I took the 40 hour OSHA course years ago, which covers respirators, etc., then promptly chose a career path knowing I'd never want to use it. So I'm no expert in the subject and I'm all for sharing information but THERE IS NO SUBSTITUTION FOR HANDS ON SAFETY TRAINING.
You can certainly do a "user seal check" yourself. Step 3
see
http://www.cdph.ca.gov/programs/olppp/Documents/respirator.pdf
But step 2 the "fit test" is a little more complicated.
In theory, if the mask is 60% effective you could argue that it's helping. However, if you're talking about putting a mask on then sweeping an area with lead dust then you're just getting a false sense of security and a big exposure.
If there are any firefighters here they can tell you first hand how important a seal is.