HEPA Air Purifiers: What to Look For

A good HEPA purifier can drop indoor PM2.5 by 50% to 80% within an hour. A bad one is an expensive nightlight. Three numbers and one filter type tell you most of what you need to know.

Why it matters

Indoor air can be two to five times more polluted than outdoor air, and during wildfire events, indoor PM2.5 typically reaches 30% to 70% of outdoor levels even with windows shut. A true HEPA filter removes 99.97% of particles at 0.3 microns, which is the hardest size to catch (smaller and larger particles get trapped more easily). For wildfire smoke, dust, pollen, and pet dander, HEPA is the right tool.

What to do

Check CADR for your room

Clean Air Delivery Rate (CADR), measured in cubic feet per minute, is the volume of filtered air the unit produces on its top setting. AHAM's "two-thirds rule" is the standard: pick a purifier whose CADR (smoke or dust) is at least two-thirds of your room's square footage at standard 8-foot ceilings. A 200 square foot bedroom needs a CADR of at least 135. A 400 square foot living room needs 270 or higher.

Look for CADR numbers verified by AHAM. Manufacturers can publish whatever they want without the AHAM seal.

Demand a true HEPA filter

"HEPA-type," "HEPA-like," and "99% efficient" are marketing terms with no standard behind them. True HEPA (or H13/H14 in the European EN 1822 scale) is the only certification that means the 99.97% spec. The label should say "True HEPA" or "HEPA H13" with no fudge words.

Skip ionizers and ozone generators

Some "purifiers" generate ozone as a feature. Ozone irritates lungs and is itself a regulated pollutant. The California Air Resources Board maintains a list of certified ozone-safe purifiers. If your unit isn't on it, check what it actually emits.

Match price to room and run time

• Small bedroom (under 200 sq ft): Levoit Core 300 or Coway AP-1512HH class, roughly $100 to $200.
• Medium room (200 to 400 sq ft): Coway Airmega Mighty or Levoit Core 400S, roughly $200 to $300.
• Large room (400 to 700 sq ft): Coway Airmega 250 or Blueair 511i, roughly $300 to $500.
• Whole-house: skip the purifier and upgrade the HVAC filter to MERV 13.

Plan to run it 24/7 during smoke events. Filters typically last 6 to 12 months at normal use, 2 to 4 months during heavy smoke.

What to avoid

• Anything sold on health claims (bacteria, viruses, "molecular") without third-party CADR data.
• UV-C bulbs as the primary mechanism. They do not meaningfully clean room-volume air.
• Purifiers without a replaceable filter. Washable "permanent" filters lose efficiency fast.
• Sizing one small unit for an open-plan house. CADR doesn't reach around corners.

Quick checklist

• AHAM-verified CADR equal to at least two-thirds of room square footage.
• "True HEPA" or "HEPA H13" on the filter, not "HEPA-type."
• CARB-certified or on the CARB safe list (no ozone).
• Replaceable filter, with replacement cost you've checked.
• Quiet enough to run on medium overnight in a bedroom.
• One unit per main room you spend time in.

Sources

• Air Purifier Buying Guide, Consumer Reports
• What to Consider When Buying a HEPA Air Purifier, Montana DPHHS
• Air Cleaners and Air Filters in the Home, US EPA
• California Certified Air Cleaning Devices, CARB
• Best Air Purifiers, CNN Underscored