Yes — but only on recirculation mode, not on fresh air intake. Most split AC units and window ACs have a recirculation setting that cools and circulates indoor air without drawing in outdoor air. This is the correct setting during AQI 300+ events. If your AC only has a fresh air mode, it is better to run a HEPA purifier without the AC than to run the AC in fresh air mode — which will pump outdoor PM2.5-laden air directly into your home. Note also that AC filters are not HEPA grade and do not filter PM2.5 — they only filter larger particles.
