I know I'm belaboring the point, but I'm on a roll, so bear with me .....
Proper fit is critical to a respirator being able to do its job. I was required to wear a respirator when I worked in chemical labs and production plants. We were checked regularly to see that our respirators were in good shape and properly fitted. I use the same fitting and testing procedures when I wear a respirator for woodworking, soap making, and such. I see a tendency in people who have never used a respirator for their job to ignore proper selection and fit. A person might as well not wear one at all if it doesn't fit right or isn't the right kind.
A disposable is more difficult to fit properly, so if you have a hard-to-fit face like me, you may need to use a reusable facepiece. If you're buying at a "big box" hardware or lumber store, the respirator selection is likely to be geared toward men, so the disposable respirators and reusable facepieces are probably going to be mostly (or all) in the medium and large sizes. Most women need a small size mask, so you may have to order from a catalog or online.
Getting back to disposables -- pay close attention to forming the nosepiece to fit snugly around the bridge of your nose and upper cheeks. There should be two bands on a disposable respirator -- one to go around the back of your head to stabilize the top part of the mask and one around the back of your neck to stabilize the lower part of the mask. Tighten each band so the mask is pulled gently against your face. Don't overtighten to avoid distorting the fit and causing headaches. (If your disposable mask has only one band, you have a mask rated only for "nuisance dusts", and it's just not going to do the job you need it to do.) When the mask is adjusted to fit, take a deep sharp breath in and pay attention to the sensation of any cool air flowing in around the edges of the mask. The top of the nose and the jawline are the usual trouble spots for me. If you feel leaks, adjust the mask as needed to improve the seal. If you can't get the mask to stop leaking no matter what you do, you may need to try a different size and/or a different style or switch to a reusable facepiece. A properly fitted disposable should "suck down" tight against the face when you take a deep breath in -- there should be no leaks around the edges, so all of the air is coming through the mask material.
Fitting a resuable facepiece is basically the same -- your goal is to have no leaks around the edges of the mask when you suck in a deep breath. To do this, first remove the cartridges from the facepiece, if needed. To remove a cartridge, twist it a quarter or half a turn and pull it away from the facepiece. Put the facepiece on and adjust the elastic bands to give a snug, but comfortable fit around your face. Put the palms of your hands over the holes where the cartridges normally go to seal those openings. Take a sharp breath in and check for leaks and adjust as needed. The process of fitting a facepiece often goes a bit easier than a disposable because a facepiece is made of soft rubbery material that seals against the face better. If you have leaks around the jawline that you just cannot get rid of no matter what you try, you might need a different size facepiece.
If you're a guy with a beard or mustache, be aware that almost all disposables and most reusable "half face" facepieces can't handle facial fuzz. You may need to go to a "full face" respirator for proper fit and effective protection.
