ISO 10121-3:2022
The first classification system for molecular filters in general ventilation applications

ISO 10121-3:2022

Created Tuesday, November 8, 2022

What is molecular filtration?

Molecules are typically 1,000 to 10,000 times smaller than the most penetrating particles that pass through HEPA and ULPA filters. Typical hazardous gases in outdoor air are volatile organic compounds (VOCs), sulphur dioxides (SO2), nitrogen dioxide (NO2) and ozone (O3).

A cost-effective method of controlling harmful gaseous pollutants is molecular filtration. It can be used both in outdoor air systems (also known as make-up air systems) to control pollutants and in recirculation systems to control internally generated pollutants.

All Camfil molecular filters utilise a technique known as adsorption. In simple terms, this means that the molecules adhere to materials with an extremely large surface area. Almost all Camfil molecular filters use activated carbon or activated alumina as the active ingredient. Molecular filters are sometimes also called chemical filters or gas phase filters.

What is ISO 10121?

The series of standards ISO 10121 provides test methods for determining the filtration efficiencies of molecular filter media (ISO 10121-1) and molecular air filters (ISO 10121-2) against various gases.

ISO 10121-3 is the first classification system for molecular air filters supplying outdoor air for general ventilation systems. It contains comprehensive filter classes for the most common air pollutants in outdoor and indoor air. This facilitates the selection of the correct molecular filter, depending on the local air quality.

Why ISO 10121-3?

A number of studies have shown that hazardous gases, which are often present in air pollutants, can be associated with numerous negative health effects.

Ozone (O3) is formed in our atmosphere by the interaction of UV light with gases produced by various combustion processes. Ozone poses a respiratory hazard. The air quality guidelines by WHO set a maximum mean exposure concentration of 60 μg/m3 for a period of 8 hours during peak season.

Nitrogen dioxide (NO2) is formed as a direct result of combustion processes. NO2 is not only responsible for haze and acid rain, but is also harmful to our lungs, aggravating asthma symptoms and increasing susceptibility to infections. The air quality guidelines by WHO set a maximum mean annual exposure concentration of 10 μg/m3.

Most sulphur dioxide (SO2) is released from the burning of fossil fuels in power plants and industrial processes. Volcanoes are another known source of SO2. The health effects of SO2 are similar to those of O3 and NO2. The WHO air quality guidelines set a maximum daily exposure concentration of 40 μg/m3.

Toluene (C7H8) is an organic molecule used by the standard to represent the very large group of Volatile Organic Compounds (VOC). The number of sources of VOC is endless and can be found both indoors and outdoors: These include solvents, paints, building materials, combustion processes, oil and gas, etc. Due to the diversity of their chemical properties, the effects of VOC can range from an unpleasant but harmless odour to a lethal effect when inhaled or causing long-term effects such as cancer.

With the clear and easy-to-understand filter classes from ISO 10121-3, it is now quick and easy to select the right molecular air filter for a particular supply air application based on how efficient molecular filters are at removing the four gases. The selection is similar to the selection of a suitable particle filter according to ISO 16890.

What does the classification mean?

POLLUTANT DN(moles per unit GPACD face area)
DN(grams per unit GPACD face area)
Ozone 1,5 6,0 24,0 72 288 1152
SO2 1,5 6,0 24,0 96 384 1538
NO2 1,5 6,0 24,0 69 276 1104
Toluene 1,5 6,0 24,0 138 553 2211

Doses LD, MD, HD predict the lifetime of the filter
(vLD (very Light Duty) are filters that do not qualify as LD)

  • LD (Light Duty) = relatively short lifetime / low capacity
  • MD (Medium Duty) = 4 times higher lifetime* / capacity
  • HD (Heavy Duty) = 16 times higher lifetime* / capacity

* compared to LD

%-value indicates the average efficiency

  • LD 60 = 60% average efficiency over short lifetime.
  • MD 60 = 60% average efficiency over medium lifetime.
  • HD 60 = 60% average efficiency over very long lifetime.

Graph: Product performance examples for toluene


A GPACD (gas phase air cleaning devices ) can be tested with all 4 reference gases.

  1. Measurement of the initial efficiency for the respective gas
  2. Measurement of the efficiency versus dose for the respective gas
  3. As soon as the efficiency falls below 50%, the test is stopped
  4. Classification for each reference gas in classes from Light Duty (LD) to Heavy Duty (HD)
  5. GPACDs that cannot qualify as Light Duty (LD) are classified as very Light Duty (vLD)
  6. Calculation of the integral efficiency (rounded down to 5% increments)
  7. Each GPACD has 4 classes in total (one class per reference gas)

Table: ISO 10121-3 classes for selected "City" family products

City-Flo XL
520 mm
CityPleat 200
44 mm
534 mm
292 mm
Reference gas
Ozone vLD 20 LD 55 HD 85 HD 80
SO2 vLD 10 vLD 30 MD 55 MD 50
NO2 vLD 20 vLD 50 LD 85 LD 70
Toluene vLD 30 LD 75 MD 80 MD 80

Can we remove it?