HEPA Filter Guide — True HEPA vs HEPA-Type, CADR Calculations, and the Air Purifier Math Most Manufacturers Hide

The “HEPA-Type” Filter in Your $40 Air Purifier Captures 60% of the Particles That a Real HEPA Filter Captures at 99.97%

“HEPA” is the most abused term in the air purifier industry. True HEPA (sometimes called “medical-grade HEPA” or “H13/H14”) is a specific filtration standard with a specific test protocol and a specific performance threshold: 99.97% capture efficiency for particles at 0.3 micrometers — the most penetrating particle size (MPPS). “HEPA-type,” “HEPA-style,” “HEPA-like,” and “HEPASilent” are marketing terms with no standardized meaning. They describe filters that look like HEPA pleated media but have not passed the 99.97% threshold test. Many capture 85-95% at MPPS — which sounds adequate until you calculate what gets through.

At 99.97% efficiency, 3 out of every 10,000 particles pass through. At 95% efficiency, 500 out of every 10,000 particles pass through — 167 times more leakage. At 85% efficiency, 1,500 out of 10,000 pass through — 500 times more leakage than true HEPA. The difference between “HEPA” and “HEPA-type” is not 5-15 percentage points. It is orders of magnitude in actual particle penetration.

The second deception is room size coverage. Manufacturers rate air purifiers for rooms far larger than the unit can effectively clean at adequate air change rates. An air purifier “rated for 500 sq ft” may achieve only 1-2 air changes per hour (ACH) at that room size — insufficient for meaningful PM2.5 reduction. The number that matters is CADR (Clean Air Delivery Rate), and the calculation that determines whether your purifier actually works is CADR versus room volume.

HEPA filter classification standards

Classification	Standard	Efficiency at MPPS (0.3 µm)	Penetration	Common name	Where used
E10	EN 1822	85%	15%	EPA filter	HVAC pre-filtration
E11	EN 1822	95%	5%	EPA filter	HVAC filtration, some consumer purifiers
E12	EN 1822	99.5%	0.5%	EPA filter	Enhanced HVAC
H13	EN 1822	99.95%	0.05%	True HEPA	Medical, pharmaceutical, premium consumer
H14	EN 1822	99.995%	0.005%	HEPA (medical-grade)	Cleanrooms, operating theaters, isolation rooms
U15	EN 1822	99.9995%	0.0005%	ULPA	Semiconductor manufacturing, high-security biocontainment
U16	EN 1822	99.99995%	0.00005%	ULPA	Extreme cleanroom applications
”True HEPA”	US DOE STD 3020	99.97%	0.03%	True HEPA	US consumer standard (between H13 and H14)
“HEPA-type”	No standard	85-99% (unverified)	1-15%	Marketing term	Budget consumer purifiers
”HEPASilent”	Proprietary (Blueair)	~99% (with electrostatic pre-charging)	~1%	Branded hybrid	Blueair purifiers (lower pressure drop)

The MPPS explanation: 0.3 micrometers is not the smallest particle size — it is the most penetrating particle size. Particles smaller than 0.3 µm are actually captured more efficiently (by diffusion/Brownian motion). Particles larger than 0.3 µm are also captured more efficiently (by impaction and interception). HEPA filters are least efficient at exactly 0.3 µm, which is why that size is used for the efficiency rating. A filter rated at 99.97% at 0.3 µm captures >99.99% of both smaller and larger particles.

Particle size context — what 0.3 µm means

Particle	Size (µm)	HEPA capture efficiency	Can you see it?	Health relevance
Virus (individual)	0.02-0.3	99.97%+ (diffusion capture)	No	Respiratory infection
Bacteria	0.5-5	99.99%+	No	Infection
Tobacco smoke	0.01-1.0	99.97%+ (most in MPPS range)	Visible as haze	Cardiovascular, cancer
Cooking oil smoke	0.03-0.5	99.97%+	Visible at high concentration	Respiratory irritation
Wildfire smoke PM2.5	0.1-2.5	99.97%+	Visible as haze	Cardiovascular, respiratory
Dust mite allergen	1-40	99.99%+	Barely (largest)	Allergic rhinitis, asthma
Pollen	10-100	99.99%+	Yes	Allergic rhinitis
Pet dander	2.5-10	99.99%+	Barely	Allergic rhinitis, asthma
Mold spores	2-20	99.99%+	No	Allergic, respiratory, toxic
Human hair	50-100	99.99%+	Yes	None (too large to inhale deep)

CADR — the only number that determines air purifier effectiveness

CADR (Clean Air Delivery Rate) measures the volume of clean air a purifier delivers per unit time, in cubic feet per minute (cfm) or cubic meters per hour (m³/h). It combines airflow rate × filter efficiency into a single number. A purifier with high airflow but low-efficiency filter may have the same CADR as a purifier with lower airflow but true HEPA filter.

CADR-to-room-size calculation

The AHAM (Association of Home Appliance Manufacturers) recommends a CADR at least 2/3 of the room area in square feet. This targets approximately 4.8 ACH with 8-foot ceilings. For health-sensitive applications, higher ACH is needed.

Room area (sq ft)	Room volume (ft³, 8’ ceiling)	CADR needed: 2 ACH (cfm)	CADR needed: 4 ACH (cfm)	CADR needed: 6 ACH (cfm)	CADR needed: 8 ACH (cfm)
100	800	27	53	80	107
150	1,200	40	80	120	160
200	1,600	53	107	160	213
250	2,000	67	133	200	267
300	2,400	80	160	240	320
400	3,200	107	213	320	427
500	4,000	133	267	400	533
700	5,600	187	373	560	747
1000	8,000	267	533	800	1067

Formula: CADR (cfm) = Room volume (ft³) × ACH target ÷ 60

Conversion: 1 cfm = 1.699 m³/h. To convert CADR from m³/h to cfm, divide by 1.699.

ACH targets by use case

Use case	Minimum ACH	Recommended ACH	Why
General air quality maintenance	2	4	AHAM “2/3 rule” targets ~4.8 ACH; adequate for background purification
Allergy sufferer bedroom	4	6	Allergen levels need faster clearance during sleep
Wildfire smoke event	5	8+	Sustained high-concentration PM2.5 requires aggressive filtration
Smoke/cooking odor room	4	6	Odor + PM2.5 clearance after cooking events
Nursery/infant room	4	6	Developing lungs more vulnerable; precautionary higher rate
Home office (productivity)	2	4	PM2.5 reduction supports cognitive function
Workshop/craft room	4	8+	Particulate-generating activities require high clearance

The manufacturer’s room size lie: When a manufacturer says their purifier covers “500 sq ft,” they typically calculate using 2 ACH or less — the minimum for any measurable effect. At 2 ACH, the purifier cycles the room air twice per hour, but PM2.5 reduction at this rate is only 30-50% of ambient levels. For the 70-80% reduction most people expect from an air purifier, you need 4-6 ACH — which means the “500 sq ft” purifier effectively covers 200-300 sq ft. Always divide the manufacturer’s room size rating by 1.5-2 for realistic coverage.

HEPA purifier operating cost comparison

Component	Typical cost	Replacement interval	Annual cost	What happens if not replaced
True HEPA filter	$30-80	6-12 months (depends on air quality and use)	$40-120	Reduced airflow → reduced CADR → reduced effectiveness; NOT a health hazard from the filter itself
Pre-filter	$5-15 (or washable)	1-3 months	$20-60 (if disposable)	HEPA filter loads faster; shorter HEPA lifespan
Activated carbon filter	$15-50	3-6 months	$40-150	VOC and odor removal drops to zero; carbon saturates and may off-gas
Electricity	—	Continuous	$15-80 (depends on wattage and speed)	—
Total annual operating cost	—	—	$75-350 per purifier	—

Power consumption by purifier speed

Speed setting	Typical wattage	Monthly electricity cost (at $0.16/kWh)	CADR (% of max)	Noise (dBA)	Practical use
Low / Sleep	5-15W	$0.60-1.75	20-40%	20-35 dBA	Overnight bedroom use
Medium	20-40W	$2.30-4.60	50-70%	35-50 dBA	Daytime background
High	40-80W	$4.60-9.20	80-100%	45-65 dBA	Cooking events, high AQI
Turbo / Max	60-200W	$6.90-23.00	100%	55-70+ dBA	Emergency (wildfire, smoke event); too loud for sustained use

The noise-performance tradeoff: Most people run air purifiers on low or sleep mode because higher speeds are uncomfortably loud. But low mode delivers 20-40% of rated CADR. A purifier rated at 300 cfm CADR delivers 60-120 cfm on low — turning your “500 sq ft” purifier into a “100-200 sq ft” purifier. The purifier you actually use at the speed you actually tolerate is the performance that matters, not the maximum CADR.

Filter type comparison beyond HEPA

Filter technology	PM2.5 removal	VOC/gas removal	Ozone generation?	Maintenance	Best for	Avoid if
True HEPA (H13+)	99.97%	None (particles only)	No	Filter replacement every 6-12 months	Particles, allergens, smoke	VOC/chemical concern is primary (add carbon)
HEPA + activated carbon	99.97% (HEPA) + some VOC (carbon)	30-80% (depends on carbon mass and VOC type)	No	HEPA: 6-12 mo; Carbon: 3-6 mo	General air quality — particles + odors/gases	Heavy chemical contamination (insufficient carbon mass)
Electrostatic precipitator (ESP)	85-95%	Minimal	Yes (0.005-0.05 ppm)	Plate washing monthly (reusable)	Low-maintenance, no filter cost	Ozone sensitivity, asthma, children
Ionizer (negative ion generator)	30-60% (particles settle, not captured)	None	Yes (0.01-0.08 ppm)	Minimal	Supplement to HEPA	Primary air cleaning (ineffective alone); ozone concerns
Photocatalytic oxidation (PCO)	Minimal	Variable (20-70%; generates byproducts)	Yes (some models)	UV lamp replacement 12-24 months	VOC destruction in theory	Incomplete oxidation creates formaldehyde and other byproducts — avoid
Plasma / PlasmaWave	50-80%	20-50%	Low (0.001-0.01 ppm; most models below FDA 0.05 ppm limit)	Minimal	Supplement to mechanical filtration	Ozone-sensitive individuals (even low levels may affect)
UV-C (in purifier)	None (does not remove particles)	None	Possible (185 nm wavelength generates ozone; 254 nm does not)	UV lamp: 12-24 months	Bioaerosol inactivation (supplement)	Primary air cleaning (ineffective alone for particles)

The ozone problem: Electrostatic precipitators, ionizers, and some UV-C systems generate ozone as a byproduct. The FDA limits ozone emission from medical devices to 0.05 ppm. California’s ARB certification (mandatory for air purifiers sold in CA) limits ozone to 0.05 ppm. Even at these levels, ozone reacts with terpenes from cleaning products and air fresheners to form formaldehyde and secondary PM2.5. For homes with any VOC sources, ozone-generating purifiers create new pollutants while removing others.

Purifier placement and room dynamics

Placement	Effectiveness	Why	Common mistake
Center of room, elevated (table height)	Maximum	Unrestricted airflow intake and output; air circulates through entire room	Rarely practical (aesthetics, tripping hazard)
Against wall, 6-12 inches clearance	Good (85-95% of optimal)	Adequate intake; wall creates some dead zone behind unit	Pushing flush against wall (restricts intake)
Corner of room	Moderate (70-85% of optimal)	Two walls restrict circulation; serves room unevenly	Relying on one purifier in L-shaped room
On floor, low placement	Moderate for floor-level particles; poor for room air	Good for settling dust; poor for cooking smoke that rises	Assuming floor placement handles all particle sizes
Inside closet or cabinet	Very poor	Restricted airflow; purifier recirculates same small volume	”Hiding” the purifier for aesthetics
Near pollution source (kitchen, entry)	High for source capture	Intercepts pollutants before room distribution	Placing near a window with outdoor smoke infiltration (pulls in more)

How to apply this

Use the ingredient-checker tool to identify VOC-emitting products in your home — if your cleaning products, air fresheners, or personal care products emit significant VOCs, you need a purifier with activated carbon in addition to HEPA, and eliminating the source is more effective than filtering it.

Calculate your actual CADR need. Measure your room (length × width × ceiling height = volume in ft³). Multiply by your target ACH (4 for general use, 6 for allergy/smoke). Divide by 60. That is your required CADR in cfm. Compare to the purifier’s tested CADR (from AHAM or independent testing), not the manufacturer’s room size claim.

Buy one size up. If your calculation says you need 200 cfm CADR, buy a purifier rated at 300+ cfm. This lets you run it on medium speed (achieving your target CADR) instead of maximum (which you won’t tolerate due to noise).

Verify “True HEPA” claims. Look for H13 or H14 classification (EN 1822), or “99.97% at 0.3 microns” specifically stated. “HEPA-type,” “HEPA-style,” and “99% HEPA” are not True HEPA. If the filter specification does not state the efficiency at 0.3 µm, it is not True HEPA.

Avoid ozone-generating technologies. If you have any VOC sources in your home (cleaning products, air fresheners, new furniture, paints), electrostatic precipitators and ionizers create secondary pollutants. Stick with mechanical HEPA filtration.

Honest limitations

CADR is measured under standardized conditions (sealed room, no new particle generation) — real-world performance is lower due to room air leakage, continuous particle generation, and purifier placement. The AHAM CADR protocol tests three particle types (smoke, dust, pollen) — results vary by particle type, and CADR for your specific concern may differ from the headline number. True HEPA captures 99.97% of particles at the filter — but frame leakage around the filter seal can reduce whole-unit efficiency to 95-99%, especially in cheaper housings. Filter lifespan varies enormously with air quality — a filter rated for 12 months in clean suburban air may last 3-4 months during wildfire season or in a home with smokers. Noise measurements (dBA) vary by testing distance and room acoustics; manufacturer-stated noise levels are typically measured at 1 meter in an anechoic chamber and will be louder in a reflective room. Smart purifier auto-modes that adjust speed based on sensor readings may under-respond to cooking spikes (sensor lag) or over-respond to harmless events (humidity changes mistaken for particles). The California ARB ozone limit (0.05 ppm) allows detectable ozone — individuals with asthma may react below this threshold. Carbon filter effectiveness for VOCs depends on the mass of activated carbon, contact time, and the specific VOC — thin carbon sheets in budget purifiers provide negligible VOC removal despite marketing claims.