According to this article[1], nanoparticles drop off quickly compared to larger particles: "However, the highest averages were recorded inside the cars and bus

According to this article[1], nanoparticles drop off quickly compared to larger particles: "However, the highest averages were recorded inside the cars and buses: the closer to the source of the pollution, the exhaust pipes spewing out the fumes, the higher the total number of nanoparticles. The difference between walking by the kerbside of the road, and by the building side, on the same pavement – just a few short steps – was an average of 82,000 particles versus 69,000. The same readings registered no change in PM2.5." 1: https://www.bbc.com/future/article/20191113-the-toxic-killers-in-our-air-too-small-to-see https://www.bbc.com/future/article/20191113-the-toxic-killer... HN discussion: https://news.ycombinator.com/item?id=21552934 https://news.ycombinator.com/item?id=21552934