One of the most frequent questions about the respirators we carry at pksafety.com is what size to buy. All our respirators come in small, medium or large, and most first-time buyers have no idea which is best for them. A proper fit is just about the most important consideration for a respirator, especially when pesticides, lead, or other harmful substances are in the air. Unfortunately the only way to know for sure if your respirator has a good seal is to do an OSHA-approved fit test, and the tests from 3M run about $235.00 The 3M FT-30 Bitrex Fit Test Kit allows you to test up to 150 employees. But if you are a small company or just want to buy a single mask, investing in the test can be prohibitively expensive. Here is what we know. Companies like 3M and Moldex operate under the assumption that the majority of people have about the same sized face. Therefore roughly 85-90% of population will fit into a medium sized respirator. If you are one of the relatively small part of the population that fits into the small and large sizes, you already have an idea because of your previous experiences purchasing hats or safety glasses. We're not willing to go out on a legal ledge either, but we can say the majority of people, as long as they don't have facial hair in the area where the mask is in contact with the skin, can get a good fit with a medium-sized, high-quality flexible-faced respirator mask. Masks such as the 3M 7500 Series have extremely flexible silicone facepieces that conform to the contours of your face. Only a fit test will tell you with any degree of certainly whether your mask is blocking all particles and fumes correctly, but these masks do provide a good fit for the majority of users. A word on disposable respirators. For the most part single- or limited-use respirators come in a one size fits all format and only a few have adjustable straps and nose clips to create a better fit. The few models that are available in large or small sizes are again for people with above- or below-average sized faces or nose bridges. You need a respirator that fits comfortably on your face and that covers most of your nose and all your mouth. You need to ensure that you feel the respirator is creating a seal all around. If you can smell or taste the items you are trying to block out with your mask, it either doesn't fit correctly, or it's time to change filters.