Insulation & Air Quality
HEPA Filter Guide: True HEPA vs HEPA-Type, MERV and Furnace Limits
A true HEPA filter captures at least 99.97% of particles at 0.3 µm, the size that is hardest to catch; larger and smaller particles are trapped even more effectively. "HEPA-type" or "HEPA-like" filters have no standard behind them. Most home furnaces and air handlers can't push enough air through a HEPA filter, so whole-house systems use MERV 11–13 media or a separate bypass HEPA unit.
What makes a filter true HEPA
HEPA stands for High Efficiency Particulate Air. In the US, the benchmark is a filter that removes at least 99.97% of particles 0.3 micrometers (µm) in diameter, tested in a way that checks both the media and the seal of the filter in its frame. Put another way, no more than 3 of every 10,000 test particles at that size get through.
Europe uses the EN 1822 classes, which test at each filter's most penetrating particle size: H13 must capture at least 99.95% and H14 at least 99.995%. Consumer air purifiers sold internationally often quote H13. Either way, a genuine HEPA claim names a standard and a test, not just the word.
Why 0.3 µm is the benchmark
It seems odd that a filter rated at 0.3 µm catches smaller particles even better, but that follows from how fibrous filters work. They are not sieves. Particles are caught by three mechanisms:
- Impaction: large, heavy particles can't follow the air as it bends around a fiber and slam into it.
- Interception: mid-sized particles following the airflow pass close enough to a fiber to touch it and stick.
- Diffusion: very small particles jitter randomly (Brownian motion) and wander into fibers.
Impaction and interception get weaker as particles shrink, while diffusion gets stronger. Somewhere around 0.1–0.3 µm neither effect is at its best, giving a minimum efficiency called the most penetrating particle size. Rating the filter there means its performance on everything else, from fine smoke to pollen, is higher.
True HEPA vs HEPA-type marketing
| Label wording | What it usually means |
|---|---|
| True HEPA, HEPA H13 or H14, with a stated test standard | Meets a recognized efficiency at the most difficult particle size |
| HEPA-type, HEPA-style, HEPA-like | No defined performance. Often 85–99% on larger particles, less on fine ones |
| Captures 99% of particles down to 2 µm (or similar) | Says nothing about the 0.1–1 µm range where smoke and fine PM2.5 live |
| Washable or permanent HEPA | Washing tends to damage the fiber mat; treat performance claims with caution |
For a portable purifier, the more useful number is the CADR, which measures the combined effect of filter efficiency and airflow in the real unit. A true HEPA filter in a housing that lets air leak around its edges can perform worse than a slightly less efficient filter that is well sealed.
MERV vs HEPA
HVAC filters are rated on the MERV scale under ASHRAE Standard 52.2, which measures efficiency across three particle-size bands. HEPA sits above the top of that scale.
| Filter | 0.3–1 µm capture | 1–3 µm capture | 3–10 µm capture |
|---|---|---|---|
| MERV 8 | not required | at least 20% | at least 70% |
| MERV 11 | at least 20% | at least 65% | at least 85% |
| MERV 13 | at least 50% | at least 85% | at least 90% |
| MERV 16 | at least 95% | at least 95% | at least 95% |
| True HEPA | 99.97% at 0.3 µm, higher across the band | over 99.97% | over 99.97% |
The full scale is on the MERV rating chart. Because a central system recirculates the whole home's air several times an hour, a MERV 13 filter in the return often removes a similar share of airborne particles over time as a portable HEPA unit does in one room.
Why you can't fit HEPA in most furnaces
HEPA media is dense. At the airflow a home blower moves, a HEPA filter sized to fit a standard filter slot would typically add 1 in. w.c. or more of resistance. Most residential furnaces and air handlers are designed for a total external static pressure of about 0.5 in. w.c. across the filter, coil and all the ductwork combined. The results of forcing it:
- Airflow collapses, so the AC coil runs too cold and can freeze, leading to warm air from the vents.
- The furnace heat exchanger overheats and trips its high-limit switch, which shortens equipment life.
- Variable-speed blowers ramp up to fight the restriction, burning more electricity and getting louder.
The practical options are a deep 4–5 inch MERV 13 media cabinet in the return, a bypass HEPA unit with its own fan that pulls a slice of air from the return duct and feeds it back cleaned, or portable HEPA purifiers in the rooms that matter most. See the furnace filter guide for filter sizes and change intervals.
Modifying the return plenum or adding a bypass unit changes system airflow and may involve line-voltage wiring. Have an HVAC contractor measure static pressure before and after.
Care and replacement
- Replace HEPA filters on the manufacturer's schedule, often every 6–12 months for room purifiers, sooner during wildfire smoke or renovation dust.
- Clean or replace any prefilter more often; it protects the expensive HEPA layer from lint and hair.
- Don't vacuum, wash or blow out a HEPA filter unless the maker explicitly says it is designed for that. You can tear the media and create bypass paths.
- Handle used filters gently, outdoors if possible, and bag them before disposal.
Purifier placement matters as much as the filter; see where to place an air purifier and the air purifier sizing calculator.
Frequently asked questions
Is H13 the same as true HEPA?
They come from different standards. H13 (EN 1822) requires at least 99.95% at the most penetrating particle size; US true HEPA requires 99.97% at 0.3 µm. In practice both are high-performance filters, and the purifier's CADR and seal matter more.
Can I put a HEPA filter in my furnace?
In almost all residential systems, no. The resistance is too high for the blower. Use a MERV 13 filter, ideally in a 4–5 inch deep cabinet, or add a bypass HEPA unit designed for ducted systems.
Do HEPA filters remove viruses?
Viruses travel in respiratory droplets and aerosols that are mostly larger than individual virus particles, and HEPA captures that size range very well. Filtration reduces airborne concentration; it doesn't replace ventilation or other precautions.
Do HEPA filters remove odors?
No. HEPA traps particles only. Odors and VOCs need activated carbon or another gas-phase medium.
Last reviewed 2026-10-28