RPD Selection Information

There is no overall rule or set time frame about when filters/cartridges should be changed. Each situation needs to be treated/assessed individually.

Particulate Filters – Particle filtering respirators have limitations to filter contaminants from the ambient air and need to be changed prior to their end of service life.

The breathing resistance of a particulate filter will progressively increase in use as it becomes clogged with trapped particles and eventually becomes so high that the filter must be replaced. The time taken for this condition to develop will vary according to the characteristics of the filter, and the type, size, work rate i.e. the flow rate of air through the filter and concentration of the particles. As a general guide, the breathing resistance can be considered too high when there is a perceived increase in resistance to breathing. Resistance of particulate filters may be considerably increased if used in damp conditions.

In the case of PAPR particulate filters, clogging of the filters is normally signaled by a fall in the air-flow rate and the blower unit will indicate/alarm a particulate filter change is required before the expected protection is compromised.

The use of a particulate pre-filter is advantageous where coarse particulates would otherwise rapidly clog the filter.

The use of back flushing, using compressed air or other methods to prolong the life of a particulate filter is to be actively discouraged since it will reduce the efficiency of the filter.

Gas/Vapour Filter/Cartridges –

Gas filters/cartridges (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. The breathing resistance does not normally rise during use. Adverse storage conditions such as high humidity, can significantly decrease filter life after filter packaging is open.

When a gas filter/cartridge is selected, a change schedule for replacing the filter before breakthrough occurs should be established. This can be accomplished with service life estimators or information from the gas filter manufacturer. Gas filter service life data can be used to set change schedules for ensuring the replacement of the gas filter before breakthrough occurs. If the service life is determined to be too short for the task duration, a gas filtering RPD with a higher capacity should be considered. If the service life of the highest capacity is still too short for the task duration, then breathable gas supplying RPD should be selected.

AS/NZS 1715:2009 Section 4.2.5.3 specifies that a gas/vapour filter/cartridge must be discarded no longer than 6 months after opening, irrespective of the number of periods of use.

Certain filters/cartridges have set time use periods like the organic vapour AX/OG low boiling point ≤65°C classification which has a single shift change requirement. This also maybe sooner based on concentrations levels as specified by the manufacturer. Mercury filters meeting AS/NZS 1716:2012 a maximum use times being Class 1 of 8 hours and Class 2 of  50 hours. These filter change times period must be follow as specified by a manufacturer and/or standard requirement.

Furthermore, if an odour or taste is perceived in the inhaled air or when the wearer coughs or experiences discomfort, e.g. if the contaminant is an irritant, this indicates filters are not being changed often enough and the filter replacement schedule should be adjusted. The sense of smell should not be relied upon as detailed further below of the limitations of the sense of smell and irritation.

Limitations of the sense of smell and irritation

For the purpose of warning users about the end of filter/cartridge life, the sense of smell is unreliable and must not be used as the primary indicator. Some of the reasons are:

• There is considerable variation between individuals, with some persons being unable to detect contaminants by smell, e.g. hydrogen cyanide has a characteristic almond odour that is not detectable by some people.
• The sense of smell in an individual may be considerably diminished temporarily by a cold in the nose or other inflammatory conditions of the nasal passages.
• The odour of a contaminant may be masked by other smells.
• The sense of smell tires (olfactory fatigue) over a period of time and fails to detect high concentrations of many contaminants particularly if such concentrations have built up gradually, e.g. hydrogen sulfide. A person working in a situation where a dangerous concentration of a contaminant has developed slowly may not detect any odour; yet a person entering the situation from outside could be aware of a very strong odour.
• The threshold of odour for some materials exceeds the level at which such materials may be considered hazardous. Thus, by the time the contaminant can be smelt, the wearer may already have been exposed to a hazardous situation, e.g. methyl bromide.
• Some gases have no odour and therefore are not detectable by this means, e.g. carbon monoxide.
• Some gases have very objectionable odours at very low concentrations but do not represent a health hazard at these concentrations, e.g. methyl mercaptan.

Some contaminants, because of their local irritant action on the upper respiratory tract or eyes will give an early indication of their presence in harmful amounts. The warning sensation experienced, although perhaps centred in the nose, is not a smell, but a discomfort, a feeling of burning or irritation, e.g. sulfur dioxide.

With some contaminants, the warning symptoms are so severe that no one would willingly remain in a dangerous atmosphere, e.g. ammonia and hydrogen chloride (HCl).
With other contaminants, the warning may be positive, but insufficient to protect a person who is willing to endure considerable discomfort, e.g. chlorine.

It is therefore of fundamental importance in respiratory protection that the sense of smell should never be relied upon to provide warning against dangerous contaminants.

Reference/s for further information and detail:

• AS/NZS 1715:2009 – Selection, use and maintenance – Section 9.5
• SA/SNZ TS ISO 16975.1:2023 – Selection, use and maintenance – 7.3.3.7, Annex A.3.3 and Annex B.2.1