I was planning on buying a filter labelled True HEPA from a well known company, that uses a fan and not ionization, and being sure to replace the filter as directed.
The HEPA standard, ensures PM0.3 are filtered out. But this article (http://now.tufts.edu/articles/big-road-blues-pollution-highw...) says ultrafine particles (PM0.1) are the biggest health risk. Do you know if any filters remove ultrafine particles?
Wikipedia seems to suggest that most do: "Diffusion predominates below the 0.1 μm diameter particle size. Impaction and interception predominate above 0.4 μm. In between, near the most penetrating particle size (MPPS) 0.3 μm, both diffusion and interception are comparatively inefficient. Because this is the weakest point in the filter's performance, the HEPA specifications use the retention of these particles to classify the filter" https://en.wikipedia.org/wiki/HEPA