Quick answer
HEPA filtration is air filtering with a high-efficiency particulate air filter, a pleated mechanical filter rated to remove at least 99.97% of airborne particles 0.3 microns across when tested under set conditions.
It captures particles, not gases. In restoration, HEPA filters sit inside air scrubbers, negative air machines and vacuums, where they catch dust and spores stirred up by drying and demolition.
Quick facts
The key facts about HEPA Filtration, with sources:
- Definition
- A pleated mechanical filter that can theoretically remove at least 99.97% of particles at 0.3 microns 1
- Federal handbook definition
- Minimum 99.97% efficiency for 0.3-micrometer particles, a maximum clean resistance of 1.0 inch water gauge at 1,000 cfm, and a rigid casing 2
- Why 0.3 microns
- It is the most penetrating particle size, the one a filter catches least efficiently 1
- What it does not do
- It addresses particles; EPA notes no widely used rating systems exist for portable cleaners that remove gases 3
- Household rule of thumb
- A CADR of 65 per 100 square feet of room, with an 8-foot ceiling 5
- Regulation
- The industry follows recommended practices from a standards group; no government agency regulates the HEPA label 6
Key takeaways
- HEPA means high-efficiency particulate air. A HEPA filter is rated to catch at least 99.97% of 0.3-micron particles 12.
- 0.3 microns is the hardest size to catch, so larger and smaller particles are captured at higher rates 1.
- HEPA filters particles. They do not remove gases, and EPA says no widely used rating exists for portable cleaners that remove gases 3.
- A lab rating is not a job rating. Airflow, fit and damage affect real performance 6.
- HEPA filtration is a supplement to fixing moisture and cleaning up. It does not remove the cause of mold 3.
HEPA filtration in plain English
A HEPA filter is a dense, pleated paper-like mat of very fine fibers. Air is forced through it, and particles stick. EPA describes it as a type of pleated mechanical air filter common in portable air cleaners, where HEPA stands for high efficiency particulate air 1.
The headline number is 99.97% at 0.3 microns. EPA says this type of filter can theoretically remove at least 99.97% of airborne particles, including dust, pollen, mold and bacteria, and calls 0.3 microns the most penetrating particle size, the size the filter catches least efficiently.
Particles of other sizes are captured at higher rates 1. The word “theoretically” matters, and later sections explain why.
The U.S. Department of Energy’s Nuclear Air Cleaning Handbook defines a HEPA filter more fully.
It is a throwaway, extended-medium, dry-type filter with a minimum particle removal efficiency of no less than 99.97% for 0.3-micrometer particles, a maximum clean resistance of 1.0 inch of water gauge at 1,000 cfm, and a rigid casing that extends the full depth of the medium 2.
A micrometer is the same as a micron.
For restoration, the filter is a component. It sits inside an air scrubber or a negative air machine, or in a vacuum, as covered below.
How a HEPA system cleans air, step by step
HEPA filtration works in stages that follow a manufacturer’s guide and EPA’s description 61.
- Air is pulled through pre-filters. They catch the larger particles, down to about 10 micrometers, so the HEPA filter does not load early 6.
- The HEPA media catches the small ones. The final filter removes particles as small as 0.3 microns 6.
- Efficiency depends on airflow. The slower the particles move, the longer they dwell in the filter and the easier they are to capture 6.
- Pressure builds as the filter loads. A higher pressure drop means less usable airflow, so loaded filters need replacing 6.
- Filtered air leaves the machine. A recirculating unit returns it to the room, and a negative air unit exhausts it outside a containment 6.
Particles passing through = particles entering × (1 − 0.9997)
0.9997- The 99.97% minimum removal at 0.3 microns, as a decimal
Particles entering- The count at the filter inlet
Particles passing through- The count that gets through, at the test size and airflow
A minimum removal rating, shown as arithmetic. It applies to the test particle size and conditions, and real jobs differ.
Where HEPA filtration is used, and what each form does
| Form | What it does | Where it is mentioned |
|---|---|---|
| Air scrubber | Recirculates filtered air in a room 6 | Used on water, sewage, mold and fire jobs 6 |
| Negative air machine | Exhausts filtered air from a sealed containment 6 | Contains contaminants during demolition and remediation |
| HEPA vacuum | Cleans dust from surfaces, recommended for final cleanup after materials are dried and contaminated materials removed 8 | NIH’s procedure; also for dust settled outside the remediation area 8 |
| Household portable air cleaner | Filters the air in one room, and can use a HEPA filter to reach a high CADR 3 | EPA and county health guidance 45 |
| Furnace or HVAC filter | Filters air throughout a home; rated by MERV, not HEPA 1 | MERV measures capture of particles from 0.3 to 10 micrometers 1 |
HEPA and MERV are different scales
EPA says MERV ratings measure how well a filter captures particles from 0.3 to 10 micrometers and are most useful for comparing furnace and HVAC filters. HEPA is a rating of at least 99.97% at 0.3 microns 1.
Example: what 99.97% means in counts
The arithmetic in this HEPA air-change example is ours. It converts the printed ratings into particle counts using a round example of one million particles entering the filter.
The ratings are the 99.97% at 0.3 micrometers in EPA’s and DOE’s definitions and DOE’s 99.9995% at 0.1 micrometer for the ultra-low penetration (ULPA) filter, which it describes as a filter of identical construction with a better medium 12.
These are test conditions, not what a particular machine will do in a house.
| Filter | Rated removal | Particles passing per million entering |
|---|---|---|
| HEPA, at 0.3 micrometers | 99.97% | 1,000,000 × 0.0003 = 300 |
| ULPA, at 0.1 micrometer | 99.9995% | 1,000,000 × 0.000005 = 5 |
| A hypothetical 80% filter | 80% | 1,000,000 × 0.20 = 200,000 |
The comparison shows why the number is high but not perfect. A filter rated at 99.97% still passes a few hundred particles per million at its hardest test size, and the 80% example in the third row is only the illustration the manufacturer’s CADR diagram uses, not a rating of any real product 6.
Real air also carries gases, which a particle filter does not touch 3.
Where you see HEPA on a restoration job
On an air scrubber. The label says HEPA, with a pre-filter and optional carbon stage 6.
On a negative air machine. The machine filters air before it is exhausted from a sealed work area.
On a vacuum. NIH recommends HEPA vacuums for final cleanup of remediation areas and for dust settled outside them, and says filters must be properly seated so all air passes through them 8.
In an estimate. HEPA equipment appears by unit and by day. See the cost guide for how equipment lines are built, and the mold remediation page for where filtration fits in that work.
Ask for the airflow behind the label
A “HEPA” label is the start of the question. Ask what airflow the machine runs at on your job and whether the filter was tested at that rate.
What HEPA filtration does well, and what it does not
What it does well
What it does not do
- Remove gases and odors. A carbon stage addresses odors, and EPA says no widely used rating exists for gases 3.
- Work when turned off 5.
- Remove the cause of mold. EPA says air cleaners do not address the cause of mold and musty odors 3.
- Guarantee the same performance on every job. The manufacturer’s guide says lab and jobsite performance differ 6.
Limits of HEPA filtration
The HEPA rating is “theoretical.” EPA’s own wording says HEPA filters can theoretically remove at least 99.97% of particles at 0.3 microns 1. The test fixes the particle size and the rate at which air passes through.
Airflow changes the answer. A manufacturer’s guide explains that IEST-RP-CC021.1 tests the media at a set speed of 5.3 centimeters per second, while the whole-filter test leaves the rate open to interpretation.
The guide says some air scrubber filters are tested at flow rates equal to as low as 150 to 200 CFM, while the units may process 500 to 2,000 CFM in real use, so a filter that passes a lab test may not necessarily be HEPA at job airflow 6.
That is the maker’s view. The IEST documents themselves are purchased and we did not read them.
Damage ruins it. The guide says a filter damaged in shipping or handling suffers a permanent loss of efficiency, because torn media leaves gaps 6.
It does not handle gases, and it does nothing when switched off 35. King County also cautions that extra features such as ionization, UV lights and oil diffusers can cause unintended health hazards unless third-party verified 5.
It does not stop moisture. The cause of mold and musty odors is a moisture source, and the fix is to remove it and clean up 3.
What drives HEPA equipment cost, and how long filters last
We publish no prices and none of the sources we loaded gives one. The cost drivers are the number of units, the days they run, filter changes and any ducting for containment.
Filters are the recurring cost. The manufacturer’s guide says to replace pre-filters and the primary filter after every job and treat filter cost as part of the cost of restoration.
It says primary filters should also be changed during a job when pressure builds and performance suffers 6. A sizing sheet from the same maker says not to clean filters 7.
King County advises a replacement schedule from the manual and a record of dates 5.
A household unit’s running cost depends on its fan speed and filters. King County notes that lower speeds or auto mode can deliver less than the listed CADR, so more units may be needed, while a higher-CADR unit can run quietly at low speed 5. Use the estimate checker to review an equipment line.
How HEPA equipment is used, and what you can do
On a restoration job, a company runs the HEPA equipment. For a household unit, the steps are yours. No permit is normally needed for a portable unit.
- Choose a unit with a CADR matched to the room. King County suggests 65 CADR per 100 square feet 5.
- Place it with the intake and outlet clear of walls, furniture and curtains 5.
- Run it with doors and windows closed during high outdoor pollution, such as wildfire smoke 5.
- Do not rely on lower speeds for the listed CADR 5.
- When changing filters, wear an N95 or KN95 mask, unplug the unit and remove dirty filters outdoors or in a well-ventilated area 5.
- On a restoration job, ask what the machine is for and what is being done about moisture.
Care of HEPA filters and units
- Follow the manual. King County says to check the user manual for filter-change timing 5.
- Clean the unit and pre-filter regularly. Use a HEPA vacuum if possible, or a damp cloth 5.
- Replace loaded filters. Pressure drop rises as a filter loads and airflow falls 6.
- Do not clean a HEPA filter. A manufacturer’s sheet says to replace filters when loaded or after use in any area with cross-contamination risk 7.
- Seat vacuum filters properly. NIH says the filter must be seated so all air passes through it, and the contents disposed of in well-sealed plastic bags 8.
Warning signs a HEPA system is not working
- Weak airflow from the outlet. A loaded filter raises pressure drop and cuts airflow 6.
- A strange smell from the unit. Dirty or wet filters can harbor microbes; the guide notes fungal spores can grow through filters over time 6.
- Torn or crushed filter media. Damage leaves gaps 6.
- A musty odor that returns. The moisture source is still there 3.
- Add-on features like ionizers or ozone. EPA says to avoid air cleaners that intentionally produce ozone 3.
A filter does not make a contaminated space safe
Never enter a flooded or fire-damaged building before it has been judged safe. Handle used filters with protection, since they hold what they caught, and let professionals contain and clean mold, sewage or fire residue. One test cannot certify a building mold-free.
Where the HEPA definition comes from
A handbook definition. The DOE Nuclear Air Cleaning Handbook (DOE-HDBK-1169-2003, Chapter 3) defines a HEPA filter by efficiency, resistance and casing, and references IEST-RP-CC001.3 performance types 2. It is a DOE handbook for nuclear air cleaning, not a rule for homes.
Industry practice. The manufacturer’s guide says, though not regulated by any government agency, HEPA filter makers conform to recommended procedures set by the Institute of Environmental Sciences and Technology, and that IEST-RP-CC021.1 and IEST-RP-CC001.3 are the most cited 6. Those documents are purchased.
EPA guidance. EPA describes HEPA filters and the MERV scale in its public pages and advises against ozone-producing cleaners 13.
Restoration standards. IICRC standards for water and mold work are purchased and voluntary, and this page does not say what they require about HEPA 10.
Your contract. Equipment language varies by company.
HEPA compared with related filter terms
| Term | What it means | How it differs from HEPA |
|---|---|---|
| HEPA | At least 99.97% at 0.3 micrometers under test 2 | The baseline |
| ULPA | A filter of identical construction with a medium retaining 99.9995% at 0.1 micrometer 2 | A higher rating at a smaller size |
| MERV | Measures capture of 0.3 to 10 micrometer particles for HVAC filters 1 | A different scale; MERV is not HEPA |
| Activated carbon | Adsorbs gas- and vapor-phase molecules such as odors 6 | For gases and odors; HEPA is for particles |
| Air mover | Pushes air across wet surfaces 8 | Moves air; filters nothing |
HEPA filter
- Catches particles
- Rated at 99.97% at 0.3 microns
- Mold spores, dust, pollen
- Does not remove gases
Activated carbon
- Adsorbs gas and vapor molecules
- Targets odors and fumes
- Life depends on type and concentration of odor
- Not a particle filter
Common myths about HEPA filtration
- Myth HEPA captures 99.97% of everything.
- Reality The rating is for 0.3-micron particles under test, and the filter catches larger and smaller particles at higher rates 1.
- Myth HEPA removes odors and fumes.
- Reality It filters particles. Carbon is the usual choice for gases, and no widely used rating exists for gas removal 3.
- Myth A HEPA machine fixes mold.
- Reality EPA says air cleaners do not address the cause of mold and musty odors 3.
- Myth Every “HEPA” label performs the same on the job.
- Reality A manufacturer’s guide says laboratory and jobsite performance can differ with airflow 6.
- Myth More features always mean cleaner air.
- Reality King County warns that ionization, UV lights and oil diffusers can cause unintended hazards unless third-party verified 5.
Smoke seasons and humid climates change how HEPA is used
The regional variable is outdoor air. King County advises closing windows and doors while a HEPA cleaner runs during high outdoor pollution, such as wildfire smoke 5, which makes filtration most useful in regions with a smoke season.
In humid regions, EPA describes outdoor air with a dew point above 70°F in Miami through most of a typical year, so moisture control matters more than filtration for preventing mold there 9.
Ozone-producing air cleaners are not approved for occupied spaces anywhere, per EPA 3.
When HEPA filtration matters, and what to do next
HEPA filtration matters when dust and spores are being stirred up, when someone is sensitive to particles, and during containment work. It matters less for a small clean spill that dries without demolition.
The next step is to ask what the filter is for and what is being done about the moisture source. The moisture control page explains the order that works, and how restoration works gives the usual job sequence.
- If Particles in the air during demolition or drying
- Then HEPA filtration helps; ask about airflow and filter changes
- If Odors or fumes
- Then Ask about a carbon stage; HEPA alone is for particles
- If Mold keeps returning
- Then Find and fix the moisture; a filter cannot
- If Final cleanup of dust on surfaces
- Then A HEPA vacuum is recommended by NIH after drying and removal
Questions about HEPA Filtration
What does HEPA stand for?
HEPA stands for high efficiency particulate air. EPA describes a HEPA filter as a type of pleated mechanical air filter common in portable air cleaners. It can theoretically remove at least 99.97% of airborne particles at 0.3 microns, including dust, pollen, mold and bacteria.
What does 99.97% at 0.3 microns mean?
It means that under test, the filter stops at least 99.97% of particles 0.3 microns across. That size is the most penetrating, so the filter catches larger and smaller particles at higher rates. For every million particles of that size entering, about 300 would pass. The rating applies to test conditions, not every real job.
Does a HEPA filter remove mold spores?
It can capture airborne mold spores, which EPA lists among the particles it filters. It does not stop mold from growing. EPA says air cleaners do not address the cause of mold and musty odors, so the moisture source still has to be fixed and the mold cleaned up.
Does HEPA remove odors or smoke smell?
Not by itself. HEPA captures particles, including smoke particles. Many odors are gases, which need a carbon filter, and EPA notes that no widely used rating systems exist for portable cleaners designed to remove gases. An odor that keeps coming back can mean the source is still present.
Is “true HEPA” different from HEPA?
The terms are used loosely. County health guidance describes a true HEPA filter as at least 99.97% effective at filtering particles as small as 0.3 microns.
A manufacturer’s guide says no government agency regulates the label and that real-world performance can differ from a lab test, so ask about the airflow the filter was tested at.
What is the difference between HEPA and MERV?
MERV is a scale used mainly for furnace and HVAC filters that measures how well a filter captures particles from 0.3 to 10 micrometers. HEPA is a specific rating of at least 99.97% at 0.3 microns. A high MERV filter is not necessarily HEPA, and the two scales are not interchangeable.
What is a HEPA vacuum used for in restoration?
NIH’s procedure recommends HEPA vacuums for final cleanup of remediation areas after materials have been thoroughly dried and contaminated materials removed, and for dust that settled on surfaces outside the area. The filter must be seated so all air passes through it, and its contents disposed of in sealed plastic bags.
When should a HEPA filter be replaced?
Follow the manual. A restoration manufacturer advises replacing pre-filters and the primary filter after every job and during a job when airflow drops, and says not to clean the filter. King County advises checking the user manual and keeping a schedule of replacements, wearing a mask while changing filters.
Do HEPA filters work when the unit is off?
No. King County’s guidance states a HEPA cleaner is not effective when turned off. Lower fan speeds also deliver less than the listed clean air delivery rate, so you may need a stronger unit or more than one. Keep the intake and outlet clear of furniture and curtains.
Are HEPA air cleaners with ionizers or ozone safe?
Be careful. EPA says to avoid portable air cleaners that intentionally produce ozone and that no federal agency has approved them for occupied spaces.
King County warns that features such as ionization, UV lights and oil diffusers can cause unintended health hazards unless third-party verified. A plain HEPA filter without those features is the safer choice.
Sources
- U.S. EPA, What is a HEPA Filter?, retrieved .
- U.S. DOE, DOE-HDBK-1169-2003 Nuclear Air Cleaning Handbook, Chapter 3: Filters for the Nuclear Industry, retrieved .
- U.S. EPA, Guide to Air Cleaners in the Home, 2nd edition (archived snapshot of epa.gov, 19 January 2025), retrieved .
- U.S. EPA, Air Cleaners and Air Filters in the Home (page updated August 3, 2026), retrieved .
- King County Public Health, HEPA air cleaners (indoor air quality and mold), retrieved .
- Legend Brands (Dri-Eaz), Guide to Air Scrubbing, retrieved .
- Legend Brands (Dri-Eaz), Air Scrubber Selection and Sizing Guide (HEPA 700 specification sheet, 2022), retrieved .
- National Institutes of Health, Moisture and Mold Remediation Standard Operating Procedures (2023), retrieved .
- U.S. EPA, Moisture Control Guidance for Building Design, Construction and Maintenance (EPA 402-F-13053, December 2013), retrieved .
- IICRC, ANSI/IICRC S500 Standard for Professional Water Damage Restoration (standard page), retrieved .
Expert review
Written by the HyreRestore Research Desk. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: The 99.97% at 0.3 micron definition was read from EPA’s What is a HEPA Filter page and the DOE Nuclear Air Cleaning Handbook, Chapter 3 (DOE-HDBK-1169-2003), which also gives the resistance and casing criteria and the ULPA description.
Household sizing and maintenance come from King County Public Health. Statements about IEST test rates and jobsite performance come from a Dri-Eaz guide, a manufacturer document; the IEST documents themselves are purchased and were not read.
The particle counts are the Research Desk’s arithmetic on example inputs. No prices and no durations are given.
Suggest a correction. We fix errors and say what changed.