Air Purifying Filters/Filtering RPD

Where the risk assessment indicates that a particle filtering Respiratory Protective Device (RPD) is suitable, the selection of the appropriate particle filtering RPD shall be determined by the required Protection level (PL) and work rate (WR). Where the risk assessment indicates that a gas/vapour or combination filtering RPD is suitable, the selection of the appropriate type and class of filter will depend on many workplace factors. These include contaminant(s), their concentration(s), work rate, task duration, temperature and humidity. Users should seek guidance from the gas filter manufacturer about the filter classification to be used and whether the contaminant can be removed by filtration at all.

These are respirators that capture the contaminant/s in the air as it passes through a filter/cartridge before being inhaled. Examples of air purifying respirator that use filters/cartridges to capture contaminants include, disposable/filtering facepiece respirators, half and full face respirators and powered air purifying respirators (PAPRs)

Air purifying/Filtering RPDs have two methods of operation — particle filtration and gas/vapour filtration.

  1. Particle filters are intended to remove solid and liquid particles (e.g. dusts, fibres, metal fumes and mists).
  2. Gas filters are intended to remove gases and vapours.

Combination filters are intended to remove both particles and gases/vapours.

A particulate filter will capture particulates only and a gas filter/cartridge will remove specific gases and vapours. It is extremely important that all potential contaminant/s have been identified to confidently confirm that any selected filter/s is suitable otherwise the wearer will still be exposed to some amount as it will still pass through the filtering material.

Particulate filters use sophisticated physics to capture hazardous particles with the main mechanisms being interception, impaction, diffusion and electrostatic attraction. Filters are tested using the “most penetrating particle size (MPPS) – which is around 0.3-0.6 microns – because this size range is the hardest to capture. If a filter performs well against the MPPS, it will perform better even better for both smaller and larger particles. The following YouTube video “The Astounding Physics of N95 Masks” from MinutePhysics explains these broad capture principles well.

Particulate filters DO NOT protect against or filter gases and vapours, oxygen-deficient atmospheres or high-level atmospheres requiring supplied air respirators e.g. Self-Contained Breathing Apparatus (SCBA). If gas/vapours are also present, then a combined filter is required

A gas/vapour filter/cartridge works very differentially from a particulate filter, where it removes hazardous gases and vapours from the air by adsorbing them onto activated carbon inside the filter/cartridge. Adsorption is where the gas/vapour molecule adhere to the surface of the carbon via van der Waals forces, physical attraction between molecules and in some cases chemical reactions. Certain hazardous gases wont’ adsorb efficiently on plain carbon, which is where manufacturers add chemical agents to the carbon that react with certain gases. Over time the carbon becomes full, meaning all adsorption sites are occupied. Once saturated contaminants will pass straight through exposing the wearer. This is why gas/vapour filters/cartridges must be changes according to a change out schedule.

Gas/vapour filters/cartridges do not protect against particles. If particles or aerosols are also present, then a combined filter is required. Gas/vapour respirators are air-purifying only. They cannot be used in oxygen-deficient atmospheres, unknown or IDLH environments, confined spaces unless monitored and safe. In those cases, SCBA or airline respirators are required.

Gas filters are available for use against groups of contaminants such as organic vapours or specific contaminants, such as mercury, and are available in different capacities. Gas filters are also available as multi-type gas filters which can be used against more than one type or group of contaminants. As a gas filter is used, it becomes saturated with the gaseous contaminant and will eventually no longer be able to remove the contaminant — this is referred to as “breakthrough” and indicates the end of service life of the filter. It is important that the gas filter be changed before breakthrough occurs; otherwise, the wearer will be exposed to the contaminant. Relying on the smell or taste of the gaseous contaminant to detect breakthrough is not suitable as the primary means of determining end of service life. The wearer’s senses may be affected for a variety of reasons or the contaminant’s warning properties may not be detectable at a safe level. However, any smell or taste of the gaseous contaminant detected by the wearer should be regarded as breakthrough. Breakthrough may occur earlier in the presence of gas mixtures or when used against different gases sequentially, than for a single gas. Certain contaminants can migrate through the filter during storage. This can lead to earlier than the expected breakthrough when the filter is next used. For this reason, some gas filters are designated for single shift use only

Limitations of filtering RPD

  • Filtering RPD purify the ambient air to be breathed by the wearer using filters to remove contaminants. They do not protect against oxygen deficiency or provide oxygen. Therefore, they are not suitable for use in actual or potential oxygen-deficient atmospheres, such as confined spaces. Filtering RPD is not suitable for entry or use in IDLH atmospheres. Filtering RPD classified as escape RPD can be used for escape from IDLH atmospheres.
  • Filtering RPD can only be used against known contaminants which the filter is able to remove from the ambient air. Filtering RPD is only suitable for use up to its maximum use concentration for the contaminant(s).
  • Gas filtering RPD cannot be used against particles; particle filtering RPD cannot be used against gaseous contaminants. However, there are combination filtering RPD, which can filter both gases and particles.
    Gas filters (including combination filters) are available in a number of capacities; however, the actual service life of gas filters depends on many factors including its designated capacity, the contaminant, the concentration, humidity, temperature and the work rate, i.e. the flow rate of air through the filter. A filter change schedule has to be calculated in advance according to the risk assessment.
  • Particle filtering RPD also have limitations to filter contaminants from the ambient air and need to be changed prior to their end of service life. Due to clogging effects, the breathing resistance might increase during use.

Many respirator manufacturers have freely accessible online resources and documents with lists of chemicals and what corresponding filter type from their range is recommended to assist workplaces with respirator/filter selection. It is important to highlight that one manufacturers filter recommendation cannot be used for a different manufacturer classed product as manufacturers will have different carbon mixes for their products each with their own recommendation which may be different. Hence the importance to contact the manufacturer for their recommendation/suitability of their products.

Below are a list of some manufacturers and their online resources

For manufacturers that are not listed above but do have an online selection guide that could be added, please email respfit@aioh.org.au with the details.

Reference/s for further information and detail:

  • AS/NZS 1715:2009 – Selection, use and maintenance – Section 4.2.2.2
  • SA/SNZ TS ISO 16975.1:2023 – Selection, use and maintenance – Section 7.3.3.6 and Annex A.2.3