MERV stands for Minimum Efficincy Reporting Value. It's a scale from 1 to
16. A MERV of 12 is currently popular with the heating and AC industry for
filters. It should do a pretty good job of stopping particles above 1 micron
in size. Smaller particles will be less well impeded. Your dirt may be
smaller particles.
Why the focus on dirt? How's the allergy doing? That's the important
thing, not dirt. Are you allergic to things that are bigger than 1 micron?

Dave M.