Filters: how long do they last?

This question is, unfortunately, tantamount to asking “how long is a piece of string?”. But if specific factors such as workplace conditions are taken into account, an appropriate and safer filter change schedule can be achieved.

The nature of today’s industry means there are many situations in which workers need to be protected from airborne contaminants. Improved controls like ventilation and contaminant are becoming more common, but there are still many jobs that require the use of personal respiratory protection.

There are a variety of respiratory protection products available for various types and levels of exposure to airborne contaminants, and advice on their selection and use is provided in AS/NZS1715 – Selection, use and maintenance of respiratory protective devices.

The most widely used types are air purifying respirators which use a filter to remove contaminants from the inhaled air and are powered by the wearer's own breathing cycle.

This category includes maintenance-free or disposable type respirators as well as reusable or ‘rubber' type respirators in both half face (covering the mouth and nose only) and full face (covering the whole face and eyes) designs.

The filters associated with these products have a limited life, and the most common question from users is always “how long will my filter last?” Unfortunately, this valid question does not have a straightforward answer, but there are ways and indications that can be used to determine a safe answer.

Air purifying filters are divided into two main categories: particle filters and gas/vapour filters.

Capturing particles

Particle filters work by capturing particles such as dusts, fumes, mists or sprays from the inhaled air stream and allowing the cleaned air to pass through to the lungs.

As they are used, these filters load up with particles and become clogged. This eventually increases the resistance to airflow, making it harder for our lungs to pull air through the filter.

At some point this effort becomes too difficult and it is then time to replace the filter or disposable respirator. The actual time it takes for this to occur will vary from situation to situation and person to person. Some of the relevant factors influencing this include:

Concentration of particles in the air

Obviously, air that has a high concentration of particles will clog up the filter more quickly than air with a low concentration of particles.

Workrate

The harder you work physically, the more air you need in a given time. The wearer will draw more air through the filter per minute, so the filter will need to capture more particles in a shorter time and will clog up and require replacement more often.

Filter performance

Any respiratory equipment used in Australian workplaces should meet the minimum performance requirements of AS/NZS1716 – Respiratory protective devices. However, different manufacturers and designs mean that there can be filters that perform even better than required by the standards in categories like breathing resistance, loading performance and even weight and size. Looking at the various products available can enable selection of a filter with more desirable performance or characteristics.

Comfort

The respirator is a necessary piece of equipment for certain jobs, but is to some extent an extra burden on the wearer. By selecting a suitable product that has a higher perceived comfort level, there is less impact on the wearer and, importantly, they will be more inclined to wear the respirator when it is required. A respirator does not provide any protection if it is left in the locker or on the workbench because it is uncomfortable to wear.

Filtering gas and vapour

Gas and vapour (G&V) filters rely on an active sorbent material to capture the molecules of certain gases or vapours. The most commonly used adsorbent is activated carbon, which is plentiful and light, and has a very high active surface area in which to capture specific contaminants.

The activated carbon is treated chemically in several ways to enable it to capture various classes of chemicals, so it is important to get the correct G&V filter for each specific application.

A typical G&V filter acts like a sponge and soaks up the target molecules out of the breathing air as it passes through the filter. It fills gradually, but exactly how long a G&V type filter will last in use is a more complicated issue than for particle filters. Factors affecting filter life include:

Concentration of G&V in the breathing air

As for particle filters, a higher concentration of the gas/vapour in the air will fill the filter more quickly.

Type of contaminant:There are thousands of different G&Vs and they will all behave differently. Some will be captured easily and permanently while others are more difficult to capture and hold and the filter will have less capability and service life for these.

Type of adsorbent. The type of adsorbent used will affect the capacity and performance of the filter. The different types are described in AS/NZS1715.

Amount of adsorbent: More adsorbent material will give a longer service life. AS/NZS1716 has four classes of G&V filters. In increasing capacity these are class Aus and Class 1, 2, and 3. They all work just as well as each other at capturing their target gases/vapours but have an increasing service life. Class 3 filters are quite large and heavy and not very practical because of this.

Environmental conditions. The temperature and humidity of the workplace environment can have a drastic effect on the service life of G&V filters. For example, organic vapour filters (called type A) are affected by humidity. The water in the air competes for the active sites on the activated carbon so high humidity applications (for example, greater than 85%) reduce the life of the organic vapour filter significantly.

Workrate

As for particle filters, higher workrates require more breathing air to be filtered so the G&V filter will fill at a quicker rate and the effective service life of the filter will reduce.

Multi gas filters

There are G&V filters that contain several different types of adsorbents in layers to give them the ability to capture a wider range of contaminants. This works well, but it should be remembered that these layers may fill at different rates depending on the types and amounts of exposures. So in a situation where solvents and ammonia are present in the breathing air, the organic vapour carbon may be relatively unused while the ammonia carbon may be exhausted and have no adsorbent capacity left. This filter will then no longer capture ammonia and should be disposed of if protection from ammonia exposure is still required.

There are more complicating factors in determining the service life of a G&V filter compared with a particle filter. In the past, many have determined G&V filter life by relying on detection of breakthrough of the gas & vapour by smell, taste or irritation.

This is unsafe in some instances, as there are many substances that do not have any warning properties. The warning can also come too late, after a significant exposure has already occurred. There are also people who have poor senses, such as in the case of a common cold, who will not be able to detect the substance when others may.

It is clear that information is needed to enable a sensible and safe filter change schedule to be determined. With knowledge of the type of work being undertaken, the contaminants produced as well as their levels, users should seek advice from the manufacturer of the G&V filters to provide an estimate of expected service life under these known conditions.

This estimate can then be factored into the specific workplace to put a sensible and simple filter change schedule in place. For example, to change every day, every Monday morning, on the first of the month, etc.

Storage

All filters (particle and G&V) should be appropriately stored to protect against physical damage, contamination, dust, extreme temperatures, excessive moisture, sunlight and damaging chemicals.

G&V filters are normally sealed in airtight packaging until needed. There should be an expiry date marked on the packaging that will indicate how long the product in storage will perform as designed without degradation in required performance.

When opened and removed from the sealed packaging, G&V filters will start adsorbing substances from the ambient atmosphere, even if they are not being actively breathed through.

When opened but not in use, G&V filters should be stored in a sealed container to reduce or prevent this sorbent action to help maintain service life. Opened G&V filters will not last for extended periods, and AS/NZS1715:1994 says “All classes of gas filter should be discarded no longer than six months after opening, irrespective of the number of periods of use”. (Section 6.2.4.3)

The answer to the question “how long will my filters last?” requires knowledge of their processes and particular workplace conditions. Determining specific exposure information will enable the selection of the most suitable product as well as the creation of a relevant filter change schedule.

This will provide workers with the information they need to ensure they change their filters appropriately. This approach, aligned with good training and correct use of the product, will help workers achieve the required levels of respiratory protection and a safer workplace.

“Industrial Workwear Solutions., Filters: how long do they last., Issue 3, 2006., Terry Gorman