Air Purifier Selection: HEPA, CADR, and Matching Filtration to Your Monitor
Volume I · May 2026 · 596 words
An air quality monitor tells you the air is bad. An air purifier does something about it. Matching the purifier to the monitor's readings — and to the room — is the difference between a device that runs and one that actually improves air quality. This guide covers filter technologies, sizing methodology, and how to verify that your purifier is working using your monitor.
Filter Technologies
| HEPA (H13/H14) | Removes ≥ 99.95% (H13) or ≥ 99.995% (H14) of particles at 0.3 µm — the most penetrating particle size. This is the gold standard for particle filtration. All purifiers recommended on this site use true HEPA, not "HEPA-type" or "HEPA-like" (marketing terms with no standard definition). |
| Activated carbon | Adsorbs VOCs, odors, and some gases onto a porous carbon substrate. The mass of carbon determines capacity — a purifier with 1 lb of carbon removes more VOCs over its lifetime than one with 2 oz. Carbon filters saturate and must be replaced; there is no indicator for saturation (unlike particulate filters which show visible loading). |
| Ionizer / electrostatic | Charges particles so they stick to surfaces. Ineffective compared to HEPA. Some ionizers produce ozone — a respiratory irritant. Not recommended. |
| UV-C | Intended to inactivate microorganisms. The exposure time in a purifier's airflow path is typically < 1 second — insufficient for meaningful inactivation. Provides negligible benefit over HEPA alone for airborne pathogens. Primarily a marketing feature. |
Clean Air Delivery Rate (CADR)
CADR is the volume of filtered air delivered per minute, measured separately for smoke (0.09–1.0 µm particles), dust (0.5–3.0 µm), and pollen (5–11 µm). Smoke CADR is the most important for wildfire and urban pollution scenarios. The AHAM standard requires CADR testing; purifiers without a published CADR should be avoided.
Sizing rule: CADR should be at least two-thirds of the room area in square feet. A 300 ft² room requires a smoke CADR of ≥ 200 CFM. This provides approximately 4.8 air changes per hour (assuming 8-foot ceilings) — the level recommended by ASHRAE for wildfire smoke events.
Using a Monitor to Verify Purifier Performance
Place the air quality monitor at the far side of the room from the purifier (not next to the outlet — you'll measure cleaned air, not room air). Record PM2.5 before turning the purifier on, then at 15-minute intervals. A properly sized purifier should reduce PM2.5 by 50% within 15–30 minutes and approach steady-state within 1–2 hours. If PM2.5 does not decrease, the purifier is undersized, the filter is saturated, or outdoor infiltration exceeds the purifier's CADR.
Filter Replacement Economics
| Purifier | Filter cost | Replacement interval | Annual cost |
| Coway Airmega 1512 | $45 (HEPA + carbon set) | 12 months | $45 |
| Blueair Blue Pure 211+ | $40 (particle + carbon) | 6 months | $80 |
| Levoit Core 300 | $30 (HEPA + carbon) | 6–8 months | $45–60 |
Filter replacement cost is the dominant lifetime cost. A $200 purifier with $80/year filters costs $600 over 5 years — the filters cost more than the device. Include replacement cost in the purchase decision. Washable pre-filters (standard on Coway and Blueair) extend HEPA filter life by capturing large particles before they reach the HEPA media.