Investigating Indoor Air Pollution Sources and Student’s Exposure Within School Classrooms: Using a Low-Cost Sensor and Source Apportionment Approach

Indoor air quality is becoming one of the most important factors for public health, as people tend to spend more time indoors, either at home or at work. This paper combines the use of calibrated low-cost particulate matter (PM) sensors with source apportionment algorithms to understand the factors that affect the indoor air quality within a typical UK school. Low-cost optical particle counter (OPC) sensors were placed in three different classrooms within the school, measuring PM concentrations during a typical school week and the subsequent holiday period, to understand the role of occupancy within schools for air quality. When students were in attendance during school hours (09:00–15:30), the classroom with the greatest average PM levels had PM_2.5 and PM₁₀ mass concentrations of 14.4 and 51.0 μg/m³, respectively. During school hours, when students were present, all classrooms had average PM_2.5 concentrations greater than 8.1 μg m⁻³ and average PM₁₀ concentrations greater than 13.1 μg m⁻³. Notably, the English studies classroom exceeded the 24-h WHO ambient PM₁₀ guideline (45 μg m⁻³). Employing the non-negative matrix factorization (NMF) algorithm for source apportionment revealed that between 93% and 98% of PM₁ observed within classrooms derived from outdoor sources. This contribution diminished as particle size increased, with outdoor sources accounting for 74%–89% of PM_2.5, and 19%–40% of PM₁₀, respectively. The differences in classroom PM concentration and source percentages are attributed to differences in lesson activities, lesson frequency, flooring (carpeted versus hard flooring), location within the school, and proximity to outdoor sources such as roads. The approach described within the paper is easily translated to other indoor locations and could also be straightforwardly scaled due to its relatively low cost. Thereby, it allows for air quality management in locations crucial for the public health and educational outcomes of children.