All the particles below 10 microns are bad your your respiratory system hence the P10 P2.5 reading on testers. The P1.0 readings are less reliable on most home pollution testers and there is a point under 1 micron (can't remember what it is) where particles that size behave like gas and come back out as easy as going in. The home testers can only be counted upon to tell you levels are going up and that you might want to do something about it. Even settled dust in the shop is stirred up by your movements in the shop, raising the particle counts. You also have to be aware that you need to take outside air readings to have a baseline for readings you take in the shop if you are opening windows or ventilating because you can't get below that air count unless you are sealed in and use filtration to get the air cleaner. It is a very involved subject but I know the best solution is to capture as much at the source so it doesn't get in the air in the first place. That means bigger DC's and large ducts which most people can't afford or are unwilling to buy.
Pete