Using an Air Monitor to Determine When to Replace a HEPA Filter

It can be difficult to tell when to replace a HEPA filter. Should one go by looks? Probably not. Going by looks turns out to be a poor measure of filter efficiency. The best way to test a HEPA filter’s efficiency and determine when to replace it is to use an air quality monitor.

Why Use an Air Quality Monitor?

Air purifier companies give recommendations on how often to replace their HEPA filters. The problem is these recommendations are rarely data-backed and seemed to be pulled out of the sky. Furthermore, how long a HEPA will last depends on many factors such as outdoor air quality levels and how often the purifier is used.

In addition, it is difficult to gauge the effectiveness of a HEPA simply by looks. Below are three HEPA filters from our 200-day test of the original DIY HEPA. On day 90, the HEPA looks quite dirty, but the data shows it had only lost around 4% effectiveness at that point. On day 200, the effectiveness had dropped significantly by 50%.

HEPA Lifespan Test Filter Air Purifier

Instead of going by looks or relying on recommendations from the manufacturer whom may have incentives for customers to replace the filters more often, using an air quality monitor gives you real-time data on the state of your HEPA filter.

Read More: The Issue with the HEPA Filter Replacement Recommendations from Air Purifier Companies

What Air Quality Monitor Should You Use?

The most dangerous particles in our air are typically some of the smallest particles. Particles under 2.5 microns in size, called PM2.5, are tiny dangerous particulates that can enter one’s bloodstream and organs and cause a variety of health problems.

Using an air quality monitor to test for PM2.5

Because of this, a PM2.5 air quality monitor is one of the best tools to ensure the air is safe to breathe and to test the efficiency of a HEPA filter in removing these particles.

Fortunately, an accurate PM2.5 air quality monitor does not need to break the bank. Test data shows the best PM2.5 air quality monitors are also some of the cheapest.

How to use an air quality monitor (Kaiterra Laser Egg)
Smart Air’s Chief Nerd Scientist Paddy with an under $150 Kaiterra Laser Egg

Looking for a PM2.5 air quality monitor? Check out our data-backed article on the best air quality monitors on the market:

Read More: Best PM2.5 Air Quality Monitors in 2021

Two Ways to Use a PM2.5 Air Quality Monitor to Test HEPA Filters

Method one is the simplest method, but comes with its flaws. Although a bit more hands on, method two is the preferred method that allows one to precisely see a HEPA’s reduction in effectiveness over time.

Method 1: Ensure Air Quality Levels Are “Safe”

One method is to use an air quality monitor to periodically test the indoor air quality levels. Once the HEPA filter can not consistently bring air quality levels to “safe” levels, it is time to replace the HEPA filter. The WHO recommended annual limit for PM2.5 is 10 micrograms. Ideally, an air purifier with a new HEPA filter should reduce air quality levels to under this limit. If not, a stronger purifier or multiple purifiers are needed to cover the intended area.

Read More: Four Steps to Choosing a Purifier

Using a Kaiterra Laser Egg to test when to change a HEPA filter.
Monitor showing air quality levels hovering around the upper limit of the WHO recommendations (10 micrograms or 42 AQI)

Once a purifier is unable to reduce levels to below this limit as it can with a new HEPA, this may indicate the filter should be replaced.

Although this is the simplest method to use an air quality monitor to test a HEPA filter, there are a few issues with it. First, recommended limits for PM2.5 are very general. In reality, PM2.5 air pollution is similar to lead, in that there is no “safe” level. Levels below 10 micrograms can also be damaging to one’s health.

Deaths from air pollution.

Read More: Here’s Why the WHO Air Quality Guidelines Are Unsafe

Method 2: Compare With Air Quality Levels Using New Filter

The preferred method for testing an air purifier’s HEPA filter is a little more hands-on. This involves testing the indoor air quality while using a new HEPA for a period of time to get a baseline for what exactly the performance of a new air HEPA filter is. As a HEPA filter is used and its effectiveness decreases, one could compare the indoor air quality levels with this baseline.

For example, below is the data collected for the effectiveness of the original Smart Air DIY filter. As you can see the original “baseline” effectiveness for this filter was reducing particulate levels by ~90%. Of course, collecting this much data is a little overboard, but the idea is to check the filter periodically to see how the effectiveness compares to the baseline.

Using an air quality monitor to test a HEPA filter's longevity

One thing to note is outdoor air quality levels can fluctuate quite a bit, which can influence indoor air quality levels. Therefore it is recommended to bring the air quality monitor outside and check outdoor air quality to ensure the comparison is being made at similar outdoor air quality levels.

At what level decrease in HEPA effectiveness should a HEPA filter be replaced? There is no “tipping point” where the effectiveness of a HEPA nosedives. A good rule of thumb is to change the filter when it becomes obvious the filter can not consistently get air quality levels to what it could when new.

Blast Mini HEPA filter air purifier in office for protection against COVID coronavirus
Smart Air office, using an IQAir AirVisual Pro monitor with our Blast and Blast Mini air purifiers

Summary: Using an Air Quality Monitor to Test a HEPA Filter

Ultimately, it comes down to a trade-off between the cost of replacing a filter and effectiveness. The more often a filter is replaced, the more effective the air purifier can filter the air. But this comes at a cost. With a PM2.5 air quality monitor, one knows exactly how effective their HEPA filter is and can replace their HEPA based on data, not the eye test.


A few caveats to using an air quality monitor to test a HEPA filter include:

  • Ensure outdoor air quality is not significantly higher than usual for your area. All filters perform worse when outdoor air quality is worse.
  • Ensure the air purifier is placed in the best position.
  • If the HEPA has a prefilter, make sure to periodically vacuum the prefilter, otherwise, the dirty prefilter may affect the HEPA’s filtration performance.

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1 thought on “Using an Air Monitor to Determine When to Replace a HEPA Filter”

  1. I believe Smart Air may have used an anemometer to measure air flow when testing air purifiers.

    Could a measure of reduced output, over time from ‘new’ (ideally under similar internal and external pollution levels), be used to guage when the HEPA filter might be replaced?

    I’ve an anemometer on order to compare the relative efficiencies of my AC and DC fans – wattages drawn for various levels of output. Anticipating DC being particularly efficient for low[er] output.
    Power Factor also being taken into account, to minimise stress on the electricity grid.

    I might also compare my existing Ikea HEPA filter against a new replacement – the old having been in operation continually for several months, though largely at my particularly efficient Medium (CADR
    90 specified, using less than 6 watts) setting.

    With usually low pollution levels in Australia, aside from extreme bushfire pollution, a powerful air purifier generally operating more efficiently at lower levels might be appropriate.


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