Although the correlation of spatial variation in NOx and particulate matter is often
high (and was high (ρ 0.83) in the ESCAPE study from which we derived
our exposure estimates (Eeftens et al., 2012)), generally stronger associations
with chronic health effects have been observed for particulate
matter (especially PM2.5) than for NOx (Beelen et al., 2014) and particulate
matter has been suggested to be a better marker for ambient air
pollution than NOx (Hoek et al., 2013).