Literature review | Indoor air quality | Sep 11, 2020

      Real-time particulate matter tracking for indoor air quality

      Our research compares the most common aerosol monitoring technique against a portable alternative.


      Testing and monitoring air quality is an essential part of our scientific research, specifically when analyzing smoke-free products, We’ve previously explored our findings on the effects of our Tobacco heating System (THS) and cigarettes on indoor air quality. While that research used a controlled laboratory setting to monitor quality, using a controlled setting isn’t always possible, especially when investigating air quality in real-world environments. Therefore, outside specialized laboratory settings, more practical solutions are needed.

      When considering the sources of air pollution such as cigarette smoke and mineral dust, the concentration of suspended particulate matter (PM) is an important element. When these aerosols are inhaled, particles smaller than 2.5 µm in diameter (PM2.5) can reach the alveoli in the lungs and be deposited there. Particles that are water non-soluble and of solid nature can be transported into the blood stream, promoting the risk of diseases such as cardiopulmonary disorders or cancer. Air pollution is attributed to PM2.5, with research suggesting that every 10 µg/m³ increase in PM2.5 exposure can lead to a 2.8% increase in mortality.

      Lit cigarette placed on the metal wheel of a laboratory machine


      Analyzing aerosol monitors

      We aimed to validate the suitability of the DustTrak DRX aerosol monitor to determine complex aerosols for air quality measurement by assessing three of the devices, compared to the gold standard, across five different factors:

      1. Linearity – the linear relationship between concentration and response, used for analysis – the calibration curve.
        The linearity response was analyzed using Emery oil (known to produce a linear photometric signal for aerosol composed of mineral dust) and the results showed that two of the DustTrak DRX devices had better linear regression models (±3%) than the expected range (±10%), but one device underestimated by 12-14% relative to the others.

      2. Selectivity – the accuracy of detecting defined PM size, like polystyrene latex spheres (PSL) of 2.5 µm.
        To track selectivity, PSL aerosol with a 2.5 µm diameter was generated and concentrations in all channels – PM1, PM2.5, PM4, PM10 and PM15 – were measured simultaneously. We found that DustTrak DRX was sensitive for selected PM2.5 size by measuring signals in channels from PM2.5 to PM15. PM1 was also measured but the device did not provide a significant signal compared to other channels, which was expected since it was not activated for this selected size.

      3. Bias – the difference between DustTrak DRX readings and ‘true’ readings from gravimetric analysis.
        Bias testing was used as a way to determine the Photometric Calibration Factor (PCF) – for calibrating light-scattering instruments. When PCF was 1.0, the default setting, the device overestimated its measurement by 150%. However when PCF was 0.38, considered suitable for measuring PM2.5 of ambient aerosol, the measurement was within 2% of the gravimetric analysis.

      4. Working range – the Limit of Detection (LOD) and Limit of Quantification (LOQ).
        LOD is the lowest concentration that can be detected, while LOQ is the lowest concentration that an analyte can be quantified, with acceptable accuracy and precision. We found that the LOD was 5 µg/m³, comparable to existing research findings, and LOQ was 15 µg/m³.

      5. Precision – determining PM counts in specific size ranges.
        The concentrations of PSL particles of different diameters for PM2.5, PM4, PM10, and PM15 were between 20-30 µg/m³.
      Illustration of various molecular structures


      What do the results tell us?

      We conclude that DustTrak DRX is a powerful tool for monitoring suspended PM mass concentration, and a good alternative to gravimetric techniques when less labor-intensive real-time monitoring is needed.

      The importance of correct calibration in the bias testing also suggests a limitation. The device has to be evaluated in the laboratory every time a new type of aerosol matrix is introduced. In our research paper, we mention the importance of appropriate calibration for light-scattering sensors. This is in contrast to a publication by Protano et al., which omitted any mention of calibration used during their study for DustTrak DRX aerosol concentration measurements.

      Our findings also suggest DustTrak DRX might lack sensitivity for particles <0.1 µm and >15 µm in diameter – because of the light-scattering method used – and may underestimate mass concentrations for highly volatile compounds. Additionally, the instrument position and aerosol dilution during monitoring may affect mass concentration measurements.

      To accurately and precisely measure PM in indoor air in real-time, it’s essential to use the right tool for with the correct calibration factor is used for each given situation.