Merv-13 means it filters at least 75% of the particles passing through it.
replyFor a standalone air filter which is pushing the filtered air into the same room its drawing from, we can assume reasonably good mixing of the intake and exhaust air. Thus to be equivalent to a HEPA-filter it only needs 1/3rd more airflow... Which really isn't much.
This doesn't follow for a a fancier setup where you're taking dirty air and blowing it into a cleanroom or something... but that's not how we're typically using household air filters.
Is it safe to assume all particles are equally as likely to be filtered, or are there particles that HEPA will filter that a MERV-13 will never filter no matter the number of cycles?
reply75% refers to the Most Penetrating Particle Size (MPPS), so they intentionally measure at the worst-case size so you don't have that problem. All other sizes are filtered at a higher percent.
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