MERV — Minimum Efficiency Reporting Value — is the ASHRAE 52.2 scale that rates how well an air filter captures particles from 0.3 to 10 microns. It runs from 1 to 16 for residential and light-commercial filters; higher numbers catch smaller particles. MERV 13 is the number to know, because it's the grade the EPA tells homeowners to target when upgrading — 'at least MERV 13, or as high as your system fan and filter slot can accommodate.'
At MERV 13 a filter captures at least 50% of the finest particles it's tested on (0.3–1.0 microns) and 85% or more of the 1.0–3.0 micron range — enough to make a real dent in smoke, fine dust, and a meaningful share of PM2.5. It does this without the airflow penalty of true HEPA, which is why MERV 13 is the sweet spot for whole-home filtration on an existing system.
The install detail that matters is depth. A 1-inch MERV 13 filter can choke airflow on some systems; the better approach is a 4-inch deep-media cabinet, which has far more surface area, a lower pressure drop, and a longer service life (often 6–12 months between changes). A reputable contractor will check that your blower can handle the upgrade rather than just selling the thickest filter on the shelf — too restrictive a filter on an undersized system trades clean air for strained equipment.