Air Purifiers Buyer's Guide: HEPA, MERV, CADR Explained

Walk into any electronics store and you'll face a bewildering array of air purifiers: HEPA this, True HEPA that, MERV ratings, CADR numbers, ionizers, UV lights, and prices ranging from $50 to $1,000+.

Which one actually cleans the air? What do all these acronyms mean? And how much should you spend?

This guide cuts through the marketing noise with science-backed explanations of what matters, what doesn't, and how to choose an air purifier that actually protects your health.

Understanding Air Purifier Technology

HEPA Filters: The Gold Standard

What HEPA Means:

High-Efficiency Particulate Air
Must remove 99.97% of particles 0.3 micrometers in size
This is a regulatory standard, not marketing hype
0.3µm is the "most penetrating particle size" (hardest to capture)

Why HEPA Works:

Removes PM2.5 effectively (2.5µm particles)
Captures even smaller ultrafine particles
Traps viruses, bacteria, pollen, dust, pet dander
Physical filtration (not chemical)
No harmful byproducts

Types of "HEPA":

True HEPA (H13/H14): ✅ Meets official HEPA standard ✅ Tested and certified ✅ Look for this

HEPA-type / HEPA-like: ❌ Marketing terms with no standard definition ❌ May capture only 85-90% of particles ❌ Avoid these

Medical-grade HEPA (H14): ✅ Even stricter: 99.995% at 0.3µm ✅ Used in hospitals ✅ Not necessary for home use ✅ More expensive

Bottom line: Insist on "True HEPA" or "H13 HEPA" minimum.

Carbon Filters: Odor and Gas Removal

What Carbon Does:

Absorbs gases, odors, and volatile organic compounds (VOCs)
Does NOT remove particles (PM2.5)
Complements HEPA, doesn't replace it

When Carbon Matters:

Cooking odors
Pet smells
New furniture off-gassing
Smoke (though particles still need HEPA)
Chemical sensitivity

Limitations:

Needs replacement (saturates over time)
Less effective than HEPA for main health concerns (PM2.5)
Can't absorb everything (formaldehyde particularly challenging)

Best Approach: Get both HEPA + carbon in a good purifier.

Technologies to Avoid

Ionizers / Ionic Air Purifiers: ❌ Produce ozone (lung irritant) ❌ Particles fall to surfaces (don't actually remove from air) ❌ Less effective than HEPA ❌ Avoid or ensure "ozone-free" certification

UV-C Light: ⚠️ Limited effectiveness for air (needs prolonged exposure) ⚠️ Can produce ozone if designed poorly ⚠️ Mostly marketing gimmick in air purifiers ⚠️ Not harmful but not necessary

Ozone Generators: 🚫 NEVER use these 🚫 Intentionally produce ozone (harmful to lungs) 🚫 EPA warns against residential use 🚫 No safe level of ozone exposure

Photocatalytic Oxidation (PCO): ⚠️ Can produce formaldehyde as byproduct ⚠️ Limited effectiveness ⚠️ Not worth the risk

Bottom Line: Stick with mechanical HEPA + carbon filtration.

Understanding CADR: The Most Important Number

What Is CADR?

Clean Air Delivery Rate:

Measured in cubic feet per minute (CFM)
Indicates volume of clean air purifier delivers
Tested and certified by independent labs (AHAM)
THE key metric for comparing purifiers

Three CADR Numbers: Most purifiers list three:

Smoke CADR: Smallest particles (0.09-1µm)
Dust CADR: Medium particles (0.5-3µm)
Pollen CADR: Largest particles (5-11µm)

For air quality/PM2.5, focus on SMOKE CADR (it's the hardest to filter).

CADR and Room Size

The Formula: Room size (sq ft) × ceiling height (ft) = Room volume (cubic ft) Divide room volume by 2 to get recommended minimum CADR

Example:

Room: 150 sq ft
Ceiling: 8 ft
Volume: 1,200 cubic ft
Recommended CADR: 600 CFM or higher

Why × 2? To achieve 5 air changes per hour (ACH)—recommended minimum for clean air.

Room Size Guidelines:

| Room Size | Minimum CADR (Smoke) | |-----------|----------------------| | 100 sq ft | 100 CFM | | 150 sq ft | 150 CFM | | 200 sq ft | 200 CFM | | 300 sq ft | 300 CFM | | 400 sq ft | 400 CFM |

Higher CADR = Better: More air changes, faster cleaning, quieter operation (doesn't need max speed).

ACH: Air Changes Per Hour

What It Means: How many times per hour the purifier cycles all room air through the filter.

Recommended Rates:

General use: 4-5 ACH
Allergy/asthma: 5-6 ACH
High pollution periods: 6+ ACH
Wildfire smoke: 8-10 ACH

Higher ACH = Cleaner Air

How to Calculate: ACH = (CADR × 60) / Room Volume

Example:

CADR: 300 CFM
Room: 150 sq ft × 8 ft = 1,200 cu ft
ACH = (300 × 60) / 1,200 = 15 ACH

This purifier would excel in that room.

MERV vs. HEPA

Understanding MERV

Minimum Efficiency Reporting Value:

Rates filters on 1-16 scale
Used for HVAC filters
Different from HEPA rating system

MERV Ratings:

| MERV | Captures | Use Case | |------|----------|----------| | 1-4 | Large particles only | Basic protection | | 5-8 | Mold, dust, pollen | Standard home HVAC | | 9-12 | Auto emissions, fine dust | Better home HVAC | | 13-16 | Bacteria, smoke, PM2.5 | Hospital-grade | | 17-20 | (HEPA territory) | Cleanrooms |

MERV 13+ ≈ Similar to HEPA in PM2.5 capture

MERV for HVAC

If upgrading your home HVAC filter:

MERV 13 or higher recommended for:

Air quality concerns
Allergies/asthma
High outdoor pollution areas
Wildfire-prone regions

Important:

Check if your HVAC can handle higher MERV (airflow restriction)
Change filters regularly (every 2-3 months)
Higher MERV = more frequent changes needed

Don't Expect Miracles:

HVAC systems aren't designed as air purifiers
Whole-house air purification is supplemental
Portable purifiers with HEPA still recommended for bedrooms

Choosing the Right Purifier

Step 1: Determine Room Size

Measure the room where you'll use it:

Bedroom: Most important (8-12 hours/night)
Living room: Where family spends time
Home office: If you work from home
Child's room: High priority

Pro Tip: Buy one sized for a larger room than yours. It'll clean faster and run quieter on lower speeds.

Step 2: Calculate Required CADR

Use the room size chart above, or:

Small room (100-150 sq ft): 150+ CADR
Medium room (150-300 sq ft): 250+ CADR
Large room (300-500 sq ft): 400+ CADR
Very large room (500+ sq ft): 500+ CADR or multiple units

Step 3: Set Your Budget

Budget Tiers:

$100-200:

Small to medium rooms
Basic HEPA filtration
Good value if from reputable brand
Examples: Levoit, GermGuardian

$200-400:

Medium to large rooms
HEPA + carbon
Better build quality
Quiet operation
Examples: Coway, Winix, Honeywell

$400-700:

Large rooms or superior performance
Premium features (smart controls, app connectivity)
Very quiet
Examples: IQAir, Blueair, Dyson

$700+:

Top-tier performance
Whole-room or multiple-room coverage
Medical-grade filtration
Commercial quality
Example: IQAir HealthPro Plus

Cost of Ownership: Don't forget filter replacement costs:

HEPA filters: $30-150, replace every 6-12 months
Carbon filters: $15-50, replace every 3-6 months
Factor into total cost

Step 4: Key Features to Look For

Must-Haves:

✅ True HEPA filtration
✅ CADR rating appropriate for room
✅ AHAM verification (independent testing)
✅ Low noise levels (<50 dB on medium speed)

Nice-to-Haves:

✅ Carbon filter included
✅ Multiple fan speeds
✅ Filter replacement indicator
✅ Timer function
✅ Sleep mode (ultra-quiet)

Optional (Convenience):

Air quality sensor (shows real-time PM2.5)
Auto mode (adjusts speed based on air quality)
App connectivity
Smart home integration
Energy Star certified

Not Necessary:

UV-C lights
Ionizer (unless ozone-free and bypassable)
Fancy designs (function > form)

Top Purifier Recommendations by Room Size

Small Rooms (100-200 sq ft)

Best Value: Levoit Core 300

CADR: 140 CFM (smoke)
True HEPA + carbon
$100-120
Quiet, reliable
Filter cost: ~$35/year

Premium Option: Coway Airmega 150

CADR: 170 CFM
True HEPA + carbon
$180-200
Air quality indicator
Very quiet

Medium Rooms (200-350 sq ft)

Best Overall: Coway Airmega 400

CADR: 350 CFM (smoke)
True HEPA + carbon
$300-350
Excellent performance
Air quality display

Budget Choice: Winix 5500-2

CADR: 240 CFM
True HEPA + carbon + PlasmaWave (ozone-free)
$150-180
Solid performance
Good value

Large Rooms (350-500 sq ft)

Top Pick: Blueair Blue Pure 211+

CADR: 350 CFM
HEPA-like (HEPASilent technology - tested equivalent)
$250-300
Very quiet
Energy efficient

High-End Option: IQAir HealthPro Plus

CADR: 300 CFM
Medical-grade H14 HEPA
$900+
Highest filtration quality
Long filter life (reduces cost/year)

Very Large Rooms or Open Concept (500+ sq ft)

Commercial-Grade: Austin Air HealthMate Plus

CADR: 400 CFM
True HEPA + 15 lbs activated carbon
$600-700
5-year filter life
Heavy-duty construction

Alternative: Use two medium-sized purifiers instead of one large

Better air circulation
Redundancy if one fails
Often more cost-effective

Special Use Cases

Wildfire Smoke

Requirements:

Highest CADR you can afford
True HEPA essential
Carbon filter helps with odor
Run 24/7 during smoke events

Top Choice:

Blueair Classic 605: CADR 500, handles large areas
Austin Air: Long filter life (important during extended smoke)

Allergies

Focus:

HEPA captures pollen, dust, pet dander
CADR less critical (allergens larger than PM2.5)
Place in bedroom for nighttime relief

Recommended:

Any True HEPA purifier sized for room
Coway and Levoit both excellent

Asthma

Needs:

True HEPA
High CADR (5+ ACH)
Quiet (for sleeping)
Run continuously

Best:

IQAir HealthPro Plus (medical-grade)
Coway Airmega series (good value, effective)

Chemical Sensitivity / VOCs

Requirements:

Lots of activated carbon (look for pounds, not ounces)
Austin Air models have 15 lbs carbon
IQAir GC MultiGas for serious chemical concerns

Note: Air purifiers help but aren't complete solution for severe chemical sensitivity.

Pets

Focus:

HEPA for dander
Carbon for odors
Pre-filter to catch hair (extends HEPA life)

Recommended:

Winix 5500-2 (good odor control)
Levoit 400S (large carbon filter)

DIY Air Purifier: The Box Fan Method

Surprisingly Effective Budget Option

What It Is: Box fan + MERV 13 filter taped to back = functional air purifier

How to Build:

Buy 20" box fan ($20-30)
Buy 20" × 20" MERV 13 furnace filter ($15-20)
Tape filter to back of fan (filter facing fan intake)
Run on medium/high speed

Performance:

Studies show 50-80% as effective as commercial HEPA purifiers
CADR roughly 200-300 CFM
Total cost: ~$40
Filter replacement: $15-20 every 3-6 months

Pros:

Extremely cost-effective
Easy to build (5 minutes)
Effective for PM2.5
Great for emergencies (wildfire smoke)

Cons:

Loud (box fans aren't quiet)
Not aesthetically pleasing
Less efficient than purpose-built purifiers
No carbon filtration (odors pass through)

Best Use:

Budget-conscious
Emergency smoke events
Secondary purifier for additional rooms
Until you can afford commercial purifier

Safety: Ensure good airflow; don't block fan intake/exhaust.

Maintenance and Optimization

Filter Replacement

When to Replace:

HEPA filters: Every 6-12 months (check manufacturer)
Carbon filters: Every 3-6 months
Pre-filters: Wash monthly (if washable) or replace

Signs You Need New Filters:

Filter replacement indicator light
Reduced airflow
Odors passing through
Visible dirt accumulation

Don't Skip Replacements: Dirty filters reduce efficiency and can become pollution sources themselves.

Placement Tips

Best Locations:

Center of room (best airflow)
Away from walls (3+ feet clearance)
Elevated (table or stand) works well
Near pollution source (next to door during smoke)

Avoid:

Corners (poor circulation)
Behind furniture (blocks airflow)
Near curtains (airflow obstruction)

Running Your Purifier

When to Run:

24/7 is ideal (air always accumulating particles)
Minimum: During sleep (8+ hours in bedroom)
During pollution events: Continuous operation essential

Speed Settings:

High: Initial cleaning or during poor air quality
Medium: Daily use
Low/Sleep: Nighttime (quieter but still effective)

Auto Mode: If available, use it—adjusts based on air quality sensors.

Measuring Success

How to Know It's Working

Air Quality Monitor:

Separate PM2.5 monitor recommended ($50-150)
Purple Air, IQAir, AirVisual brands
Place in same room as purifier
Should see PM2.5 drop significantly within 30-60 minutes

Expected Results:

Good purifier should bring indoor PM2.5 to <10 µg/m³
Even when outdoor AQI is 150-200
If not achieving this, purifier undersized or filters need replacement

Subjective Indicators:

Less dust accumulation
Reduced allergy symptoms
Better sleep
Less sneezing/congestion

Common Mistakes to Avoid

❌ Undersizing: Most common mistake. Bigger is better.

❌ Not running enough: Purifier can't work if it's off.

❌ Forgetting filter replacement: Sets calendar reminders.

❌ Blocking airflow: Give it space to breathe.

❌ Falling for gimmicks: Stick to proven HEPA + carbon.

❌ Buying based on room size claims alone: Check CADR, not just "covers up to X sq ft" marketing.

❌ Neglecting multiple rooms: One purifier only cleans one room.

Your Air Purifier Action Plan

Prioritize bedroom: Start where you spend 8 hours/night
Calculate required CADR: Use room size guidelines
Set realistic budget: Factor in ongoing filter costs
Buy True HEPA: Don't compromise on filter type
Run continuously: At least during sleep
Monitor results: Get PM2.5 monitor
Maintain regularly: Replace filters on schedule
Expand gradually: Add purifiers to other rooms as budget allows

Calculate your current exposure using our AQI to Cigarettes Calculator to understand why investing in air purification matters.

The Bottom Line

Air purifiers work—if you choose the right one and use it properly. Focus on True HEPA filtration, appropriate CADR for your room size, and continuous operation.

Don't get distracted by flashy features or marketing gimmicks. The basics—mechanical HEPA filtration with adequate CADR—are what matter.

A good air purifier can reduce your indoor PM2.5 by 80-90%, even when outdoor air quality is poor. That's the difference between breathing hazardous air and clean air, all from a device that fits in the corner of your bedroom.

Your lungs will thank you.

Information sources: EPA, AHAM, peer-reviewed air quality research, consumer testing organizations, manufacturer specifications.