Can Primary-, Medium-, and High-Efficiency Filters Be Washed?

Washability of Primary-, Medium-, and High-Efficiency Filters

1. High-Efficiency Filters (HEPA/ULPA): Never Wash

Construction and materials: High-efficiency filters normally use glass-fiber paper or polypropylene ultrafine fibers. The media is arranged in tightly spaced pleats to increase filtration area, secured with hot-melt adhesive or sealant, and enclosed in a metal frame made from aluminum or galvanized steel, or in a plastic frame.

Main reasons they cannot be washed:

Media damage: Glass-fiber paper is fragile. Washing with water or blowing with compressed air can break fibers and deform pleats, directly damaging the filtration layer. Efficiency may fall from at least 99.97% to below 90%. An H13 HEPA filter, for example, may deteriorate to medium-efficiency performance after cleaning.

Irreversible increase in resistance: After washing, fibers can stick together and reduce porosity. Airflow resistance may rise by 50%-100%. A HEPA filter with an initial resistance of 200 Pa could increase to 300-400 Pa, sharply raising fan energy use or disrupting the cleanroom pressure balance because of insufficient airflow.

Microbial contamination risk: HEPA filters are frequently used in sterile environments such as pharmaceutical and semiconductor facilities. Washing can introduce secondary contamination, and damp media can become a breeding ground for microorganisms, violating GMP and similar requirements.

2. Medium-Efficiency Filters (F5-F9): Some Are Washable, with Strict Limits

Washable media types:

Synthetic-fiber media, such as polyester nonwoven: This material has good water resistance and can be washed with water or soaked in a neutral detergent at no more than 40 degrees C. Once air-dried, it can recover part of its filtration performance.

Metal mesh and polyurethane-foam composite media: Foam supported by a metal frame can be reverse-cleaned with compressed air at no more than 0.4 MPa or rinsed with a high-pressure water jet, taking care not to tear the foam.

Non-washable media types:

Glass-fiber filter paper, which is used in some medium-efficiency filters: As with HEPA media, washing causes fibers to shed and efficiency to decline.

Electret media: This media captures dust through electrostatic attraction. Washing disrupts the charge distribution and can reduce filtration efficiency from at least 95% at F7 to 80% or less.

3. Primary-Efficiency Filters (G1-G4): Most Are Washable, but the Number of Cleanings Must Be Limited

Common washable materials:

Nylon mesh and polyester nonwoven: These materials resist wear and water. They can be washed in water with laundry detergent at no more than 60 degrees C and then air-dried. They normally should not be reused after more than five wash cycles.

Metal wire mesh, including galvanized-steel mesh: It can be rinsed directly with a high-pressure water jet or soaked in a disinfectant, such as a chlorine-containing solution diluted 1:100. It is suitable for coarse filtration under heavy dust loads, including air-conditioning fresh-air intakes.

Cleaning limits:

The dust-holding capacity of a primary filter can fall by 20%-30% after cleaning. The filter must be replaced after more than five cleanings or whenever the media becomes damaged or hardened.

Washable Filter Types and Maintenance Guidelines

Filter grade | Washable media | Cleaning method | Precautions | Applications

Primary efficiency, G1-G4 | Nylon mesh, polyester nonwoven, or metal wire mesh | 1. Water washing: soak in neutral detergent for 10-15 minutes, rinse with clean water, and air-dry. 2. Compressed-air cleaning: blow backward from the downstream face at no more than 0.4 MPa. | Inspect the media for damage after cleaning and replace rusty metal mesh. Reuse no more than five times, with a total service period no longer than three months; shorten it to one month in dusty environments. | Fresh-air filtration for air conditioning, equipment pre-filtration, and coarse filtration in general workshops

Medium efficiency, F5-F9 | Synthetic-fiber nonwoven, such as polyester or polypropylene, or metal-framed foam media | 1. Water washing: use water at no more than 40 degrees C and detergent at a concentration no greater than 5%; wash gently, then spin off excess water. 2. Chemical disinfection: spray the surface with 75% alcohol for pharmaceutical applications. | Never machine-wash or heat-dry because the media can deform. Efficiency loss after cleaning should not exceed 10%; replace the filter if efficiency falls below 80% of its rated value. | Medium-stage air-conditioning filtration, cleanroom pre-filtration, and intermediate filtration in food workshops

High efficiency, H10-U15 | None; no high-efficiency filter is washable | None | Once a high-efficiency filter is clogged or its resistance exceeds the limit, such as reaching twice its initial resistance, replace the entire filter. Never attempt to clean it. | Class 100 cleanrooms, aseptic workshops, semiconductor production lines, and other critical filtration applications

Key Risks of Cleaning Filters and Alternative Solutions

1. Three Major Risks of Improper Cleaning

Efficiency loss: After a medium-efficiency filter has been cleaned three times, its efficiency for 0.5 micrometer particles may fall from F7, at least 90%, to F5, at least 80%. This increases the load on the downstream HEPA filter and can shorten its life by more than 50%.

Secondary contamination: If media is not completely dry after washing and its relative humidity remains above 60%, mold can begin growing within 24 hours. When contaminated air is blown into a cleanroom, it can affect products, including those on pharmaceutical aseptic filling lines.

System failure: If a cleaned filter is reinstalled incorrectly, for example with a damaged sealing strip, the air-leakage rate can rise above 5%, compared with a normal limit of no more than 1%. This can disrupt the cleanroom pressure gradient and cause cross-contamination.

2. Alternatives: Prefer Replaceable Media or Self-Cleaning Designs

Filters with replaceable media: A modular primary filter, for example, can use a metal frame with a removable filter bag. Replacing only the bag avoids performance loss from washing and costs less; a replacement bag may cost about one-third as much as a complete filter.

Electrostatic self-cleaning filters: These filters collect dust in a high-voltage electrostatic field. Periodic electrical operation releases the dust into a collection box, eliminating the need to remove and wash the filter. They are suitable for public spaces such as shopping malls and office buildings and can provide a maintenance interval of up to six months.

Summary

Never wash: All high-efficiency HEPA and ULPA filters, as well as medium-efficiency filters made with glass fiber or electret media, must be replaced at the end of their service life. Washing can cause filtration failure.

Clean cautiously: Although primary filters and some medium-efficiency filters can be washed, cleaning must follow the requirements of the media and should be limited to no more than five cycles. After cleaning, efficiency and resistance should be tested, preferably with an airborne-particle counter, to confirm that the filter still meets operating requirements.

Preferred approach: In demanding applications such as pharmaceutical and semiconductor facilities, use disposable media with scheduled replacement, or choose self-cleaning filters or filters with replaceable bags to reduce maintenance risk and labor cost.

Accurately distinguishing filter grades and media prevents system failures caused by improper cleaning. It also helps control maintenance costs while maintaining filtration performance and stable cleanliness over the long term.