What is an appropriate and effective filtration technology for a healthy indoor environment?

What is an appropriate and effective filtration technology for a healthy indoor environment?

Indoor Air Quality in the Home by Dr. Anna O'Donovan, Allergy Standards

Taking care of the air we breathe is the most fundamental part of taking a proactive approach to a healthy environment. For many people the health risks posed by the indoor air environment can often be significantly higher than outdoor air. Particles released by cooking and burning fuel, volatile chemicals emitting from paint and cleaning products as well as allergenic particles such as animal dander, pollen, mold and dust mite particles, can all impact negatively on a healthy indoor air environment. Poor indoor air quality can be particularly harmful to vulnerable groups such as the very young and old, and those suffering from chronic respiratory diseases. 

Fortunately there are a number of approaches that can be taken to improve and maintain the quality of your indoor air. A number of these focus on the removal of particulates from the air by passing them through some kind of filter, and can be called air filters or air purifiers. In general these terms are used interchangeably, however air purifiers generally have more than one mode of filtration.

Modes of Filtration

Air cleaners or air purifiers are designed to remove particulates and sometimes odors from the air we breathe. There are a number of different ways that they can do this.

  1. Media based filter: Filters are generally ranked according to the particle size that they will trap. In line with this, the smaller the particle size, the more power that is required to draw air through the filter. Over time filters will need to be replaced as they become clogged and, in a similar way to vacuum cleaners, the exposure of a sensitive individual needs to be considered when establishing a removal and replacement protocol. The efficacy of a filter is dependent upon the integrity of the seals into which it is placed and you should ensure that replacement of filters does not result in compromising the filter system. Dependent upon how effective a filter is, it may be described as a HEPA filter (high energy particulate air).
  2. Electrostatic filters: electrostatic filters work on the basis of electric charge. As the particles in the air pass through the electrostatic filter, they receive a charge, this allows them to be stuck to metal plates in the air filter, preventing them from being released out of the air purifier. The principle of electrostatic filtration can also be applied to media based filters, giving them an added advantage for filtration.
  3. Activated carbon filters: these filters are made of porous carbon. As odors and gases pass through the carbon filter, they become trapped in the pores, unable to pass through. The pores may become clogged over time and will then need to be replaced.
  4. UV light: UV light is capable of killing micro-organisms, UV light sources can sometimes be present in room air cleaners, and also in HVAC systems. UV light requires time to kill micro-organisms, it doesn’t happen immediately. As such, where a UV system is in place, you should make sure that it actually is effective, rather than just being a sales pitch!

In addition to the type of filtration system selected, you should ensure that the air cleaner that you select is powerful enough for the size of room in which it is to be operated. A useful way to determine this is by an air cleaner’s CADR rating. CADR, or Clean Air Delivery Rate is the performance metric to indicate the effectiveness of an air cleaner. If you purchase an air cleaner with a CADR rating too low for your room, you will find it ineffective.

While CADR is a key measurement for selecting an air cleaner, MERV (minimum efficiency reporting value) is important for selecting the right filter. The MERV rating is a measure of the size of particle that the filter will remove; HEPA filters are normally rated MERV 13. The below chart is a useful guide:


Air-Cleaners-Allergy-Standards-Healthy-Indoor-EnvironmentWhile CADR and MERV are useful tools to assist in selecting an appropriate air cleaner, you should also consider the environment in which it is to operate. Your indoor air is very much a function of your outdoor air and so if you live in an area with high environmental particulate load, this will transfer to your indoor environment. This will impact how often you need to change your filter, and how long you need to leave your air cleaner on for. An example of this is the recent tragedy of the wildfires in California, smoke from these fires can contain a wide range of particles at very small size, down as low as 0.4 micron. Carbon monoxide, a dangerous gas, can also be produced during fires and so precautions that could be followed in the home would be the use of an air cleaner with a media based HEPA filter (to capture particles as low as 0.3 micron) and some type of carbon or zeolite filter to capture any gases that may enter the indoor environment. Due to the high load in the air following these fires, filters would need to be changed more regularly and air intake should be set to re-circulate rather than using fresh air intake.

PM2.5 Vs PM10

When discussing air pollution, 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 materials.

Certain parts of the population are more susceptible to particle pollution:

  • Children
    • Lungs still developing
  • Senior Citizens
    • May have undiagnosed heart or lung conditions
  • People with existing heart or lung diseases
    • Particle pollution exacerbates these conditions
  • People who exercise or breathe outdoors
    • Breathe faster and deeper than sedentary adults

While these particles are very small, their impact on the economy can be large – air pollution in the US on average costs between $40-50 billion and 50,000-120,000 premature deaths are associated with exposure to air pollution.

Other methods of air purification:

Ionizers and Ozone generators: while these produce different types of molecules, they work in a similar way. They produce ions or ozone which is released from the air cleaner/purifier to be distributed around the room. Allergenic particles normally floating around a room have a neutral or no charge, when they come into contact with ions or ozone, they become charged and can stick to surfaces around the home, removing them from the air. They can then be removed from these surfaces by normal day to day cleaning.


Ozone (O3) is a gas that is highly reactive, it can be formed naturally or as a result of human activity. The ozone layer in the atmosphere is formed naturally through the interaction of solar ultraviolet light with oxygen (O2). In the ozone layer, ozone has a highly protective role – it absorbs UV light and prevents excessive exposure of humans to harmful UV rays. If ozone is inhaled however, it can have a number of harmful effects. It can be transported all the way to the lower respiratory tract where it can interact with biological molecules in the lung to cause:

  • Cough
  • Throat Irritation
  • Pain or discomfort in the chest when breathing
  • Chest tightness or wheezing

For the general population this is not very comfortable, for those already suffering from lung problems, such as asthma, it can have a significantly harmful effect.

Many electrical implements (including air cleaners) produce ozone as a by-product of functioning, and this cannot be avoided. All electrical implements are tested for the ozone levels that they produce to ensure that they aren’t produced at harmful levels (greater than 0.05 parts per million). As such, an air cleaner that relies on ozone as an air purification function could not be recommended.


Ionizers release ions into the environment. Ions can be positively or negatively charged and their main function in air cleaning is to pass their charge onto neutral air particles. By passing on this charge, it causes the particles in the air to stick to surfaces, thereby removing them from the breathing zone. Research in the area of negative ions supports the safety of these ions, however there is not as much research performed on positive ions. Anecdotal, unsupported research indicates that positive ions may have a detrimental impact on health. Air cleaners with an ionising function must therefore produce predominantly negative ions to comply with certification. 

Certification of Air Cleaners and Purifiers

Selection of the right air cleaner or purifier can be difficult, and there are a number of Certification Marks that can help you make the right selection.

clean-air-delivery-air-aham-sealCADR – 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 area.


European Centre for Allergy Research Foundation allergy test

European Centre for Allergy Research Foundation allergy test

ECARF – The European Centre for Allergy Research Foundation (ECARF) 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 emission.


asthma & allergy friendly® Certification program USA

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. Air Cleaner/Purifier certification is based on the impact on the indoor environment for those suffering from asthma and allergies. The standard measures for the performance of air cleaners/purifiers against real dust and allergens (cat, dust mite and pollen) and the air cleaner is measured in a room sized chamber in order to ensure that the air cleaner actually removes the allergen, rather than just re-distributing it. In addition to the use of real allergen, the asthma & allergy friendly® testing also includes room disturbance as part of its test protocol, to simulate testing of the air cleaner under real life conditions. Validation of ozone emissions is also part of this testing standard. While media based air filtration forms the main part of certification, other types of air cleaning are also considered, including ionizers.


Sick building syndrome Indoor Air Quality Allergy StandardsAir cleaners or purifiers play an essential role in maintaining a healthy indoor air environment. However selection of the right air cleaner, with the appropriate effective filtration technology is essential. Filter efficiency, size of room, outdoor environment and alternative technologies are all things that need to be considered. It is important that prior to purchase of an air cleaner or air purifier for your home, that you educate yourself as much as possible in order to make the right choice!

Dr. Tim Yeomans photo

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.


air care, air purifier, air cleaner, ioniser, ozone, HEPA, indoor environment, allergy, asthma, certification, allergy insights, healthier home, indoor air quality.

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By |2021-10-20T12:00:08+00:0015 July 2019|Tags: , , |Comments Off on What is an appropriate and effective filtration technology for a healthy indoor environment?