iso 29461-1:2021
Turbomachinery Air Intake Filter Efficiency Standard

ISO 29461-1:2021 - The first International test standard for turbomachinery filter efficiency & dust holding capacity

ISO 29461-1 is the latest international test standard for reporting the efficiency and dust holding capacity for the entire range of air intake filters for gas turbines, compressors, and other turbomachinery applications.

The ISO 29461-1 presents a uniform method for testing filters that enables turbomachinery operators to more easily compare and select air inlet filters.


Gas turbines and other turbomachinery are sensitive equipment that require protection against harmful particulates that could diminish life and performance. Air inlet filters are the most cost-effective way to protect your assets since they can prevent contaminants from reaching your turbomachinery.

Operators currently use filters with a wide range of particle capture efficiencies, from coarse pre-filters to ultra-fine final filters designed to keep engines clean and running at peak performance. There are currently multiple standards used in the industry to help select filters; most prevalent are the EN779, ASHRAE 52.2, and ISO 16890 for lower efficiency filters and ISO 29463/ EN1822 for higher efficiency filters. The ISO 29461-1 has been designed to replace these four standards and as such is the first filter classification that can be applied to the entire range of filters in a single uniform standard that is more relevant to real-world performance.

The ISO 29461-1 standard offers the following:

All of the existing standards have been developed for other industries leading to compromises when applied to turbomachinery applications. Navigating the standards were also confusing since there was no clear correlation between them. The new ISO 29461-1 eliminates this confusion with a single standard specially designed for turbomachinery applications with one classification system.
It more closely models the efficiency of filters operating in the field by measuring their mechanical efficiency which is not influenced by an electrostatic charge. Electrostatic charged efficiency has little relevance for gas turbine and other turbomachinery applications since the charge dissipates in a short period of time relative to the filter service life.
In order to better compare filter life, ISO 29461-1 now requires a standardized dust loading procedure for all filters. This is the first standard that dust loads (H)EPA filters for filter life comparison.


The ISO 29461-1:2021 classification system shown in the table categorizes filters in groups and classes using test protocols from ISO 16890 and ISO 29463:

  • Classes T1 - T4 are tested per ISO 16890 with arrestance measured on first 100g of dust loading.
  • Class T5 is tested per ISO 16890.
  • Classes T6 - T9 are tested per ISO 16890 where only minimum efficiency is considered.
  • Classes T10 – T13 are tested per ISO 29463.
Class Group MPPS efficiency ePM1, min ePM2,5, min ePM10 Initial gravimetric arrestance A100 
ISO T1 Coarse
    20% < A100 < 50% 
ISO T2         ≥ 50 % 
ISO T3         ≥ 70 %  
ISO T4         ≥ 85 % 
ISO T5 ePM10       ≥ 50 %   
ISO T6 ePM2,5      ≥ 50 %     
ISO T7   ePM1    ≥ 50 %      
ISO T8    ≥ 70 %       
ISO T9    ≥ 85 %       
ISO T10  EPA ≥ 85 %         
ISO T11 ≥ 95 %         
ISO T12 ≥ 99,5 %        
ISO T13  HEPA ≥ 99,95 %        


The following table outlines the ISO 29461-1 rating for Camfil filter categories.
Filter Categories ISO 29461-1
Bag T4 - T9
Panel  T2 - T6
Compact  T7 - T13
Cartridge  T5 - T10, T12
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