Choosing an Air Purifier
HEPA vs ionizer vs activated carbon — what actually works for clean air
The Four Main Technologies
HEPA Filters
The gold standard for particle removal. A true HEPA filter captures 99.97% of particles 0.3 microns and larger — dust, pollen, pet dander, mold spores, and many bacteria. HEPA is mechanical: air passes through dense fibers and particles get trapped. No byproducts.
HEPA does not remove gases, VOCs, or odors — only solid particles.
Activated Carbon Filters
Activated carbon adsorbs gases and VOCs onto its porous surface. Effective against odors, smoke, formaldehyde off-gassing, and chemical fumes. Often paired with HEPA for comprehensive air cleaning.
Carbon filters become saturated over time and must be replaced regularly. A saturated filter does nothing.
Ionizers
Ionizers release negative ions that attach to particles and cause them to settle onto surfaces. The problem: many produce ozone as a byproduct. Even “ozone-free” models may produce trace amounts. Settled particles are not truly removed — they sit on walls, floors, and furniture until you clean them.
UV-C Light
UV-C light kills bacteria, viruses, and mold spores in a light chamber. Does nothing for dust, allergens, gases, or odors. Most standalone units do not expose air long enough for thorough disinfection. UV-C works best as a supplement inside a HEPA unit, not as a primary technology.
What to Actually Buy
For most homes, a HEPA purifier with an activated carbon pre-filter is the best choice. It handles both particles and gases.
Room Size Calculation
Every air purifier has a Clean Air Delivery Rate (CADR) in cubic feet per minute. The purifier should cycle all room air 4-5 times per hour.
- Match the purifier’s rated room size to your room or go larger.
- High ceilings (over 8 feet) require sizing up.
Filter Replacement Costs
Factor in ongoing filter replacement — not just the purchase price:
- HEPA filters: 12 months, $30-$80
- Carbon filters: 3-6 months, $15-$40
- Pre-filters: often vacuumed and reused, extending the HEPA filter’s life
Check replacement filter prices before buying. Some brands charge premiums for proprietary filters.
What to Avoid
- Ozone-producing ionizers. Check the CARB certified list. Unlisted units may produce unsafe ozone.
- UV-C-only units. UV-C alone is insufficient for general air purification.
- Vague marketing claims. Look for independently tested CADR ratings, not “removes 99.99% of everything.”
- “Permanent filter” models. Less effective than true HEPA and cannot be restored to original performance.
Placement Tips
- Place it where you spend the most time — usually the bedroom.
- Keep at least a foot of clearance from walls and furniture.
- Close windows and doors so the purifier cycles room air instead of pulling in outdoor air.
- Run higher when you are out; lower at night if noise is a concern.