Air Pollution Primer
- By Peter Crona & Flora Crona
- January 15, 2021
We care about air pollution because of its tremendous impact on health. If you live in a polluted area it can be a matter of life and death . If you suffer from allergies or a respiratory illness such as asthma, you might already directly feel the impact of pollution. If you are lucky and don’t directly notice the effects of air pollution, perhaps you just want to do what you can to stay healthy, or perhaps you want to raise your productivity and other mental abilities. We take between 17,280 and 23,040 breaths every day . Simply put, what we breath matters, and it matters for everyone.
With this guide we want to familiarize you with some key concepts to give you a idea of what you can do to protect yourself and your family, but also a good basis for further learning on your own.
Outdoor Air Pollution
Let’s start by looking at common outdoor pollutants; where they come from and what impact on your health they can have.
Particulate Matter 2.5 (PM2.5)
Perhaps the most commonly mentioned pollutant in the media. According to EPA , particulate matter is defined as a mixture of solid particles and liquid droplets. The “2.5” stands for that the diameter must be 2.5 micrometers or less. To give you an idea of how small this is, one centimeter equals 10,000 micrometer (1 inch = 25,400 micrometer). But as commonly said, one image says more than a thousand words:
PM2.5 is dangerous since it is small enough to enter the body where it can cause damage . In fact, if you are pregnant, it can even cross the placental barrier (which is supposed to protect the fetus from potentially harmful substances) . And looking at yourself, studies suggest that it can bypass your blood-brain barrier and reach your brain , which is likely to not be ideal for your brain health.
Limits for PM2.5 vary. Below follows a list of some common limits     :
- WHO: 10μg/m3
- EPA: 12μg/m3
- EU: 25μg/m3
- China: 35μg/m3
- India: 40μg/m3
- WHO: 25μg/m3
- EPA: 35μg/m3
- EU: No limit
- China: 75μg/m3
- India: 60μg/m3
PM2.5 can come from various types of combustion, for instance of diesel fuel and gasoline. It is important to note that the danger can vary depending on the composition of the PM2.5. Studies  have shown that anthropogenic sources (caused by human activity) were associated with oxidative potential, whereas other sources were influencing the mass concentration more.
Particulate Matter 10 (PM10)
Essentially PM10 is the same as PM2.5 but made up of larger particles. The “10” stands for particles with a diameter of 10 micrometers or less. Thus, it is a superset of PM2.5, i.e. it also includes PM2.5.
A difference is that PM10 includes things such as pollen, fragments of bacteria and various kinds of dust, for instance from construction sites and landfills . PM2.5 is considered more dangerous when it comes to mortality and consequences of long-term exposure .
Just as for PM2.5, limits for PM10 vary. Below follows a list of some common limits     :
- WHO: 20μg/m3
- EPA: 50μg/m3 (this was revoked though, due to lack of evidence )
- EU: 40μg/m3
- China: 70μg/m3
- India: 60μg/m3
- WHO: 50μg/m3
- EPA: 150μg/m3
- EU: 50μg/m3
- China: 150μg/m3
- India: 100μg/m3
Nitrogen Dioxide (NO2)
NO2 comes mainly from combustion of fuel, eg. from traffic and power plants. It can irritate your respiratory system, and for instance aggravate respiratory diseases, such as asthma. It can also contribute to the development of asthma as well as increase the risk of respiratory infections . There are also indications that for instance NO2, as well as particulate matter, can increase the severity of COVID-19 .
Just as for PMx, the limits vary. Below is a list of some common limits     :
- WHO: 40μg/m3
- EPA: 99.64μg/m3
- EU: 40μg/m3
- China: 40μg/m3
- India: 40μg/m3
- WHO: 200μg/m3
- EPA: 188μg/m3
- EU: 200μg/m3
- China: 200μg/m3
- India: 80μg/m3
EPA’s limits were converted from ppb to μg/m3 by multiplying by 1.88 as described in
EPA has an excellent page  in which they describe that high up in the atmosphere ozone protects us from UV radiation. But at ground-level, and in our homes, it can cause a multitude of health problems. EPA summarizes the harmful effects as:
Breathing ozone can trigger a variety of health problems including chest pain, coughing, throat irritation, and airway inflammation. It also can reduce lung function and harm lung tissue. Ozone can worsen bronchitis, emphysema, and asthma, leading to increased medical care. 
Furthermore, regarding the sources, Ozone is created through a reaction with sunlight, heat, NOx and VOC. These in turn come from multiple sources, one big being traffic (gasoline and diesel combustion). EPA has created a picture summarizing this:
As for all other pollutants, limits vary. Below below are some common limits     :
- WHO: 100μg/m3
- EPA: 137.2μg/m3
- EU: 120μg/m3
- China: 160μg/m3
- India: 100μg/m3
EPA’s limits were converted from ppm to μg/m3 by multiplying by 1000 and then by 1.96, based on
You can limit your exposure to some pollutants to some extent when outdoor, for instance by opting for a walk through a park or in a forest, rather than walking next to a heavily trafficked road. However, if pollution is severe, a common recommendation is to simply stay indoors.
Naturally, there’s a relation between indoor and outdoor pollution. The air comes from the outdoors after all. But, when looking at indoor air quality, it is helpful to treat “indoor” as its own system, where “outdoor” is one source of both fresh air and pollution; there are also other sources that originate from the “indoor”. In this way you can reduce the total pollution level indoor by focusing on the sources with the greatest impact.
Let’s now look at some common indoor pollutants. For those pollutants that have been presented already (in the outdoor section), we refer you to the previous description for more details. Here we focus on what indoor sources they might come from.
Particulate Matter 2.5 (PM2.5)
It is not uncommon to have very high levels of PM2.5 indoors, significantly higher than outdoor depending on where you live, both based on that outdoor pollution levels vary, but also based on how air tight your home is and what you do inside it. Energy efficient homes tend to be well insulated, which means that pollutants can build up and reach very high levels, unless the ventilation is adequate.
A good example is our home, which is a roughly 50m2 apartment located in central Berlin. It is very well insulated (we rarely need heating during winter) and has a kitchen hood circulating the air back into the apartment through a carbon filter. When cooking, PM2.5 levels can reach several hundreds μg/m3, even if air pollution outdoors is only around 25μg/m3, as shown in figure 3.
With that said, high temperature cooking is a significant source. Other sources include candle burning and usage of microwave. We have measured high PM2.5 levels after making popcorn in my microwave. Making waffles with a waffle maker also quickly raises PM2.5 levels in my flat. This likely holds true for making toasts as well. In fact, in  candle burning, cooking and toasting are mentioned.
On the contrary, we have seen very low increases in PM2.5 levels when cooking with low temperature, for instance frying eggs on the lowest heat (with our electric stove).
If you have a PM2.5 meter, we encourage you to simply light a match, let it burn for a little and blow it out. The PM2.5 level is elevated for a surprisingly long time. This can help you to appreciate the effect of just a little indoor combustion. As further reading, we also recommend you to check out
The outdoors can naturally be a great source of PM2.5 too, in which case you might consider improving insulation. Having limited places where air enters your home makes it easier for you to clean it if necessary, and if the outdoor air is heavily polluted, you might want to keep the air exchange at just the right level, and not higher than necessary. In a home in Shanghai we observed that the windows lacked sealing strips, which allowed a lot of polluted air to sip in and made it hard to tackle (since the pollution source was spread out in the apartment).
One reason that you can’t just insulate as much as possible is that humans exhale CO2, which quickly builds up indoors if the ventilation is not adequate.
Regarding limits, we are not aware of any specific limits for indoor exposure. Personally, we just aim for below 10μg/m3, and after for instance cooking, we look at how quickly we can get back to below 10μg/m3. Our rough goal is that the average indoor exposure over a day should always be below 10μg/m3. However, note that we are lucky and live in a place with fairly good air outdoors, making this feasible for us. Remember, any reduction of PM2.5 is valuable, even if you remain at a level exceeding WHO limits.
Total volatile organic compounds (TVOC)
Berkeley Labs  writes that TVOC is a subset of VOC, and that VOC includes thousands of compounds, some man-made and others natural, that are mostly present as gases at room temperature. It is important to note that not all VOCs are toxic, therefore a high TVOC doesn’t necessarily mean that the air is dangerous. In fact, Berkeley Lab writes:
In general, TVOC measurements in buildings have not been useful in predicting health effects .
Nevertheless, Bayerisches Landesamt für Gesundheit und Lebensmittelsicherheit  has created some limits (we simplified and translated these to English for your convenience):
- <=300µg/m3: Very low
- >300-1000µg/m3: Low
- >1,000-3,000µg/m3: Moderate
- >3,000-10,000µg/m3: High
- >10,000µg/m3: Very High
- >25,000µg/m3: Avoid the room completely
High levels of some VOCs have adverse health effects. Berkeley Labs writes:
The suspected health effects cover a broad range including, but not limited to, sensory irritation symptoms, allergies and asthma, neurological and liver toxicity, and cancer. .
Indoor sources include paint (so, if you have a newly painted room, TVOC levels are likely high), cleaning products, markers , new products (eg. furniture) and perfume or scented products . This is an argument for being careful with renovations when expecting a child, as during pregnancy and when a newborn is around, you want the air as good as possible.
Formaldehyde is actually a common VOC, and this section therefore overlaps a bit with the previous section about TVOC. However, formaldehyde is often singled out and given more attention. For instance, WHO’s guidelines for indoor air quality includes a chapter about formaldehyde . Also “The Index Project” by the european commission includes a chapter about formaldehyde . There are different limits around  provides a 1 hour and 8 hours limit:
- 1 hour: 123μg/m3 (100ppb)
- 8 hours: 50μg/m3 (40ppb)
Lastly, a somewhat surprising (and we haven’t tested it) paragraph we found in “The Index Project” report is:
There might also be formaldehyde sinks in indoor environments. Zwiener et al(1999) reported reduction of formaldehyde concentration between 80% and 87% in chamber air in two hours after the test had begun. This reduction is due to chemical reaction of formaldehyde and the wool proteins. 
It appears that wool can capture formaldehyde. In  it is mentioned that wool can act as a buffer.
We’re definitely going to get some wool to play with! Perhaps a carpet, as it was pointed out in  that it has the potential to capture formaldehyde, nitrogen oxides and sulphur dioxide.
Generally, ozone indoor comes from the outdoor. However, some office equipment, such as laser printers, can produce ozone indoors. Also air purifiers can produce ozone. Combined with poor ventilation, indoor ozone sources can be problematic as ozone could build up, especially if the space is small.
CARB requires air purifiers to not exceed outputting 50ppb (0.05ppm) of ozone . An air purifier that might produce ozone may only be sold in California if it is certified. The California Air Resources Board has a list of certified air purifiers. Even if this requirement is for california specifically, you will often see air purifier manufacturers write that ozone emission is <0.05ppm or less than 50pbb, regardless of where they are sold. For your convenience, 50ppb ozone equals 98μg/m3 ozone, based on the same calculation as above for the EPA limit.
Carbon Monoxide (CO)
CO is very dangerous since you can not detect it with your senses. It generally comes from some sort of combustion. Minnesota Department of Health has created a list  of common sources:
- Clothes dryers (not using solely electric)
- Water heaters (not using solely electric)
- Furnaces or boilers (not using solely electric)
- Fireplaces, both gas and wood burning
- Gas stoves and ovens
- Motor vehicles
- Grills, generators, power tools, lawn equipment
- Wood stoves
- Tobacco smoke
They also recommend that you get an CO alarm to protect yourself.
CO is infamous for that it can lead to death. It can be hard to notice when you are suffering from CO poisoning as the early symptoms are similar to those of flu . We’d recommend you to follow Minnesota Department of Health’s recommendation of getting a CO alarm if you believe you have a CO source in your home, for instance a gas stove.
We recently used our fondue equipment to make egg dumplings. It has a container where you use a kind of fire gel. Our sensor, roughly 5 meters away, picked up elevated CO levels, as can be seen in figure 4. This is the first time ever the sensor has registered CO levels over 0ppm.
Carbon Dioxide (CO2)
CO2 is mainly a side-product of human metabolism; we exhale it. You might already know about CO2 through climate change. But, looking at CO2 is also used as a way to confirm adequate ventilation, i.e. that you get enough fresh air. This is both since high CO2 indicates a too low air exchange rate, but also since CO2 can be problematic in itself.
An example of a study that looked at the impact of CO2 is , where they let participants perform various tasks and looked at how performance changed as the CO2 concentration was varied; see figure 5, which comes from the study and shows how performance varies with different CO2 concentrations.
CO2 levels can be very high in energy efficient (well insulated) modern buildings. In particular in bedrooms if the door is closed . In fact, the simple act of keeping the bedroom door open has been shown to improve sleep .
Improving Indoor Air Quality
Unlike outdoor, indoor we typically do have some control over the air quality. We can take actions to improve it.
Generating Less Pollutants
Perhaps obvious, but if we generate less pollutants indoor air quality will improve. Some ideas of what you could do:
- Do not smoke indoors
- Use your kitchen hood at highest setting when cooking
- Opt for lower temperature when cooking
- Do not light candles or do any other combustion indoor
- Use a vacuum cleaner with a HEPA filter, as it otherwise might blow up a lot of particles.
Regardless of the quality of the outdoor air, ventilation is very important. As mentioned, the primary source of CO2 indoor is us - humans. And there are other indoor sources that can quickly raise pollution levels so they exceed outdoor levels. Adequate ventilation also ensures that humidity levels are good.
Generally, ventilation is handled by your landlord or left to professionals. However, we’ve both seen and heard of people that block their ventilation because they felt a draft, wanted to save money on heating or for other reasons. We’d advise people to be careful and remember that there’s a reason for the ventilation being there.
Keeping vents clean is another thing you might be able to do yourself. If you see an air vent covered with dust, removing the dust could significantly improve your ventilation.
If the outdoor air is clean and the climate allows, good ventilation and/or often opening windows might be all you need. But, more than 90% of the population on our planet unfortunately live in areas with dirty air (as in exceeding WHO limits) . An air purifier is an easy way to improve indoor air quality. There are both portable air purifiers and filters you can install in your ventilation system. Generally, it is easier to get a portable air purifier, as you just need to plug it in. There are also special air purifiers that you can install in air ducts.
Most Common Air Purification Technologies
Even if there are many technologies out there, the vast majority of air purifiers will use either one or a combination of the technologies listed here:
In principle an air purifier is a very simple machine. Essentially, it blows air through some kind of filter, in which pollutants get trapped. Some companies, for instance Smart Air, have used this to produce highly effective low-cost air purifiers by simply putting a filter on a fan.
Mechanical filtration is just about this. There are different kinds of filters, a very famous is, a bit imprecisely, called HEPA-filter. HEPA is actually an efficiency standard for air filters, and not a filter itself. Unfortunately there are different definitions, for instance in the USA and Europe. Generally, you will see air purifiers with HEPA filters of class H13. The classes go from E10, E11, E12, H13, H14, U15, U16, U17, in order of their efficiency, i.e. U17 is the one capturing most particles. However, it is rare to as a private person find anything beyond H13 or possibly H14.
HEPA-filters are very good for capturing particles and can thus reduce both PM10 and PM2.5. However, they do not capture gases, for instance VOCs. For this typically activated carbon filters are used.
The above mentioned classification is what we see most in webshops selling air purifiers. This comes from a standard called EN 1822. However, there is actually a newer standard called ISO 29463, where for instance H13 is split into two classes, ISO 35 H and ISO 40 H. We recommend you to have a look at this summary of filter classes by EMW filtertechnik if you want the details.
Another common technology is ionization. It works by electrically charged particles. Some ionizers also have collectors, which are surfaces charged with the opposite charge, so that they attract the charged particles. Several studies support that ionizers can lower PMx and ultrafine particles, see for instance table 3 in  or . Our own experience is consistent with this (well, we only have a PM2.5 meter, so unfortunately we haven’t been able to experiment with ultrafine particles).
It is important to not confuse ionizers with ozone generators. Ozone generators are generally used for sanitization, they should not be operated with humans around, and the operator must ensure to follow safety guidelines. Ionizers do emit ozone too, but good ionizers emit very low levels, well below existing limits. Our experience is that ionizers with carbon fiber emitters (as opposed to metal “needles”) tend to have low ozone output, something supported by . However, we have also experience with ionizers that use metal “needles” that have low ozone output, for instance the bipolar ionizer in our Sharp KC930EUW. See figure 6 for a picture of the ionizer module.
This is air purification with light. Generally TiO2 is used, which when exposed to ultraviolet light acts as a catalyst that can break down many pollutants. This technology doesn’t work well with particles, but with gases, for instance VOCs. It can be seen as an alternative technology to activated carbon filters, with the potential benefit of significantly lower maintenance, but the drawback of not necessarily being as efficient as activated carbon.
We are quite interested in developments here. Our experience is that the efficiency of activated carbon filters quickly falls (admittedly, we only have basic sensors). There are some innovative air purifiers using modified TiO2 or other materials combined with LED. IKEA has also incorporated TiO2 in one of their curtains (Gunrid) to make it purify air when exposed to sunlight.
Not so common in our experience, but there are air purifiers using heat as well. The technology is sometimes called Thermodynamic sterilization (TSS). But based on test data , it seems to be reducing viruses, bacteria and mould. Something that surprised us is that it can also reduce ozone, in one test by the Swedish National Testing & Research Institute, a 26% Ozone Reduction was reported. This is perhaps not so surprising when looking at the half-life of ozone at different temperatures. As shown in , at 20°C ozone has a half-life of 3 days. But at 250°C it is just 1.5 seconds.
Often you will see combinations of various technologies. For instance, ionizers are often used to enhance filter efficiency; an example of a study looking at this is , which showed that ionizers can significantly enhance filter performance. An example of a company using this in commercially available air purifiers is Blueair, which by many is considered a premium brand for air purifiers. As HEPA-filters can’t capture VOCs, you’ll often see air purifiers with a HEPA-filter for particles and an active carbon filter for VOCs.
Clean Air Delivery Rate (CADR)
Finally, a very useful number to look at when evaluating air purifiers is CADR. It defines three classes, namely pollen, dust and smoke. These are just names for your convenience, they have more concrete definitions based on particle sizes:
- Smoke: 0.09–1.0µm
- Dust: 0.5–3µm
- Pollen: 5–11µm
CADR makes it easier to compare air purifiers without knowing all the details. The higher value the better.
We hope that you found this, and the rest of the article helpful. Stay tuned for more blog posts! We tried to make a good selection of what to cover in this blog post. But, we also want to discuss for instance mold and dust mites in future blog posts, as many people are exposed to both.
 “Ella kissi-debrah death: Air pollution recorded as cause of nine-year-old’s death in first ever case for uk,” Sky News, Dec. 2020, [Online]. Available: https://news.sky.com/story/ella-kissi-debrah-death-air-pollution-recorded-as-cause-of-nine-year-olds-death-in-first-ever-case-for-uk-12164155.
 A. Brown, “How many breaths do you take each day?” EPA. Environmental Protection Agency, Apr. 2014, [Online]. Available: https://blog.epa.gov/2014/04/28/how-many-breaths-do-you-take-each-day/.
 “Particulate matter (pm) basics,” EPA. Environmental Protection Agency, Oct. 2020, [Online]. Available: https://www.epa.gov/pm-pollution/particulate-matter-pm-basics.
 N. Liu, L. Miyashita, G. Mcphail, S. Thangaratinam, and J. Grigg, “Late breaking abstract - do inhaled carbonaceous particles translocate from the lung to the placenta?” European Respiratory Journal, vol. 52, no. suppl 62, 2018, doi: 10.1183/13993003.congress-2018.PA360.
 L. Peeples, “News feature: How air pollution threatens brain health,” Proceedings of the National Academy of Sciences, vol. 117, no. 25, pp. 13856–13860, 2020, doi: 10.1073/pnas.2008940117.
 “Ambient (outdoor) air pollution,” World Health Organization. World Health Organization, May 2018, [Online]. Available: https://www.who.int/news-room/fact-sheets/detail/ambient-(outdoor)-air-quality-and-health.
 “NAAQS table,” EPA. Environmental Protection Agency, Dec. 2016, [Online]. Available: https://www.epa.gov/criteria-air-pollutants/naaqs-table.
 “Air quality standards,” Standards - Air Quality - Environment - European Commission. [Online]. Available: https://ec.europa.eu/environment/air/quality/standards.htm.
 “China: Air quality standards,” Transport Policy. [Online]. Available: https://www.transportpolicy.net/standard/china-air-quality-standards/.
 “India: Air quality standards,” Transport Policy. [Online]. Available: https://www.transportpolicy.net/standard/india-air-quality-standards/.
 K. Daellenbach et al., “Sources of particulate-matter air pollution and its oxidative potential in europe,” Nature, vol. 587, pp. 414–419, Nov. 2020, doi: 10.1038/s41586-020-2902-8.
 “California air resources board,” Inhalable Particulate Matter and Health (PM2.5 and PM10) | California Air Resources Board. [Online]. Available: https://ww2.arb.ca.gov/resources/inhalable-particulate-matter-and-health.
 “Health effects of particulate matter,” World Health Organization. 2013, [Online]. Available: https://www.euro.who.int/__data/assets/pdf_file/0006/189051/Health-effects-of-particulate-matter-final-Eng.pdf.
 “What are the air quality standards for pm? | air quality planning unit | ground-level ozone | new england | us epa,” EPA. Environmental Protection Agency, Oct. 2019, [Online]. Available: https://www3.epa.gov/region1/airquality/pm-aq-standards.html.
 “Basic information about no2,” EPA. Environmental Protection Agency, Sep. 2016, [Online]. Available: https://www.epa.gov/no2-pollution/basic-information-about-no2.
 B. Paital and P. Agrawal, “Air pollution by no2 and pm2.5 explains covid-19 infection severity by overexpression of angiotensin-converting enzyme 2 in respiratory cells: A review,” Environmental chemistry letters, pp. 1–18, Sep. 2020, doi: 10.1007/s10311-020-01091-w.
 “Ground-level ozone basics,” EPA. Environmental Protection Agency, Sep. 2020, [Online]. Available: https://www.epa.gov/ground-level-ozone-pollution/ground-level-ozone-basics.
 G. Bekö et al., “Ultrafine particles: Exposure and source apportionment in 56 danish homes,” Environmental Science & Technology, vol. 47, no. 18, pp. 10240–10248, Sep. 2013, doi: 10.1021/es402429h.
 “Introduction to vocs and health,” Introduction to VOCS and Health | Indoor Air Quality (IAQ) Scientific Findings Resource Bank (IAQ-SFRB). [Online]. Available: https://iaqscience.lbl.gov/voc-intro.
 H. Fromme, “Beurteilung nach dem tvoc-konzept,” Gesundheit: Beurteilung nach dem TVOC-Konzept. Dec. 2013, [Online]. Available: https://www.lgl.bayern.de/gesundheit/arbeitsplatz_umwelt/chemische_umweltfaktoren/beurteilung_tvoc_konzept.htm.
 R. Castorina et al., “Volatile organic compound emissions from markers used in preschools, schools, and homes,” International Journal of Environmental Analytical Chemistry, vol. 96, pp. 1–17, Nov. 2016, doi: 10.1080/03067319.2016.1250892.
 C. Potera, “Scented products emit a bouquet of vocs,” Environmental health perspectives, vol. 119, p. A16, Jan. 2011, doi: 10.1289/ehp.119-a16.
 W. H. O. R. O. for Europe, WHO guidelines for indoor air quality: Selected pollutants. World Health Organization. Regional Office for Europe, 2010.
 S. K. Dimitrios Kotzias Kimmo Koistinen and B. Seifert, “The index project - critical appraisal of the setting and implementation of indoor exposure limits in the eu,” European Commission. 2005, [Online]. Available: https://ec.europa.eu/jrc/en/publication/eur-scientific-and-technical-research-reports/index-project-critical-appraisal-setting-and-implementation-indoor-exposure-limits-eu.
 “Residential indoor air quality guideline,” Health Canada. 2006, [Online]. Available: https://www.canada.ca/content/dam/canada/health-canada/migration/healthy-canadians/publications/healthy-living-vie-saine/formaldehyde/alt/formaldehyde-eng.pdf.
 E. Mansour, S. Curling, and G. Ormondroyd, “Absorption of formaldehyde by different wool types,” Sep. 2015.
 S. McNeil, “The removal of indoor air contaminants by wool carpet.” Oct. 2015, doi: 10.13140/RG.2.1.1155.6324.
 List of CARB-Certified Air Cleaning Devices. [Online]. Available: https://ww2.arb.ca.gov/list-carb-certified-air-cleaning-devices.
 “Carbon monoxide (co) poisoning in your home,” EH: Minnesota Department of Health. [Online]. Available: https://www.health.state.mn.us/communities/environment/air/toxins/index.html.
 U. Satish et al., “Is co2 an indoor pollutant? Direct effects of low-to-moderate co2 concentrations on human decision-making performance,” Environmental health perspectives, vol. 120, Sep. 2012, doi: 10.1289/ehp.1104789.
 A. Mainka and E. Zajusz-Zubek, “Keeping doors closed as one reason for fatigue in teenagers—a case study,” Applied Sciences, vol. 9, p. 3533, Aug. 2019, doi: 10.3390/app9173533.
 A. Mishra, A. Ruitenbeek, M. G. L. C. Loomans, and H. Kort, “Window/door opening-mediated bedroom ventilation and its impact on sleep quality of healthy, young adults,” Indoor Air, vol. 28, Oct. 2017, doi: 10.1111/ina.12435.
 “9 out of 10 people worldwide breathe polluted air, but more countries are taking action,” World Health Organization. May 2018, [Online]. Available: https://www.who.int/news/item/02-05-2018-9-out-of-10-people-worldwide-breathe-polluted-air-but-more-countries-are-taking-action.
 S. Jiang, M. A. Ali, and S. Ramachandran, “Negative air ions and their effects on human health and air quality improvement,” International Journal of Molecular Sciences, vol. 19, p. 2966, Sep. 2018, doi: 10.3390/ijms19102966.
 B. Lee, M. Yermakov, and S. Grinshpun, “Removal of fine and ultrafine particles from indoor air environments by the unipolar ion emission,” Atmospheric Environment, vol. 38, pp. 4815–4823, Sep. 2004, doi: 10.1016/j.atmosenv.2004.06.010.
 H.-J. Kim, B. Han, C. Woo, and Y.-J. Kim, “Ozone emission and electrical characteristics of ionizers with different electrode materials, numbers, and diameters,” IEEE Transactions on Industry Applications, vol. PP, Sep. 2016, doi: 10.1109/TIA.2016.2606362.
 “Airfree scientific test reports,” Breathing Space. [Online]. Available: https://www.breathingspace.co.uk/airfree-scientific-test-reports-i86.
 T. BATAKLIEV, V. GEORGIEV, M. Anachkov, S. Rakovsky, and G. ZAIKOV, “Ozone decomposition,” Interdisciplinary toxicology, vol. 7, pp. 47–59, Oct. 2014, doi: 10.2478/intox-2014-0008.
 B. Shi, L. Ekberg, A. Trüschel, and J. Gustén, “Influence of filter fiber material on removal of ultrafine and submicron particles using carbon fiber ionizer-assisted intermediate air filters,” ASHRAE Transactions, vol. 118, pp. 602–611, Jan. 2012.