Breathe Naturally - January 2, 2024

The Role of Pre-Filters in Air Purifiers


Key Takeways:

  • Pre-filters are the first line of defense in an air purifier, catching large particles like hair and dust before they reach the more delicate filters.
  • They help prolong the life and performance of the other filters like HEPA and activated carbon.
  • Pre-filters should be checked and changed frequently, about every 1-3 months depending on use.
  • Common types of pre-filters include mesh and foam filters. High efficiency pleated filters can also be used.
  • Pre-filters improve air quality by trapping large particles, but do not capture small particulates like pollutants and allergens.

Air purifiers have become increasingly popular in homes and offices as a way to improve indoor air quality. Most air purifiers use a multi-stage filtration system, with the first stage being the pre-filter. As the name suggests, pre-filters are designed to filter out large airborne particles before the air reaches the more delicate components of the air purifying system. Pre-filters play an important role in extending the life and performance of the other filters, saving money and hassle by decreasing the frequency at which they need to be replaced.

What Are Pre-Filters?

Pre-filters, sometimes referred to as coarse filters, are designed to trap large particles like dust, dirt, hair and dander. They usually consist of a fibrous mat or mesh that captures particles through interception and impaction. Pre-filters are meant to handle the bulk of large debris in the air, acting as the first line of defense before air reaches higher efficiency filters like HEPA (high efficiency particulate air) filters or activated carbon. This protects the performance of the downstream filters and prevents them from getting prematurely clogged with large particles.

A higher quality of air for the ones you love

Common materials used for pre-filters include:

• Polyester - Provides good filtration at low cost
• Fiberglass - Durable material with reasonable efficiency for the price
• Foam - Can trap large particles; some models are washable and reusable
• Mesh screens - Allows high airflow while catching large debris
• Pleated filters - Offer more surface area for higher particle capture than flat filters

The Role of Pre-Filters

Pre-filters have three main roles in an air purifying system:

1. Protect the main filters - By trapping hair, dust and other large particles, pre-filters help maintain airflow through the system and prevent clogging of the HEPA, activated carbon and other filters. This extends the usable lifespan of the more expensive filters.

2. Improve air quality - Pre-filters do capture many large airborne particles that can irritate allergies and asthma. While they allow smaller particles like smoke and pollen to pass through, pre-filters still contribute to cleaning indoor air.

3. Increase cost-effectiveness - Rather than change the entire filter set, you only need to periodically replace the pre-filter, saving money. They allow the other filters to work at their full potential longer before a change is needed.

Pre-filters are an important component, but do have limitations. They do not remove gases or odors, nor do they capture fine particles like smoke, bacteria or pollutants. Upgrading to a high efficiency pleated pre-filter can provide better filtration, though the other filters are still needed to purify air to the highest standards.


Checking and Replacing Pre-Filters

Pre-filters should be checked frequently, about once a month, to see if they need to be changed. With regular use, most pre-filters will need replacement every 1-3 months. Signs it's time for a new pre-filter include:

  • Increased noise when the purifier is running - This indicates restricted airflow due to a clogged filter.
  • Visible dirt and debris - If you can see a thick layer of hair and dust, the pre-filter is no longer trapping particles efficiently.
  • Reduced airflow - Measure the air output from the vents to see if it has decreased, signaling filter restriction.
  • Odors - If smells are starting to pass through the pre-filter, it needs to be replaced.

Follow the manufacturer's instructions for removing and replacing pre-filters. Make sure the arrows indicating airflow direction on the filter are pointing the right way. Position the filter securely in its housing and run the air purifier to check for increased airflow, noise reduction and proper operation.

The Bottom Line

Equipping an air purifier with a pre-filter extends the life of the more expensive HEPA and carbon filters. Pre-filters remove large airborne particles, acting as the first defense against indoor irritants like dust and pet dander. To maintain performance, they should be checked about once a month and changed every 1-3 months depending on use.

While they improve air quality by trapping debris, pre-filters do not eliminate small particles, gases and odors like higher grade filters. Overall, pre-filters are an essential component of an effective multi-stage air purification system.


United States Environmental Protection Agency. "Introduction to Indoor Air Quality." Accessed November 20, 2023. "Air Cleaning." Accessed November 20, 2023.

Centers for Disease Control and Prevention. "Air Cleaners, HVAC Filters, and Coronavirus (COVID-19)." Last modified February 23, 2021.