Understanding Air Filter Test Methods: ASHRAE, EN and ISO Compared

When you compare air filters, the rating on the label only tells part of the story. Different standards use different test dusts, particle sizes and reporting methods, so two filters that look similar on paper may perform differently in service.

For facility managers, mechanical contractors and technical buyers, understanding the test method is just as important as understanding the rating itself. It helps you choose filters that suit the system, the site conditions and the maintenance approach you need.

Why air filter test methods matter

Air filters are tested in laboratories so their performance can be compared in a controlled way. The challenge is that not all laboratories use the same method.

A filter rated under one standard may be assessed using a different dust loading method, different particle size ranges or a different way of calculating efficiency than another standard. That means the numbers can look familiar while the underlying test is not.

In practice, this matters when you are:

• •matching a filter to an HVAC system designed under a particular standard
• •comparing products from different suppliers
• •checking whether a replacement filter is suitable for an existing housing
• •specifying filters for new builds, refurbishments or maintenance contracts
• •working to site requirements linked to AS 1668.2:2024 or the building owner’s specification

If the standard, test method or filter class is unclear, it is worth confirming details with the mechanical services engineer or building owner before ordering.

ASHRAE ratings: common in North American specifications

ASHRAE-based ratings are widely used in specifications that reference the American testing approach, especially where MERV is listed.

What ASHRAE testing focuses on

ASHRAE methods generally assess how well a filter removes particles across defined size ranges. The result is usually presented as a MERV value, which gives a broad indication of performance across those particle sizes.

That makes MERV useful for comparing filters within the same rating system. It is less useful when trying to compare directly with EN or ISO results, because the test dust, particle classification and calculation method are not the same.

What to watch for

• •A higher MERV rating does not automatically mean the filter is the right fit for your system.
• •Pressure drop still matters, especially where fan capacity or energy use is a concern.
• •The housing, gasket arrangement and sealing method can affect real-world performance.
• •If you are replacing an existing filter, confirm the dimensions and frame style using the manufacturer instructions or by how to measure an air filter.

EN filter classes: widely used in Australia

European test methods have long been common in Australia, particularly the older EN 779 classes such as G3/G4 and F5-F9. These classes are still widely referenced in existing systems, even though they have been superseded by ISO 16890 for many new specifications.

How EN 779 differs from other methods

EN 779 grouped filters into coarse and fine classes based on test results using laboratory dust. The classes were useful for specifying products in HVAC systems, but they do not convert directly to MERV or ISO 16890 ratings.

That point is important because a filter class name alone does not tell you everything about its performance in a current Australian site. A legacy specification might call for an EN class that no longer maps neatly to modern documentation.

HEPA under EN 1822

Where higher-efficiency filtration is required, HEPA filters are tested differently again under EN 1822.

Under that standard:

• •H13 is at least 99.95% efficient at the most penetrating particle size (MPPS)
• •H14 is at least 99.995% efficient at the MPPS

These are laboratory efficiencies at a specified test point, not a blanket statement about every particle size or every installation. Real performance still depends on correct installation, sealing and system design.

ISO 16890: the current international approach

ISO 16890 is now the main international standard used to classify general ventilation filters. It was introduced to give a more practical picture of how a filter performs against different airborne particle size groups.

What ISO 16890 measures

Instead of only assigning a single class based on one type of dust loading result, ISO 16890 sorts filters according to their efficiency for particle size groups commonly found in indoor and outdoor air.

You will often see classifications such as:

• •ePM1
• •ePM2.5
• •ePM10
• •ISO coarse

That structure helps buyers understand what part of the particulate range a filter is most effective against, while still recognising that laboratory results are only one part of the selection process.

Why ISO 16890 is useful

For technical buyers, ISO 16890 often makes specification easier because it is aligned more closely with particle size categories rather than a single legacy class name. That said, the number on the label still needs to be considered alongside:

• •system airflow and allowable pressure drop
• •the dust load at the site
• •whether the filter is used as pre-filtration or final filtration
• •the physical dimensions and frame type
• •maintenance access and replacement practicality

Why the ratings do not convert directly

It is tempting to look for a simple conversion between MERV, EN 779 and ISO 16890. In reality, there is no reliable one-to-one conversion because each standard tests filters differently.

The differences include:

• •the dust used in the laboratory test
• •whether the filter is tested by average efficiency, particle count or other criteria
• •the size ranges measured and how they are grouped
• •how dust loading affects the result over the life of the test
• •the final format used to report performance

That is why a conversion table can be misleading. Two filters that appear similar on paper may behave differently once installed, especially if the system has variable airflow, limited space or a high dust load.

Choosing the right filter for your site

A good filter selection starts with the system, not the rating alone. The right answer depends on the application, the casing, the fan capability and the site’s maintenance strategy.

Practical points to check

• •System design: Confirm what the equipment manufacturer allows.
• •Site requirements: Check the building specification and any relevant ventilation requirements.
• •Dimensions: Measure carefully before ordering; small differences can cause leakage or installation problems. Our guide on how to measure an air filter can help.
• •Pressure drop: Monitor it where gauges are fitted and replace filters according to the manufacturer instructions and site conditions.
• •Dust load: Construction sites, workshops and coastal locations may need closer inspection.
• •Washability: Some filters are designed to be washable - check the product instructions before cleaning or reusing.

If you are unsure whether a filter should be a coarse pre-filter, a general ventilation filter or a higher-efficiency final filter, it is usually better to check the application first rather than simply choosing the highest rating available.

Getting the specification right

For existing plant, the most common problems happen when the replacement filter matches the label but not the installation.

Watch for:

• •an incorrect depth or face size
• •a frame style that does not seal properly in the holding frame
• •a filter class that creates too much resistance for the fan
• •a legacy specification that still refers to EN 779 without clarifying the intended performance under ISO 16890
• •a high-efficiency filter being used where the system was never designed for that pressure drop

If you need replacements, modifications or a custom size, it helps to work with a supplier that understands both the standard and the system. You can review our products or ask about custom air filters if the size or construction is not standard.

Talk to Total Air Filtration

Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, established in 2009. If you need help comparing ASHRAE, EN or ISO-rated filters, we can help you choose or make the right filter for your application.

Call 07 3800 3448 or request a quote. You can also contact us or explore filter servicing and commercial filter servicing if you need support beyond supply.