Airborne pollutants and contaminants come from a variety of sources. Although they may be in low concentrations, they can exhibit harmful and toxic properties.

In chemical and industrial facilities, laboratories and other places where people gather or work, airborne contaminants can be present in the atmosphere. In such environments, personnel wear respirators and/or breathing apparatus may be required to protect health and wellbeing.

In potentially more hazardous environments such as terrorist incidents, conflicts, and emergencies requiring first responders or CBRN defence, respirators are worn for personnel safety and protection against any toxic gases and vapours that may have been released into the atmosphere.

Chemviron’s long-established expertise in developing new impregnated carbon techniques and products has enabled military and first responders to meet new challenges in respirator protection.

Respirators, Airborne Contaminants and Activated Carbon

The main role of a respirator is to protect the wearer from inhalation of harmful gases or vapours. They remove the airborne contaminants, thus purifying it and making it safe and clean to breathe.

Such air-purifying respirators, called respiratory protective equipment (RPE), come in many forms. Many include tight-fitting facemasks which incorporate replaceable cartridges or canisters. These small filters contain a bed of specialised activated carbon (historically often referred to as activated charcoal.)

How do Respirator Carbon filters work?

Respirator activated carbon filters are designed to remove harmful gases or vapours from the air through physical and chemical adsorption processes.

Physical adsorption is where the airborne contaminant is fixed in the pore structure of activated carbon by attractive Van der Waals forces. The strength of these attractive forces is dependent on the nature of the contaminant. Many organic or Volatile Organic Compounds (VOCs) contaminants are readily removed through physical adsorption.

Certain inorganic gases, such as ammonia (NH₃) and sulphur dioxide (SO₂), however, are not significantly attracted to the activated carbon’s pore structure so an additional chemical adsorption process is required. This involves impregnation of the high-activity activated carbon base with specialised chemical solutions.

Chemical adsorption is a chemical reaction in the activated carbon pore structure between the impregnant compound and the airborne contaminant. This reaction changes the properties of the contaminant and fixes the resulting compound in the activated carbon structure.

The quality and properties of the activated carbon feedstock used for the impregnation process, typically in granular or pellet forms, are essential to realising the finished product’s performance and quality. A consistent and high-quality production process is then critical to ensure that the impregnant is finely dispersed throughout the activated carbon to balance both physical and chemical adsorption mechanisms.

The chemical impregnation process to create a particular impregnated carbon will vary according to the requirements of the adsorption challenge. Given the critical nature of respirator applications, the impregnation technique applied in each case, has to be a fine balance between the impregnant type required, the impregnant amount and the nature and form of the base carbon feedstock.  

Given that many respirators often have both organic and inorganic gas challenges, it is important to factor in the loss of physical adsorption capacity due to the addition of impregnant versus the enhanced chemisorption gained from the impregnation process.  As an impregnated activated carbon is capable of both physical and chemical adsorption, that functionality makes it ideal for respirator and filter applications.

Certain compounds such as short-chain aldehydes can benefit from physical and chemical adsorption processes. In these cases, additional attention is required to carefully select chemical impregnants with consideration given to their effect on the carbon’s physical adsorption characteristics.

Respirator Filter Standards

For personnel protection, industrial respirators and hygiene applications, the British and European Standard BS EN 14387 is the most common standard covering the performance criteria for respirator filters.

The filter types covered by this standard are as follows:

Type A: For use against certain organic gases/vapours with a boiling point >65^oC as specified by the respirator manufacturer. 

Type B: For use against certain inorganic gases and vapours as specified by the manufacturer.

Type E: For use against sulphur dioxide and other acidic gases and vapours as specified by the manufacturer.

Type K: For use against ammonia and organic ammonia derivatives as specified by the manufacturer.

Type AX: For use against certain organic gases/vapours with a boiling point < 65^oC as specified by the manufacturer. 

Type SX: For use against specific named gases and vapours as specified by the manufacturer.

Special Filter Type NOP3: for use against nitrogen oxides e.g., NO, NO₂, NO_X.

Special Filter Type HgP3: for use against mercury.

Multi-type gas filters e.g., ABEK Filters are a combination of two or more of the above types excluding type SX and which meet the requirements of each type separately.

In addition to types, filters are also defined by Class as follows:

Class 1: Low-Capacity Filters

Class 2: Medium-Capacity Filters

Class 3: High-Capacity Filters

In more challenging situations i.e., for first responders and CBRN filters, more demanding standards are required. An activated carbon filter usually has increasingly challenging adsorption requirements and often specific gas performance capabilities may apply. These may be country, EU, NATO or military-specific, such as MIL-SPEC or NIOSH CBRN.  Such standards may apply to filters for respirator gas masks, vehicles, ships, buildings, and shelters with filters for collective protection.  

This is a very specialised application, so please call the experts at Chemviron for help and support.

Note that respirator and gas mask filters are smaller, usually thin bed filters using a finer mesh carbon and filters for vehicles or buildings are deeper bed filters using a coarser bed carbon for improved pressure drop purposes.

In addition, for many vehicles, both passenger and goods, carbon filter protection as a cabin air filter is increasingly used to protect the passengers and driver from unwanted odours and harmful chemicals. These include VOCs and gases such as SO₂ and difficult-to-remove chemicals such as formaldehyde, H₂S, and ammonia. 

What Contaminants Can Be Removed Using Activated Carbon?

The list of contaminants – other than typical VOCs – that can be effectively removed in respirator filters using such specialised impregnated activated carbons includes the following:

Acetaldehyde	Chloroacetophenone	Nitrogen Oxides – NO, NO2, NOx
Acid Gas	Chloropicrin (PS)	Organic Vapour
Acrylonitrile	Choking Agents	Ozone
Agent VX	Cyanogen	Pepper Spray (OC)
Aldehydes	Cyanogen Chloride (CK)	Phosgene (CG)
Ammonia	Cyclohexane	Phosphine
Arsine (SA)	Dimethyl methyl phosphonate (DMMP)	Riot gas (CS)
Basic Gas	Formaldehyde	Sarin (GB)
Blister Agent	Hydrogen Chloride (HCl)	Soman (GD)
Blood Agent	Hydrogen Cyanide (AC)	Sulphur Dioxide
Carbon Disulphide	Hydrogen Sulphide	Tabun (GA)
Carbonyl Sulphide	Mercury vapour	Tear Gas (CN)
Carbon Tetrachloride	Methylamine	Toxic Industrial Chemicals (TICs)
CBRN Agent	Mustard Gas (HD)	Toxic Industrial Materials (TIMs)
Chemical Warfare Agents (CWA)	Nerve Gases	VOCs
Chlorine	NIOSH Gas

Note: The designation indicated in brackets after the gas description is the military reference to this contaminant, often referred to in standards and specifications.

How can we help?

The most effective carbon to be used will depend on the application where the carbon is to be used, the filter type, and the gas performance required. It is important to advise if specific contaminants or standards are applicable.

As a European producer of such specialised impregnated activated carbons for over eighty years, Chemviron can provide experienced advice and help. In addition, the Chemviron Cloth Division UK produces FLEXSORB® activated carbon cloth for a range of speciality applications. This includes impregnated carbon cloths for defence and other air filtration applications

So, if you need help improving your filter performance, the choice of activated carbon, or for further advice, contact our team.

For further information on VOCs and other air pollutants, see our related articles on ‘’What are Volatile Organic Compounds’’ & “What are Air Pollutants’’.