In the particulate matter and "Frac Sand" (sandstone that is crushed and used as proppant in hydraulic fracturing) project (see https://publiclab.org/wiki/frac-sand), we are interested in low cost, accessible ways to learn more about the particulate matter in the air we breathe. We're especially interested in whether or not there are easy, accessible ways to see if there are respirable airborne particles (less than 5 micrometers in diameter), and if those particles are crushed silica or are particles formed from condensates (see more about that here: https://publiclab.org/wiki/silica-monitoring#Exposure+to+silica+in+occupational+and+non-occupational+settings). In this specific question, I'm wondering if any DIY microscopes have high enough resolution to accurately measure the size of particles collected on filters or other surfaces, and identify if they are small enough to be respirable. If it is not possible to accurately measure the size of individual particles, is it possible to estimate the general size of particles? Mathew Lippincott started this work and has done some particle sizing (such as this note: https://publiclab.org/notes/mathew/09-03-2015/optical-imaging-of-passive-particle-monitors). Here I'm curious about other DIY microscopes and improved resolution of this set up.