According to the EPA, medium efficiency filters with a MERV of 5 to 13 are reasonably efficient at removing small to large airborne particles. Filters with a MERV between 7 and 13 are likely to be nearly as effective as true HEPA filters at controlling most airborne indoor particles.
Medium efficiency air filters are generally less expensive than HEPA filters, and allow quieter HVAC fan operation along with higher airflow rates than HEPA filters since they have less airflow resistance.