Diagnostic barriers to using PM2.5 concentrations as metrics of indoor air quality

Particulate matter with a diameter of ≤2.5µm (PM2.5) has been shown to be present in many buildings at concentrations that are harmful to human health. Accordingly, they should be used as metrics of indoor air quality (IAQ) and included in standards or norms. This paper uses measurements of PM2.5 concentrations made in three different environments using three different devices to show that there are barriers that must be before they can be considered viable diagnostics. Optical particle counters (OPCs) are a common device used to measure temporal changes in PM2.5 concentration. The PM2.5 concentration is inferred from the light scattering properties of the particles sampled, properties which vary by source and composition Accordingly, OPC measurements must be scaled by a calibration factor, a simple multiplier derived from concurrent gravimetric sampling. A gravimetric sampler uses an impactor to remove large particles and measures the total mass of the collected on a filter during a sampling period allowing the calculation of an average PM2.5 concentration. OPCs are often most sensitive to PM2.5 with a particular size, and so OPCs capable of disaggregating PM2.5 concentrations by their diameter can be used to select an appropriate OPC for a particular source. Knowledge of these factors is important if a measurement of PM2.5 concentration is to be used to describe the quality of indoor air using a metric. In order to be useful, a standard or norm should provide clear guidance on device selection, device calibration, and calibration factors (for optical particle counters). For in-situ measurements, it must specify an appropriate measurement location, a sampling frequency, and time-averaging period. After measurements have been made, appropriate statistics and an error analyses should be prescribed to quantitatively describe the variance in the data. 
Ambient PM2.5 concentrations have been measured in many countries for decades and so there is a mature library of guidance and standards. The indoor community should consider how it might adopt the methods it uses for regulatory monitoring by gravimetric sampling, and how it demonstrates the equivalence of real time OPCs to the gravimetric method so that they too can be used for regulatory purposes.