There is a testing sequence you should use when putting on a respirator, they come in different sizes and you need one that fits your face. Put it on, then cover the filters with your hands. You shouldn't be able to breathe in any air, it's a very unnerving feeling when you can't take a breath. Next, cover the outlet with a hand, you shouldn't be able to breathe out. If air leaks by in either of these tests, air is leaking in and/or out and the respirator isn't doing you any good. This is particularly important when working with solvents, chemicals, pesticides, etc.
Body heat can still fog your glasses, though. There are anti fog products for the interior of windshields, helmet face shields, goggles, etc. Back in the day, we used to use a wax called Montecristo.