Despite the relatively short period of time spent in cars, exposure levels are of concern given the immediate proximity to motor vehicles, plus in urban areas, high ambient concentrations compared to other micro-environments. For vehicle-related pollutants such as particulates, this contribution is particularly important.

The adverse health effects of particulate matter (PM) have been clearly established in the scientific community, and it has frequently been proposed that ultrafine particles (UFPs) -those with an aerodynamic diameter of ≤0.1 μm- have a more significant effect on human health per unit mass of similar chemical composition than larger particles (Delfino et al. 2005; Harrison et al. 2012; Seaton et al. 1995). The development of a Solid Particle Number (SPN) measurement programme -under the Particle Measurement Programme (PMP) has standarised the measurement protocol for vehicular UFPs. The PMP protocol has been extensively scrutinized, with its repeatability and reproducibility being widely lauded. However, several studies have shown that a significant number of sub-23 nm particles can remain present downstream from the PMP system (Giechaskiel et al. 2009; Herner et al. 2007; Johnson et al. 2009). Consequently, a number of studies were conducted to investigate the composition of these sub-23nm particles, demonstrating that most of them were formed through the renucleation of semi-volatiles, and therefore not of a solid state (Zheng et al. 2011; Zheng et al. 2012).

In developing informed public policy to protect public health, it is important that the concentrations of SPN as well as Total Particle Number (TPN) -including particles of a non-solid state- within vehicles are better quantified, as it is understood that the health concerns associated with PM are a function of particle size, rather than particle state of matter.