Practical considerations for using low-cost sensors to assess wildfire smoke exposure in school and childcare settings.

Background: More frequent and intense wildfires will increase concentrations of smoke in schools and childcare settings. Low-cost sensors can assess fine particulate matter (PM_2.5) concentrations with high spatial and temporal resolution.

Objective: We sought to optimize the use of sensors for decision-making in schools and childcare settings during wildfire smoke to reduce children's exposure to PM_2.5.

Methods: We measured PM_2.5 concentrations indoors and outdoors at four schools in Washington State during wildfire smoke in 2020-2021 using low-cost sensors and gravimetric samplers. We randomly sampled 5-min segments of low-cost sensor data to create simulations of brief portable handheld measurements.

Results: During wildfire smoke episodes (lasting 4-19 days), median hourly PM_2.5 concentrations at different locations inside a single facility varied by up to 49.6 µg/m³ (maximum difference) during school hours. Median hourly indoor/outdoor ratios across schools ranged from 0.22 to 0.91. Within-school differences in concentrations indicated that it is important to collect measurements throughout a facility. Simulation results suggested that making handheld measurements more often and over multiple days better approximates indoor/outdoor ratios for wildfire smoke. During a period of unstable air quality, PM_2.5 over the next hour indoors was more highly correlated with the last 10-min of data (mean R² = 0.94) compared with the last 3-h (mean R² = 0.60), indicating that higher temporal resolution data is most informative for decisions about near-term activities indoors.

Impact statement: As wildfires continue to increase in frequency and severity, staff at schools and childcare facilities are increasingly faced with decisions around youth activities, building use, and air filtration needs during wildfire smoke episodes. Staff are increasingly using low-cost sensors for localized outdoor and indoor PM_2.5 measurements, but guidance in using and interpreting low-cost sensor data is lacking. This paper provides relevant information applicable for guidance in using low-cost sensors for wildfire smoke response.