Evaluation of mass-balance models as particle exposure prediction tools in occupational environments

The challenge: Combustion processes generate unwanted products, such as particles and gases, which have been linked with adverse health effects. Moreover, combustion processes are the most important pathway for the formation of ultra¬fine particles (diameters ≤ 0.1 micrometers), which have been under increasing focus, due to their likelihood of affecting health. In ambient air, particles are defined by their diameter and ambient air exposure limits are set for mass concentrations. Particulate matter (PM) in ambient exposure has defined limits for PM10 and PM2.5 (particles with aerodynamic diameters ≤10 and ≤2.5 micrometers, respectively, with PM2.5 comprising a portion of PM10), for which exist solid evidence for harmful effects to human health. The fraction of ultrafine particles has usually small contribution to the mass of PM2.5, and they have a shorter lifetime (hours vs days or weeks for PM2.5) and a larger spatial and temporal variation, precluding their assessment with the use of the same methods. Moreover, ambient conditions with low levels of PM10 and PM2.5 do not necessarily correlate with low levels of ultrafine particles.

The study of the associations of particle exposure and health effects has many challenges, especially in relation to the ultrafine fraction. The human exposure to combustion-derived particles is ubiquitous. There is great uncertainty pertaining to the assessment of personal exposure and epidemiological studies and designs depend on exposure gradients. Furthermore, ultrafine particle exposure present a large spatial and temporal variation, which will intrinsically imply misclassification of exposure, if one considers stationary monitoring data to investigate population effects. Besides the uncertainty in exposure data for the ultrafine particles, the health effects may have multiple causes and origins (1). Consequently, it is a challenge to assess human health effects from exposure to combustion-derived particles, dominated by ultrafine size mode.

Controlled human exposure studiesare experiments where humans agree to be intentionally exposed in a controlled scenario to provide information on biological measurable changes. This particular study design allows investigating short-term effects and mechanisms from exposure to combustion-derived particles. The measured biological effects are early, transient and reversible indicators, without inducing disease, ensuring the safety of the participants and following international ethical standards. These studies are usually performed in an exposure chamber that allows controlling the concentration, aerosol composition, ventilation, temperature and humidity. The combustion particles are typically generated by devices outside the chamber and directed into the chamber as well-characterized aerosol. Some studies select susceptible participants with known clinical status or characteristics (for example patients or elderly). This may create stronger responses to the combustion exhaust, but other variability factors can then be introduced. Therefore, many researchers choose to expose healthy subjects and control for confounders by selective inclusion criteria, limiting generalization but facilitating comparisons among studies.

The present text is a summary of a systematic review on controlled human exposure to combustion-derived particles, which has been published as a book chapter (2). We performed a literature search on ambient air particle in controlled studies, using PubMed as search engine (January 2020). This resulted in 102 scientific articles reporting effects following exposure of healthy, non-smoking participants to fine or ultrafine particles from wood smoke, diesel exhaust, concentrated ambient particles (CAPs) and indoor sources (candle burning, cooking and printing).