So where exactly then do 'nanoparticles' fall on the PM exposure list then? The standard measurements separate 'small' (PM2.5 exposure is exposure to particles with their largest length being less than 2500 nanometers. The link says PM2.5 include, "combustion particles, organic compounds, metals, etc." - wherein those classes of particulate indisputably qualify as nanoparticles.
If you want to be really pedantic, we could instead say that PM2.5 include 'nanoparticles, clumps of nanoparticles, irregularly shaped nanoparticles, and fine particles'. But I suspect most people do not have common mental distinction between a 'fine' particle and a 'nanoparticle', even though they are (physiologically) more similar to each other in most regards than they are to larger, visible particles, hence the difference in measuring both PM10 and PM2.5 atmospheric particulate [1].
Regardless, the article is talking about a huge amount of pollution, indistinguishable from 'nanoparticles' at the point of measurement, that have caused health problems for a truly massive number of humans.
[1] https://en.wikipedia.org/wiki/Particulates#Size.2C_shape_and...
> Similarly, so called fine PM, (often referred to as PM2.5), tend to penetrate into the gas exchange regions of the lung (alveolus), and very small particles (> The smallest particles, less than 100 nanometers (nanoparticles), may be even more damaging to the cardiovascular system. Nanoparticles can pass through cell membranes and migrate into other organs, including the brain. Particles emitted from modern diesel engines (commonly referred to as Diesel Particulate Matter, or DPM) are typically in the size range of 100 nanometers (0.1 micrometer). These soot particles also carry carcinogens like benzopyrenes adsorbed on their surface.