Air Conditioners and the indoor environment

Air Conditioners and the indoor environment

Contact us to discuss your requirements

Air Conditioners and the indoor environment

Air quality is a term that incorporates a range of features that can include particulate matter, allergens, chemicals (VOCs) and relative humidity. The concentration and impact of these must be controlled in order to ensure a healthy indoor home environment. Those with asthma and allergies in particular can be affected by the quality of indoor air.

Healthy Indoor Air

There are a range of approaches that can be taken in order to support a healthy indoor environment. Air conditioners are one of these and can play a significant role in good indoor air quality. Greater asthma morbidity – specifically a larger number of hospitalizations, wheezing episodes and night symptoms due to asthma – have been associated with the presence of moisture, mildew and cockroach allergen in homes1. Air conditioners provide a dual function of controlling temperature and relative humidity in home environments, as well as potential removal of allergens.

Temperature extremes can have a negative impact on those suffering from asthma and allergies. While the mechanism of action of temperature on asthma and allergy exacerbation has not been absolutely defined, the relationship that temperature has to relative humidity is likely to play a role. A 2013 article reported on increased emergency department admissions for childhood asthma during periods of extreme temperature2and higher temperatures have also been associated with lower lung function in children with asthma3. Control of temperature to median ranges is therefore essential and ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) recommend a temperature range of 68-75°C (20-24°C) in winter time and 73-79°F (23-26°C) for the summer time4.

Relative humidity (RH) is in essence is a measure of how much moisture is in the air, and is related to the temperature of the air. It is important in the context of its impact on the body and on the surrounding environment. RH of 30-60% is a comfortable physiological range and is recommended by ASHRAE5; if RH starts to drop below 30% however it can have a negative impact on a range of physiological functions. Some viruses can survive for longer in dry, cool conditions and lower RH levels can result in drying out of the eyes and mucous membranes. These two impacts combined can cause a greater risk of infection6. Very dry air can also result in dry skin, worsening of eczema and irritation of the airways.

As RH levels increase above 60%, the potential detrimental impact is more from an environmental perspective. As RH increases, it creates a more suitable environment for dust mites and mold to grow. Dust mites thrive at RH levels above 70% and controlling RH to below 60% is essential to prevent their proliferation. Generally, ideal mold growth conditions are above 70%, however any RH level above 55% will allow mold to grow. For those with asthma and allergies, controlling exposure to allergens is essential and ensuring unfavourable conditions for mold and dust mites is essential.

The ideal relative humidity for comfort and health is between 40-50%, an air conditioner is best suited to provide this control.

Asthma and allergies

Use of air conditioning can help mitigate asthma and allergy symptoms caused by poor air quality. A 2011 study found a stronger association between asthma and allergy symptoms among children sleeping in non-air-conditioned homes, suggesting air conditioning in the bedroom may modify the health effects of indoor air pollutants7. In an earlier study, on days with high concentrations of particulate matter, cities with a high prevalence of air conditioner use report fewer hospital admissions for respiratory and cardiovascular disease compared with cities with lower air conditioner use8.

In addition to temperature and RH control, most air conditioning systems will have a particle removal system in place. While it is not the primary function of air conditioners to remove allergens, many air conditioners do incorporate filters that are capable of this. In this context, change of filter should be considered as allergen may have built up on the filter during use. The types of filters that an air conditioner may have, include:

  1. Coarse filter: the most basic filter is a coarse filter for removal of larger particles such as large dust particles and pet hair that may damage or clog the air conditioner unit. These filters are generally composed of plastic or polypropylene.
  2. Activated carbon filter: carbon filters can absorb contaminants (like VOCs) and some odors from the air. They have very porous structure and the compounds become trapped; over time the carbon filter will eventually become full, and will need to be replaced.
  3. Electrostatic filter: 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.
  4. Media based filter: as the air is drawn through the filter, the pore size of the filter stops specific sizes of particles being able to pass through the filter. The smaller the particle size, the more energy that is required to draw the air through the filter. Some of the most effective media filters, are HEPA filters (high efficiency particulate air) which can trap extremely small particles. This type of filter can remove a minimum of 99.97% of airborne particles of 0.3 microns or greater which will address the majority of indoor air contaminants such as dust, pollen, mold and bacteria9.

If the air conditioner does filter the air, the particle size it retains should be determined. 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 when such a system is put in place, you should ensure that replacement of filters does not result in compromising the filter system. In general filters which can filter out a minimum of 10µm (this is 1/100th of a millimetre or less than 1/1000th of an inch) are appropriate for allergen removal.

While media based filters can provide very effective particulate removal, as outlined above, other technologies may also be used in air conditioner units (eg electrostatic technology and carbon filters). Users should always ensure that there is validated data to support the efficacy of any technology that they are to reply on.

Certification of Air Conditioners
Selection of the right air conditioner can be difficult, and there are a number of Certification Marks that can help you make the right selection.
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 conditioners. The AHAM air conditioner standard measures specific characteristics that an air conditioner should achieve, mostly based around maintaining relative humidity in a standard room sized chamber.

asthma & allergy friendly® – The asthma & allergy friendly® Certification Program is operated by Allergy Standards Ltd in collaboration with the Asthma and Allergy Foundation of America (AAFA), Asthma Canada and is also operated internationally through a global certification mark. Air Conditioner certification is based on the impact on the indoor environment for those with asthma and allergies. The standard measures for the performance of air conditioners in a room sized chamber and addresses control of temperature and relative humidity, disturbance and removal of allergen, ozone production and general operation such that those with asthma and allergies should be appropriately protected.

Air conditioners can play an essential role in an allergy management plan. However selection of the right air conditioner, with effective relative humidity control is essential. Filter efficiency, size of room, practical operation of machine and alternative technologies are all things that need to be considered. Prior to purchase of an air conditioner for your home, it is important that you educate yourself as much as possible in order to make the right choice.

Contact us to discuss your requirements

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.


asthma & allergy friendly®, Allergy Standards, allergies, asthma, air conditioning, humidity, certification program, indoor air, indoor environment, indoor air quality, allergen, air conditioners, filter, air filter, HEPA filter, particles, dust, particulates


  1. Bonner S, Matte TD, Fagan J, Andreopoulos E, Evans D. Self-reported moisture or mildew in the homes of Head Start children with asthma is associated with greater asthma morbidity. J Urban Health. 2006; 83:129-37
  2. Xu Z, Huang C, Hu W, et al. Extreme temperatures and emergency department admissions for childhood asthma in Brisbane, Australia. Occupational and Environmental Medicine 2013;70:730-735.
  3. Li, S, Baker, PJ, Jalaludin, BB, Marks, GB, Denison, LS and Williams, GM. Ambient temperature and lung function in children with asthma in Australia. European respiratory Journal 2014 43 (4): 1059-1066.
  4. American Society of Heating, Refrigerating and Air-Conditioning, Inc. ASHRAE Standard, Ventilation for acceptable air quality, 2007, ANSI/ASHRAE Standard 62.1-2007.
  5. ASHRAE Handbook—HVAC Systems and Equipment, 2016; Chapter 22.
  6. Wolkoff, P and Kjaergaard, SK. The dichotomy of relative humidity on indoor air quality. Env Int 2007; 33 (6): 850-857.
  7. Zuraimi MS, Tham KW, Chew FT, Ooi PL, Koh D. Home air-conditioning, traffic exposure, and asthma and allergic symptoms among preschool children. Pediatr Allergy Immunol. 2011;22(1 Pt 2):e112‐e118. doi:10.1111/j.1399-3038.2010.00992.x
  8. Janssen NAH, Schwartz J, Zampbetto A, et al. Air conditioning and source-specific particles as modifiers of the effect of PM10 on hospital admissions for heart and lung disease. Environ Health Perspect. 2002;110:43-9
  9. Environmental Protection Agency [online] accessed June 2020.


Related Internal Links