Air is one of the most fundamental requirements for life, but is often unnoticed due to the fact that it is everywhere. Man-made and natural compounds can make air unsuitable for certain applications and people. In these circumstances, filters can be an excellent way to improve air quality, indoors and on the go.
People who suffer from hay fever should protect themselves from pollen and mold spores, and people who work in busy cities may wish to protect themselves from the pollutants that are ubiquitous in these environments. Filters can be an effective way to do this.
When discussing indoor air quality, the terms ‘PM2.5’ and ‘PM10’ are often used. PM stands for particulate matter and the number that follows it refers to the size of the particulate matter in microns. A micron is one thousandth of a millimetre, for comparison, the period at the end of this sentence is about 400 microns! Although both these types of particles are very small, PM2.5 are referred to as ‘fine particles’ and PM10 as ‘coarse particles’ (as they are larger). The primary sources of these particles are cooking, automobile emissions, dust and incomplete combustion (fires). When particles are inhaled, larger particles are normally captured by the hairs in the nose, however particles of size less than PM10 can bypass this defence. The smaller the particle, the deeper it can make its way into the respiratory system, and potentially the more damage it can cause. Particles that are defined as PM10 include dust, pollen and sea salt, particles defined as PM2.5 are generally particles emitted from fossil fuel burning such as coal, wood, rubber and other materials1.
In hospital environments high filter efficacy is required to ensure that vulnerable patients are not exposed to particles that may exacerbate their illness. For example, when hospitals are undergoing renovation, great care has to be taken to ensure that Aspergillus mold spores are not introduced into the breathing zones of immunocompromised patients. Due to their weakened immune system, certain strains of Aspergillus mold can cause serious disease and death2.
In the pharmaceutical sector significantly high spec filters are required to ensure that the particle number in clean rooms is kept as low as 12/m3 (in outdoor air the particle number can be as high as 35,000,000/m3)3. This level of cleanliness is required to ensure that products intended as medicines do not get contaminated in any way.
There are a range of different types of filtration systems:
- Media based filters
- Electrostatic filters
- Activated carbon filters
- Media based filters are a basically a solid material with holes. The holes allow air to pass through, but will prevent certain sizes of particles passing through. The size of particle prevented going through will be dependent upon the application for the filter. In the home, prevention of mold spores (2-20µm) may be sufficient while in a clean room, prevention of viral particles (0.1µm) may be required. Media based filters are used in vacuum cleaners, air cleaners, HVAC systems and so on. The type of material that is used in these filters will affect price and also performance.
- Fiberglass – this is one of the most affordable types of media based filter and is disposable. These types of filters are mainly focussed on protecting the air cleaning device from large particulates, rather than filtering out contaminants from the air.
- Pleated filter – are generally made from cotton or polyester, a higher number of pleats will result in a greater surface area and therefore greater filtering performance.
- Washable filter – some air filters may be washable, and while it can save money in the long run, you also need to factor in time to wash the filter and ensuring that it is completely dry prior to use. Replacing the filter while damp may lead to mold and bacterial growth
- HEPA filters – high efficiency particulate air filters are the most appropriate ones for people with allergies as they have the capability to remove small particles to a high efficiency. HEPA filters can remove in excess of 99.97% of particles and have a MERV rating (see Table 1) of 17 and above4.
- Electrostatic filters use an electric charge to stick the particles to metal plates. As the particles are drawn into the filter, they are given a charge, oppositely charged particles (positive and negative) stick to each other and so if the particles are given a negative charge, they will stick to a positively charged metal plate.
- Activated carbon filters are used to remove odours and volatile organic compounds from the air, but do not have any specific particle removal efficacy.
Media based filters are the most common type and can be used as a first line of defence against a range of indoor air contaminants. Maintenance of indoor air quality is essential for a healthy home and can be achieved using a range of approaches.
Heating, Ventilation and Air Conditioning (HVAC) systems
A HVAC system works by drawing in air from outside and delivering it to the home environment. Filtration is a key part of this process; pollen and fungal spores are all carried through the air as part of their natural dispersal process, and will find their way into a home. If you are allergic to pollen or fungal spores, the filtration part of your HVAC system plays a central role in maintaining good indoor air quality. Filters work by having a pore size small enough to prevent these allergens from passing through but that will still allow air passage. As the air is filtered, the pores can become blocked and make the system much less efficient, ultimately causing the filter to fail. As such proper maintenance and monitoring of HVAC filters in essential.
The type of media filter used for HVAC systems will have a direct impact on the purity of the air that is delivered to the home. Media based filters are often classified according to the MERV rating. MERV, or minimum efficiency reporting value, is a measure of the size of particle that the filter will remove. The higher the MERV rating, the smaller the particle that the filter will remove, and the greater amount of that particle that will be removed. For example, A MERV 5 filter will remove 20-34% of particle sizes between 3 and 10µm, this means that anywhere from 35-80% of these particles sizes (which include pollen and dust mite particles) will pass through the filter.
Selection of the correct filter should be carried out carefully, based on you and your family’s individual needs as well as your surrounding environment. While selecting the highest rated MERV filter may seem to make sense, there are a number of considerations. Higher rated MERV filters will cost more and this should be considered. In addition, if you live in area with a very high pollen count, it is likely that the pollen will quickly clog up a MERV 17 filter, leading to greater turnover of filters as well as reduced efficacy. If the filter becomes clogged and is not changed, a build-up of pressure across of the filter can lead to failure of the filter as the air forces itself through, or around the filter.
As the seasons change, the exposure of particles to the filter will also change. You should make yourself aware of this and maintain your filter accordingly. During Spring and Summer, pollen counts will be higher and will result in a greater load for your filtration system.
Car Cabin Air Filters
While there are few scientific publications on how long people spend in their cars, there does seem to be a general consensus that it is between 8-9 hours per week6,7,8. That is the equivalent of a full working day each week spent in your car. Because of this, it is just as important that you take care of your indoor air quality in your car, as well as your home. An essential part of maintaining indoor air quality in your car, is the car cabin filter. This filter keeps dust, dirt, pollen, bacteria, mold spores and exhaust gases from entering the ventilation system in your car. The filters are generally media based, and similarly to HVAC filters prevent passage of a variety of contaminants based on pore size of the filter. Also similarly to HVAC filters, car cabin filters can become blocked and so regular checks and replacement are recommended in order to make sure that they operate correctly. If a filter does become blocked, apart from potentially allowing particulate matter through, ultimately it will make the air conditioning system work harder and may cause mechanical failure9. Some filters may have an activated carbon element to them – this will help to prevent odours and exhaust gases from entering the car cabin.
Filters are generally recommended to be replaced every 12,000 miles, or at a regular service interval. However if you are driving in areas with heavy particulate or pollen load, they may require replacement more regularly.
Air Conditioner Filters
An air conditioner at it most basic converts warm air to cold air, to keep the indoor air environment comfortable. It also affects relative humidity (RH); RH is the amount of water vapour that is in the air and should be maintained between 40 and 50%. If the levels drop below 40% the air can become very dry and affect the comfort of the room, resulting in dry and irritated eyes, flaky skin and also can dry out the mucous membranes of the respiratory tract. If the levels go above 50% they can start to provide an environment where dust mites and mold can proliferate. The relative humidity control impact of an air conditioner is therefore very important10.
The way in which air conditioners can help to maintain a healthy indoor air environment is by replacing warm air with cold, this is achieved through constant circulation of the air in a room through the air conditioning unit. As the air is drawn in, it is passed through a filtration system. This filtration system is generally media based, although may be in parallel with other mechanisms such as electrostatic precipitation.
Similarly to HVAC and car cabin filters, the pore size of the media based filters is an essential aspect to the efficacy of the filtration system. Filter pore size should be selected based on what you need to remove from the air. For smaller particles such as bacteria, higher efficiency filters are required. Maintenance of air conditioner filters is just as important as HVAC and car cabin filters. Should the filter become over-loaded, in addition to a greater power requirement to drive the air through, the filter may ultimately fail, providing you with little to no air cleaning capacity.
The selection of the right filter is clearly very important. If you don’t have allergies, and are just concerned with indoor air quality, a lower grade filter in addition to a carbon filter would most likely be fine. If you need to be more proactive about your indoor air quality a HEPA filter would be a more appropriate filtration system. Electrostatic filters are also a fine entry level filtration system however would be mostly redundant where a high grade HEPA filter was in place.
There a range of certification systems that are available to assist consumers with making decisions on managing their indoor air quality. These may be based on the product as a whole (ie air conditioner) or filters as individual units (ie HVAC and car cabin filters).
CADR – The Association of Home Appliance Manufacturers (AHAM), established more than 50 years ago, represents the appliance manufacturing industry by through leadership, education and advocacy. Part of this role involves setting standards for the performance of these appliances, including air cleaners and filters. The standard for determining this is the Clean Air Delivery Rate (CADR). This measure defines the amount of clean air that an air cleaner delivers, based on scores for smoke, pollen and dust. The higher the CADR, the faster the rate of air cleaning. This CADR number, as mentioned above can be used to determine the size of room that the air cleaner is suitable for; the CADR of your air filter should be equal to at least two-thirds of the room’s area11.
asthma & allergy friendly® Certification program
asthma & allergy friendly®– The asthma and allergy friendly® Certification Program is operated by Allergy Standards in collaboration with the Asthma and Allergy Foundation of America (AAFA), Asthma Canada (AC) and is also operated internationally through a global certification mark. The program certifies a range of consumer products that they are more suitable for those suffering from asthma and allergies, based on their impact on indoor air quality. Their certification addresses both chemical and biological contaminants, as well functionality of the products.
ECARF – the European Centre for Allergy Research Foundation is a not for profit foundation that certifies products based on specific testing standards. ECARF certifies a range of consumer products, including air cleaners/purifiers. These are tested based on removal of both fine and coarse indicator particulates that would represent the size of bacteria, pollen, spores as well as much smaller particulates. The standard also allows for testing of ozone and noise emission13.
All food and drink that enters your body is tested to ensure that it is safe for consumption. A human could last up to 3-4 days without water14, and just 3-4 minutes without air15. This is how important air is to us, and yet the food and drink we consume is tested to a much higher level than the air we breathe at home. Taking some proactive steps to improve your indoor air quality is a way to ensure that you keep this fundamental life support as clean as possible. You should always educate yourself as fully as possible as to your own needs in terms of maintaining a healthy indoor air environment. Selection of products to support your indoor air quality can be difficult, however selection of products that are certified is certainly a step in the right direction.
Dr. Tim Yeomans
About the author
Thanks to Dr. Tim Yeomans for this insightful article.
Dr. Tim Yeomans is the Centre Manager for Shannon Applied Biotechnology Centre, a collaboration between two third level colleges in Ireland. Tim holds a PhD in Microbiology and postgraduate qualifications in Technology Commercialisation and Innovation Management. Tim has worked in research and development for 20 years, both in industry and academia. In his role in Shannon ABC, Tim is responsible for the scientific direction of the Centre, intellectual property management and business and technology development.
asthma & allergy friendly®, indoor environment, air filters, HEPA, car cabin, allergy, asthma, certification, allergy insights, healthier home, indoor air quality.
- Zhisheng Li, Qingmei Wen, Ruilin Zhang. Sources, health effects and control strategies of indoor fine particulate matter (PM2.5): A review, Science of The Total Environment, Volume 586, 2017, Pages 610-622.
- Wirmann, B. Ross, O. Reimann, J. Steinmann, P-M. Rath, Airborne Aspergillus fumigatus spore concentration during demolition of a building on a hospital site, and patient risk determination for invasive aspergillosis including azole resistance, Journal of Hospital Infection, Volume 100, Issue 3, 2018, Pages e91-e97.
- BHATIA: “HVAV Design for Cleanroom Facilities”, Continuing Education and Development. Available in: Click here.
- American Society of Mechanical Engineers, ASME AG-1a–2004, “Addenda to ASME AG-1–2003 Code on Nuclear Air and Gas Treatment”, 2004.
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- Peder Wolkoff, Indoor air humidity, air quality, and health – An overview, International Journal of Hygiene and Environmental Health, Volume 221, Issue 3, 2018, Pages 376-390.
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