Best UV-C Air Purifier for Viruses: The Honest Guide to HEPA + UV Filtration
The first time I bought an air purifier specifically for virus protection, I spent more than I should have on a unit with a UV-C lamp the size of a finger and a spec sheet full of claims I couldn’t verify. It sat in my bedroom for eighteen months, and I had exactly zero way of knowing whether it was doing anything useful. What I know now — after going deeper on the science and testing several units over the past two years — is that the UV-C lamp in most consumer air purifiers is not the main event. It’s a supporting cast member. The HEPA filter is the main event, and the UV-C is a meaningful bonus when the unit is designed properly. That distinction saves you real money and prevents you from buying something that sounds impressive and performs poorly.
The good news: a handful of well-tested, properly certified units genuinely combine true HEPA filtration with effective UV-C germicidal technology at prices that don’t require a second mortgage. The bad news: most of the market is noise. UV-C claims on air purifier listings are not regulated the way HEPA filtration is, which means that “kills 99.9% of viruses” can appear on packaging for units whose UV exposure dwell time is far too short to achieve that result at real-world airflow rates.
This guide cuts through that noise. What follows is what the science actually says about UV-C and airborne viruses, which certifications mean something versus which are marketing decoration, and the three specific air purifiers worth putting in your home if reducing airborne viral transmission is the goal.
Quick Comparison: Best UV-C Air Purifiers for Viruses
| Model | Coverage Area | Filter Grade | UV-C Stage | Certifications | Warranty | Best For |
|---|---|---|---|---|---|---|
| GermGuardian AC5900WCA | Up to 1,760 sq ft/hr | True HEPA (0.1 micron) | Yes — optional toggle | CARB, ETL, Zero Ozone | 3 years | Large rooms, whole living areas |
| Pure Enrichment PureZone | Up to 300 sq ft | H13 True HEPA | Yes — always on | ETL, Zero Ozone | 5 years | Bedrooms, home offices |
| Pure Enrichment PureZone Elite | Up to 990 sq ft | H13 True HEPA | Yes — with ionizer | ETL, Zero Ozone | 5 years | Medium-large rooms, auto mode |
| UV-C Only (no HEPA) | Varies | None | Yes — but insufficient dwell | Often unverified | Varies | Not recommended for virus protection |
| HEPA Only (no UV-C) | Varies | True HEPA | No | AHAM, ETL | Varies | Good baseline; add UV-C for extra protection |
What UV-C Light Actually Does to Viruses
UV-C light occupies the 200–280nm band of the ultraviolet spectrum. At these wavelengths, the radiation penetrates the outer membrane of microorganisms — viruses, bacteria, mold spores — and damages their nucleic acids (DNA or RNA). A virus that has had its RNA scrambled by sufficient UV-C exposure cannot replicate. It’s not destroyed in the way a chemical disinfectant might dissolve it; it’s rendered biologically inert. The distinction matters because “kills viruses” in UV-C marketing usually means “prevents viral replication,” which is the functional definition of inactivation.
The EPA describes UVGI (Ultraviolet Germicidal Irradiation) as a well-established air treatment method, noting that upper-room UVGI can provide a germicidal effect when properly installed and maintained. The critical phrase there is “properly installed.” Professional-grade upper-room UVGI systems installed near ceilings, designed by engineers who calculate exposure dosage and airflow rates, work reliably. Consumer air purifiers with a small UV-C bulb tucked inside a plastic housing are doing something less precise.
That doesn’t mean consumer UV-C units are worthless. It means the air purifiers that combine UV-C with a true HEPA filter are the ones that perform. The HEPA filter is doing the heavy physical lifting — trapping aerosol particles that carry virus — while the UV-C works on what passes through. Together, the combination creates a more complete defense than either technology provides alone.
The Dwell Time Problem Most Brands Won’t Mention
Here’s the inconvenient science. Achieving meaningful viral inactivation via UV-C requires exposing a microorganism to a sufficient dosage of UV-C radiation, measured in mJ/cm². For influenza and SARS-CoV-2, effective inactivation typically requires several millijoules per square centimeter. In a consumer air purifier with a 6–10 watt UV-C bulb and typical airflow rates, the air passing through the unit spends somewhere between 0.3 and 1 second near the lamp. At standard bulb intensities and that dwell time, the dose delivered falls short of what independent testing has shown necessary for reliable inactivation.
The units that overcome this limitation do so in a few ways: a more powerful lamp, a chamber designed to reflect UV-C light and increase effective exposure, slower airflow through the UV stage, or — most practically — a HEPA filter that traps the particles first and keeps them near the lamp longer than freely moving air would. This is why you should never buy a UV-C-only air purifier and expect meaningful virus protection. The HEPA filter isn’t a bonus feature; it’s what makes the UV-C stage functionally relevant.
Certifications That Actually Matter
CARB and Zero Ozone Verified
Some UV-C lamps, particularly cheaper ones, emit wavelengths below 200nm that react with atmospheric oxygen to produce ozone — a respiratory irritant that can worsen asthma and lung conditions, the exact problems you’re trying to avoid. CARB (California Air Resources Board) certification and the Zero Ozone Verified mark both confirm that a unit’s ozone emissions fall below thresholds considered safe for continuous indoor use. Don’t buy a UV-C air purifier without one of these certifications clearly listed. A unit generating ozone while supposedly cleaning your air is worse than nothing.
ETL and UL Listing
ETL Listed and UL Listed are electrical safety certifications issued by Intertek and Underwriters Laboratories, respectively. They confirm that the product has been tested to recognized safety standards — important for any device you’re running continuously in a bedroom. These are baseline requirements, not differentiators, but their absence on a listing is a warning sign.
AHAM VERIFIDE
AHAM VERIFIDE certification (from the Association of Home Appliance Manufacturers) independently verifies a unit’s CADR — Clean Air Delivery Rate — rather than letting manufacturers self-report numbers. A CADR of 200 CFM means 200 cubic feet of air per minute is actually being delivered clean, not just that the fan is moving 200 CFM. For virus protection purposes, where air changes per hour directly affect airborne particle concentration, this is the specification to scrutinize. If a product claims AHAM-verified CADR, those numbers are real.
Recommended UV-C Air Purifiers for Viruses
GermGuardian AC5900WCA — The Large-Room Workhorse with Verified Performance
The GermGuardian AC5900WCA is a 21-inch console-style air purifier built around a 4-layer filtration system: a carbon pre-filter that catches large particles and handles odors, a True HEPA filter rated to capture 99.97% of particles as small as 0.1 microns, an optional UV-C germicidal stage, and an activated carbon filter targeting VOCs. The UV-C lamp is independently toggleable — you can run the HEPA filtration with the UV-C off or on depending on your preference, which is a useful feature for parents or households where someone prefers to minimize UV lamp exposure time. The unit carries CARB certification and Zero Ozone Verified status, so the ozone concern that plagues cheaper UV-C purifiers is addressed here.
On coverage: the AC5900WCA circulates a 365 square foot room 4.8 times per hour, and a 1,760 square foot space once per hour. For a bedroom or home office, running it on medium delivers meaningful air changes with noise levels quiet enough for sleep. The three-speed fan with an 8-hour auto-off timer makes it genuinely easy to manage on a schedule. The filter change indicator removes the guesswork — you’ll know when both the HEPA filter and UV-C bulb need replacement rather than estimating by calendar. GermGuardian has been independently tested against Staphylococcus albus, E. coli, Aspergillus Niger, and Phi-X174 (a bacteriophage used as a surrogate for harder-to-test human viruses).
The honest limitation: its footprint is wider than a tower-style unit, which matters in small rooms where floor space is already claimed. The HEPA filter replacement cost, while reasonable, applies to a relatively large filter surface — plan for that ongoing expense. And for very large open-plan spaces above 1,000 square feet, running this unit on its highest speed continuously is the only way to achieve meaningful air change frequency; at low speed in a big room, the protection drops substantially.
Best for: Families who want a proven, well-certified UV-C + HEPA unit for a bedroom, living room, or large home office. The combination of verifiable certifications, independently tested germ reduction, and the practical flexibility of a toggleable UV-C stage makes this the easiest well-rounded recommendation at its price tier.
Pure Enrichment PureZone — The Bedroom Staple with a 5-Year Warranty
The PureZone from Pure Enrichment is a compact 4-stage tower purifier covering rooms up to 300 square feet, built around an H13 True HEPA filter — one grade above the standard True HEPA specification used in most consumer units. H13 classification captures 99.97% of particles at 0.3 microns and performs meaningfully better at smaller particle sizes, including the fine aerosol range where virus-laden droplets concentrate after larger respiratory droplets evaporate. Stage 1 is a washable pre-filter for large particles. Stage 2 adds activated carbon for odors and VOCs. Stage 3 is the H13 HEPA. Stage 4 is the UV-C lamp, which operates continuously whenever the unit is powered on.
Independent testing against H1N1 (influenza A), Staphylococcus albus, E. coli, and Aspergillus niger showed 99.9% elimination rates in controlled lab conditions — the kind of specific pathogen data that separates meaningfully tested products from ones making generic claims. The 5-year manufacturer’s warranty is genuinely unusual at this price point; most competitors offer 1–2 years. Pure Enrichment’s customer service reputation holds up across a large review base, and the warranty isn’t just a marketing statement — they honor it. The unit runs at 36dB on its lowest setting, which most people describe as quieter than a soft fan, making it a legitimate option for running beside a bed all night.
The trade-off is coverage area. 300 square feet is a typical medium-sized bedroom or home office — not a living room, not an open floor plan. Using this unit in a 500-square-foot open space expecting meaningful air cleaning is optimistic. The filter replacement, while only needed every six months, is a combined H13 HEPA and activated carbon unit sold together — convenient but slightly more expensive per replacement cycle than units with separately replaceable filter stages. The UV-C bulb also needs annual replacement to maintain germicidal effectiveness, which is easy to forget until the unit reminds you.
Best for: Anyone who wants a compact, well-tested, H13-grade UV-C air purifier for a bedroom or home office at a mid-tier price, backed by the best warranty in the class. The combination of H13 filtration, independently tested virus elimination data, and a 5-year commitment from the manufacturer is hard to beat for a single-room setup.
Pure Enrichment PureZone Elite — The Smart Tower That Reads the Room
The PureZone Elite is the full-room upgrade of Pure Enrichment’s UV-C line — a tower-style unit covering up to 990 square feet per hour with an integrated air quality sensor that automatically adjusts fan speed based on real-time particulate readings. The filtration architecture is 4-stage: activated carbon pre-filter, H13 True HEPA filter, UV-C germicidal lamp, and an ionizer stage. The color-coded air quality display gives a live reading on PM2.5 levels, which is genuinely useful for understanding when cooking, outdoor air events, or activity spikes are degrading indoor air quality. Set it to auto mode and it handles the response without intervention.
The same H1N1 and bacterial test data that applies to the standard PureZone applies here — the UV-C lamp specifications and germicidal performance are consistent across the Pure Enrichment line. The Elite’s larger HEPA filter surface area and higher airflow capacity mean more air passes through per hour, which matters for the overall reduction in airborne virus concentration. At 990 square feet, this covers most modern living rooms, open-plan kitchen-dining areas, or the main common space of a house. The 5-year warranty carries over from the standard model.
One thing to note about the ionizer stage: it produces a small amount of negative ions to help charged particles clump together and fall out of the air faster. Most independent tests find this mildly beneficial for particulate removal but not a significant addition for virus protection specifically. The unit is still Zero Ozone Verified, meaning the ionizer operates within safe ozone thresholds. If you’re particularly sensitive to ionizer technology or live with someone who is, the ionizer can be disabled while the HEPA and UV-C stages continue running independently — that flexibility is worth noting.
Best for: Anyone who wants large-room UV-C and HEPA coverage with the convenience of automatic mode and an air quality display, without stepping up to a significantly more expensive smart unit. The 990 sq ft coverage at the $100–$150 price range makes this the best value option for main living areas where you want passive, hands-off protection.
Understanding HEPA Grades: True HEPA vs H13
The term “HEPA” gets applied very loosely on air purifier listings. True HEPA, to meet the HEPA standard established by the U.S. Department of Energy, must capture at least 99.97% of particles at 0.3 microns in diameter — the most penetrating particle size, known as MPPS (Most Penetrating Particle Size). H13 is a step above, defined under the EN 1822 European standard, and must capture at least 99.95% of particles at the MPPS and 99.75% overall. In practice, H13 filters perform better at sub-0.3-micron particle sizes — which includes the fine aerosol fraction where airborne viruses accumulate after respiratory droplets partially evaporate.
For virus protection specifically, H13 is the grade worth prioritizing when budget allows. Both True HEPA and H13 will physically capture virus-containing particles. H13 does it with finer margins at the most relevant particle sizes. Neither is inferior as a daily air purifier; H13 simply provides an additional margin at the small end of the size scale that matters most for viral aerosols.
How Air Changes Per Hour (ACH) Affects Virus Concentration
The single most overlooked factor in air purifier performance for virus protection is air changes per hour. An air purifier that cycles the air in a room four to six times per hour provides meaningfully better protection than one cycling it once. Each pass through the filter removes a fraction of the remaining airborne particles — the more passes, the lower the concentration gets. A unit technically rated for your room’s square footage but running on its lowest speed setting might be delivering only 1–2 ACH, well below the 4–6 ACH range that independent indoor air quality research associates with significant pathogen reduction.
The practical implication: a smaller air purifier running on high in a correctly-sized room outperforms a larger unit running quietly in an oversized space. Match coverage area ratings generously — if your room is 300 square feet, look for a unit rated for 400–500 square feet so it can achieve meaningful ACH without running at full power continuously. Anyone curious about supplementing home air quality monitoring with actual measurement tools will find our coverage of precision testing meters for home use worth a look — the same principle of measuring rather than guessing applies equally to air quality as to electrical diagnostics.
UV-C and Ozone: The Risk Most Buyers Miss
Not all UV-C lamps are equal. Standard germicidal UV-C lamps emit primarily at 254nm — the wavelength most effective against microorganisms and safe for enclosed air purifier chambers. Some cheaper lamps, or units that generate ozone deliberately as an additional “purification” step, emit below 200nm where oxygen molecules split and form ozone. Ozone at even modest concentrations is a respiratory irritant. The EPA and CDC both advise against ozone-generating air purifiers, particularly for anyone with asthma or other respiratory conditions.
The certifications to check for — Zero Ozone Verified, CARB EO-certification — confirm that a unit’s ozone output falls below 0.050 ppm, the threshold set by California’s strict air quality regulations. Units sold as “ionic air purifiers” or “ozone generators” and marketed for virus elimination are a different category entirely and should be avoided for health reasons. Every unit recommended in this article carries verified ozone-safe certifications.
“A professionally designed system of ultraviolet germicidal irradiation (UVGI) that is well installed and maintained can effectively kill the virus that causes COVID-19 and help protect people from the disease indoors. By itself, using a UVGI system is not enough to protect people. When used along with other best practices, UVGI can be part of a plan to protect people indoors.” — U.S. Environmental Protection Agency, Indoor Air Quality FAQ
Running an Air Purifier Effectively: Placement and Operating Habits
Where you put an air purifier matters almost as much as which one you buy. Place it in the room where you spend the most time — typically a bedroom or home office — not in a hallway or corner where airflow is restricted. The unit should have clearance on all sides, ideally placed centrally enough that its intake isn’t pulling from only one corner of the room. Running it on a higher speed when the room is actively occupied and dropping to a lower, quieter setting overnight gives you effective air changes when it matters most without noise disruption during sleep.
One habit that consistently surprises people: keeping doors and windows closed while the unit runs. An air purifier in a room with open windows is competing with the entire outdoors — it cannot maintain meaningful air changes when the room’s air volume is functionally unlimited. Seal the space you want to protect. If you’re concerned about ventilation and fresh air alongside purification — a reasonable concern in smaller rooms — the right approach is scheduled ventilation (windows open for 20 minutes, then closed) rather than running a purifier with windows perpetually cracked. Households thinking about humidity alongside air quality may find our review of the HoMedics TotalComfort Ultrasonic Humidifier with UVC technology relevant — it addresses the related question of maintaining humidity at levels that reduce viral transmission in dry indoor environments.
Filter and UV-C Bulb Replacement: The Ongoing Cost Nobody Advertises Upfront
A UV-C air purifier has two consumables that need scheduled replacement: the HEPA filter and the UV-C bulb. HEPA filters for most consumer units need replacement every 6–12 months depending on usage intensity and ambient air quality. A household with pets, near a busy road, or in a dusty environment will need closer to 6-month cycles. The UV-C bulb, which physically appears to be working long after its germicidal effectiveness has degraded, should be replaced annually. Its 254nm output weakens over time even when the bulb still glows, so replacement on schedule rather than visual assessment is the right approach.
Factor the replacement costs into your budget before purchasing. For units in this article, HEPA filter replacements run $20–$40 depending on model and whether you buy genuine or compatible third-party versions. UV-C bulbs are typically $10–$20. Annual operating costs for a single-room unit come to $40–$60 on average — not a major expense, but one that a lot of buyers overlook when comparing initial purchase prices. Using third-party compatible filters is generally fine for mechanical filtration; for UV-C bulbs specifically, stick to manufacturer or verified-specification replacements to ensure the correct wavelength output.
What UV-C Air Purifiers Won’t Do
A consumer UV-C air purifier will not eliminate all risk of airborne virus transmission in a room. It reduces the concentration of airborne particles, including those carrying viruses, through continuous filtration and UV-C inactivation. It does not create a sealed sterile environment. It does not protect against direct droplet transmission from close contact. It does not replace ventilation, masks, or other protective measures in high-risk settings. If someone in your household is actively ill with a respiratory virus, an air purifier helps reduce the concentration in shared spaces — it doesn’t prevent transmission from the source.
It also won’t substitute for regular maintenance of the HVAC system. The CDC’s NIOSH guidance on germicidal ultraviolet is explicit that GUV/UVGI is a supplemental layer in a broader ventilation and filtration strategy — not a standalone solution. In a home setting, that broader strategy means a well-maintained HVAC filter (at minimum MERV 13), reasonable ventilation habits, and a quality air purifier in the spaces where people spend concentrated time. Good air quality is a layered result, not a single device’s job. For households also managing respiratory health — asthma, allergies, COPD — the combination of a quality UV-C purifier with a well-calibrated humidifier has more supporting evidence than either device alone. Our look at the HoMedics Cool Mist Ultrasonic Humidifier with Aromatherapy is a natural companion read for anyone building out that kind of home air quality system.
Our Verdict
Here’s the thing most air purifier buying guides quietly avoid: the UV-C lamp in a consumer unit is not doing most of the work. The HEPA filter is. Every reputable piece of independent research on UV-C in consumer air purifiers arrives at the same conclusion — the lamp provides a meaningful supplement to filtration, not a replacement for it. If you encounter a UV-C-only air purifier without a quality HEPA filter, skip it regardless of price. The UV-C claims are plausible in theory and underperforming in practice without the HEPA stage to trap particles first.
The buyer behavior that most “best of” lists push — upgrading to the most expensive unit with the highest-wattage UV lamp and the most marketed features — is often not what the average household needs. A $70–$150 unit with verified H13 HEPA filtration, a certified ozone-safe UV-C stage, and a CADR appropriately matched to the room being purified outperforms a $300 unit running in an oversized space on a low setting. The math is in the air changes, not the spec sheet superlatives.
For most households, the Pure Enrichment PureZone in a bedroom and the GermGuardian AC5900WCA in the main living area covers the spaces where people are most exposed to each other’s respiratory air. Both are verified by independent labs, certified ozone-safe, and backed by warranties that suggest their manufacturers believe in the product long-term. That combination — verified performance, certified safety, real warranty — is the actual checklist, and it’s shorter than the product listings make it look. For anyone who wants to dig further into home health equipment that pairs naturally with air quality management, the guide to combustible gas and refrigerant leak detectors covers related home safety tooling worth knowing about.
One more thing: replace the UV-C bulb on schedule. A faded bulb that still glows is the most common reason UV-C air purifiers underperform. The HEPA filter keeps working as it loads; the UV-C bulb quietly loses its germicidal punch after the first year without any visible sign. Set a calendar reminder. It costs less than $20 and it’s the single maintenance step with the most impact on continued virus protection performance.
UV-C Air Purifier Performance by Room Type and Risk Level
| Room Type | Typical Square Footage | Recommended CADR (min) | Target ACH | Suggested Unit | Notes |
|---|---|---|---|---|---|
| Small bedroom | 100–200 sq ft | 100 CFM | 4–6 ACH | Pure Enrichment PureZone | Ideal for overnight operation; quiet at low speed |
| Master bedroom / home office | 200–400 sq ft | 150–200 CFM | 4–5 ACH | Pure Enrichment PureZone or PureZone Elite | Run on medium overnight; high during work hours |
| Open living / kitchen-dining | 500–1,000 sq ft | 250–400 CFM | 3–4 ACH | GermGuardian AC5900WCA or PureZone Elite | Central placement critical; consider 2 units for irregular floor plans |
| Home gym / basement | 400–800 sq ft | 200–350 CFM | 4–5 ACH | GermGuardian AC5900WCA | Higher aerosol output from exercise; run on high during activity |
| Small office / studio | 200–500 sq ft | 150–250 CFM | 4–6 ACH | PureZone Elite or GermGuardian AC5900WCA | Multiple occupants increase aerosol load; size up generously |
Frequently Asked Questions – Best UV-C Air Purifier for Viruses
Do UV-C air purifiers actually kill viruses?
UV-C light can inactivate viruses, but its effectiveness inside a consumer air purifier depends heavily on how long the air stays in contact with the lamp. Most consumer units expose passing air for only a fraction of a second — often not long enough to achieve meaningful viral inactivation on their own. A UV-C air purifier works best when paired with a true HEPA filter, which physically traps virus-laden particles before the UV-C light supplements the process. Units with both HEPA filtration and UV-C are substantially more effective than UV-C alone, which is why every unit recommended here uses both technologies together.
What certifications should I look for in a UV-C air purifier?
Look for CARB (California Air Resources Board) certification to confirm zero-ozone output, ETL or UL listing for electrical safety, and AHAM VERIFIDE for independently verified CADR ratings. Zero Ozone Verified certification is especially important for UV-C units, since some cheaper lamps produce ozone as a byproduct that can aggravate respiratory conditions. ENERGY STAR certification indicates lower power draw for units running continuously. If a listing doesn’t clearly state ozone certification, treat that as a red flag and keep looking.
What is CADR and why does it matter for virus protection?
CADR stands for Clean Air Delivery Rate, measured in cubic feet per minute. It reflects how quickly an air purifier actually delivers clean air — not just how fast the fan moves air, but how much filtered output the unit produces per minute. Higher CADR means more air cycles per hour, which directly affects how quickly virus-laden particles are removed from a room. For meaningful protection, aim for at least 4 air changes per hour in the room being purified. A quick rule of thumb: the purifier’s CADR in CFM should be at least two-thirds of the room’s square footage for adequate hourly air cycling.
Is HEPA or UV-C more important for killing airborne viruses?
HEPA filtration is arguably more critical, because it physically captures virus-containing aerosol particles regardless of UV-C dwell time or lamp intensity. UV-C is effective but requires sufficient exposure time to deliver inactivating dosage — a condition that most consumer units achieve imperfectly. The most effective consumer air purifiers use both: HEPA traps the particles and UV-C inactivates microorganisms it reaches. Either technology alone is less effective than the combination, which is why a cheap UV-C-only device doesn’t belong in the same conversation as a properly engineered HEPA + UV-C unit.
How often should I replace the UV-C bulb in my air purifier?
Most manufacturer guidelines recommend replacing UV-C bulbs every 10 to 14 months, or between 9,000 and 12,000 hours of operation. Even when a UV-C bulb still appears to glow, its germicidal output at germicidal wavelengths degrades significantly after the first year. This is the single most overlooked maintenance task for UV-C air purifier owners. A HEPA filter that’s loading will often be replaced on schedule because filter change indicators make it visible; a UV-C bulb losing its effectiveness is invisible. Set a calendar reminder annually and replace it whether it looks dim or not.
Can UV-C air purifiers produce ozone?
Yes — some UV-C lamps emit wavelengths below 200nm that split oxygen molecules and create ozone, a lung irritant that can worsen asthma and other respiratory conditions. Quality UV-C air purifiers use lamps engineered to emit germicidal UV-C wavelengths around 254nm without producing ozone. The CARB EO certification and Zero Ozone Verified mark both confirm that a unit’s ozone output falls below the California safety threshold of 0.050 ppm. Every unit reviewed in this article carries one or both of these certifications. Any UV-C listing that doesn’t clearly confirm ozone-safe operation should be avoided.
What room size do I need to cover for effective virus protection?
For meaningful virus reduction, target 4 to 6 air changes per hour (ACH) in the room being purified. The calculation: room length × width × ceiling height gives you cubic feet; divide by 60 to find the CFM needed for 1 ACH per minute; multiply by 4–6 for the ACH range you want. As a practical shortcut, a standard bedroom under 200 square feet needs a purifier with a smoke CADR of at least 100 CFM; a 300-square-foot bedroom needs 150 CFM; a 600-square-foot living area needs 300 CFM or more for adequate protection. Always buy slightly above your room’s size to give yourself a useful performance buffer.
