could be utilized for outdoor environments, with Honeywell's HPMA115 performing well. However, the calibration process must be performed for each specific environment. Our findings highlighted the limitations of low-cost sensors for AQ monitoring and the need for further research to develop reliable sensors.