What kind of air purifier I should buy?

by Maija, Santrupti engineers

This is the question that you see people asking daily in social media. It is difficult to give any list of “good room air purifiers” as there is no official room air purifier testing standards or third party verification schemes available yet. This is why real independent data for comparison is missing. But here are some thoughts related to the topic which may help you to compare different products and solutions.

I will start with some background information and later I give few tips to select an air purifier in India. First concern of course is that do I need an air purifier and will I lose my immunity when using it? Different air pollution impacts our body differently. Humans have a relatively good immunity against microbiological contamination that refers to the infectious or allergenic substances like bacteria, yeast, mould, fungi, virus and pollen or their toxins and by-products. But we have no “immunity” against particulate matter that we breathe in. Ultra-fine particulates travel deep into alveoli and stays there starting to reduce the active surface area of lungs leading to reduced lung capacity and asthma. Larger particulates are of less concern, although they can irritate eyes, nose, and throat. According to a study in Hong Kong, nearly 20% of ultra-fine particulate samples collected in the city carried hazardous metals. They get through lungs into our blood stream causing e.g. cardiovascular disease and cancer.

One of the most important considerations is to know what pollution needs to be removed from the air as that specifies the required filtration technology. Air pollution can be categorized to two main groups: particulate matter (PM) and gases.

There are different kind of particulates in the air: dust (solid matter that is big enough to be seen by eyes), fine & ultra-fine particulates (solid or liquid matter that cannot be sensed or seen by humans) and microbiological contamination (contain living organisms or were released from living organisms). The main sources of dust (particulates bigger than 75 µm), fine particulates (size ranging from 5 to 50 µm) and ultra-fine particulates (size of 2.5 µm or smaller, also known as PM 2.5) are traffic, biomass burning, construction works and some industrial processes. Also copy machines and printers releases ultra-fine particulates. Microbiological particulates covers the entire range of sizes starting from 0.01 µm viruses to 100 µm pollen.

Typical gases in the polluted outdoor air are e.g. ground-level ozone, nitrogen dioxide, sulphur dioxide, benzene, lead and carbon monoxide. These are generated in vehicle and power plant combustion and in some industrial processes. Also an open sewage drainages, polluted rivers and landfills releases harmful gases for humans.

Everywhere around the world particulates and gases are in the outdoor air. The main question is how much and what kind of gases, particulates and microbiological contamination? Outdoor air is the biggest source of indoor air pollution in case there is no air purification system in the air intake. Another main pollution group is the chemical and microbiological emissions (e.g. Volatile Organic Compounds (VOC), mould, fungi and bacteria) from construction materials, furniture and cleaning products. People produce carbon dioxide and can also be a pollution source as sometimes they carry viruses or wear strong perfumes (VOC).

Dust and particulate removal is mainly done using mechanical particulate filters that remove particulates from the air when particulates come in contact with the surface of filter media and adhere to the fibers. Filtration efficiency is depending on the density of media and tightness of filter casing (how much air is passing the filter without being filtered). Filters are classified based on the filtration efficiency to coarse, fine and HEPA (High Efficiency Particulate Air) filters. Coarse filters removes mainly dust (particulates that are bigger than 10 µm). Fine and HEPA filters removes efficiently also smaller particulates (0.1-10 µm) depending on the filtration class. Fine filters are in the range of EU 7 to EU 9 (European classification) or respectively MERV 13-15 (American classification).

As an example EU 9 / MERV 15 fine filter removes all particulates that are bigger than 2.5 µm and more than 95% of particulates that are size of 0.4 µm. HEPA filters are designed to remove 99.9% of particulates that are bigger than 0.4 µm. However due to more dense filter media, the pressure loss of HEPA filter is much higher than in fine filters and therefore the fan energy required to circulate the same amount of air is higher. HEPA filters are also more expensive than fine filters.

Mechanical filters are either filter media sheets or bags or pleated panels. Typically filter sheets are used only as coarse filters. Proper fine filters are either bag filters or pleated cassettes. Many filters are marketed as fine or HEPA filters, even they do not fulfil the international filter standards. In case filters have an independent third party certification (Eurovent or AHRI), you can be sure that the filter performs as promised. Good filter is also classified either based on EU or MERV rating system, not e.g. as 10 or 5 micron filter.

Another technology to remove particulate matter is to use electrostatic precipitator (ESP) device. It removes fine particulates and smoke from air by first negatively charging the dust particulates (ionization unit) and then collecting them using the positively charged collector plates, where particulates build up and form a layer of dust. Disadvantage of this technology is that the ionization unit creates ozone. According U.S. Environmental Protection Agency (EPA) ozone can be harmful to human health whether pure or mixed with other chemicals and therefore ozonizing air cleaners cannot be recommended in occupied spaces. However, ozone can be removed using carbon or chemical filter after ESP device. ESP unit requires weekly maintenance of dust collection tray and collector plates in order to avoid dust to be carried back to the room air.

Harmful gases are removed using carbon or chemical filters. Activated carbon has been used in the purification and filtration of gases for a long time. Gas molecules in an air stream enters the large pores at the carbon surface and moves towards the smallest pores in internal surface. There are also new innovative chemical filters available where mixture of carbon and other ingredients (like desiccant or metal silicate) are used. The right mixture of absorption materials is depending on the chemicals that needs to be removed from the air. Life cycle of carbon and chemical filters are depending on the amount of media in a filter and the volume of gases in the air passing through the filter.

Many other technologies used in room air purifiers are used to kill microbiological contamination that is alive and therefore to sterilize the air. Examples of such technologies are UV-lights, cold plasma technology, photo-hydro ionization (PHI), ozonizing and ionizing. Each technology has different impact on various microbiological contaminants and therefore cannot be directly compared. The main question is do they only kill the harmful contamination or do they also remove e.g. the good, healthy bacteria? These technologies have either no or very small impact on particulate pollution removal even some of them contributes in ultra-fine particulate removal. Ultra-fine particulates adhere and form larger particulates that are less harmful for our health and easier to be filtered. But separate particulate filter is required to remove them.

HEPA filters are used to remove both alive and dead microbiological contamination from the air as they filter efficiently also the smallest particulates (0.01 µm). This is the typical reason to use the HEPA filters in the room air purifiers – rather than the ultra-fine particulate matter (PM2.5) removal that can be done with fine filters too.

Typical outdoor air flora of microbiological contamination is not harmful to our health and the immunity of healthy people can protect against them. But in case there is moisture damages or mould growth inside the building, the harmful bacteria and mould spores are emitted to the indoor air. Same is with badly operated and maintained ventilation systems or any standing water that is a base for harmful bacteria growth like legionella. In these cases the air purifiers that kill and remove microbiological contamination is recommended to be used before the damaged materials has been removed. However, the preventive maintenance is the appropriate way to protect occupants from these rather than sterilizing the air unnecessarily in normal conditions.

Some of the latest “mould” research findings in North Europe recommend not to use any kind of sterilization in the occupied areas and even clean the spaces with water only. If the mould and related bacteria is sterilized either with cleaning products, sterilizing liquids or air purifiers, only the less harmful mould gets destroyed leaving more living space for the harmful moulds that will not die during the sterilization. Therefore after some time the toxic metabolic substances of moulds are rather increased in the air than reduced.

Another important principle to maintain clean indoor air besides the air purification is to pressurize the spaces. Positive pressure ensures that air leaks from inside to out preventing dirty outdoor air to enter inside the building. This requires the mechanical ventilation where fan supplies the cleaned air into the building. Unfortunately room air purifiers has no impact on pressurization as they only circulates the air inside the space.

What kind of air purifier is required in India?

In India we have extremely high levels of dust, fine and ultra-fine particulates in the outdoor air. In some specific areas we also have high levels of harmful gases. But the microbiological contamination in the outdoor air is in normal level and is therefore less of concern. But in the spaces with mould or mildew growth, there may be harmful levels of microbiological contamination.

First challenge in many room air purifiers is that they do not have a sufficient coarse filters to remove biggest dust particulates. If the purifier only have a fine or HEPA filter, it gets chocked very quickly and that increases the maintenance requirement and filtration cost. In case the filter is not changed often enough, pressure loss increases and air flow rate reduces. This reduces the filtration efficiency of unit.

In case the main concern is the removal of particulate matter, the HEPA filter is not necessarily required, but fine filter can do the job. However most of the room air purifiers has an integrated HEPA filter as the original application was to remove microbiological contamination in Western buildings with moisture damages and mould growth. HEPA filter can of course be used in India too, but the maintenance have to be sufficient and good coarse filtration is required before it.

Carbon or chemical filter is required if there are harmful gases in the room air or in case unit has the electrostatic precipitator or other technology that generates ozone. Ozonisers are not recommended to be used as room air purifiers in occupied spaces.

Most of the room air purifiers have various technologies to sterilize the air. They are only required in special cases if the microbiological contamination is very high or the user is already allergic or asthmatic. Additional technology makes the units unnecessarily expensive and each one of us should consider our immunity against microbiological air pollution. If we still have a good immunity, do we want to destroy it? Of course, we can always just not to use these functions in our room air purifier.

Typical room air purifier can clean the air in approximately 30-40 m2 and some bigger units slightly more. It is important to follow the manufacturers’ recommendations regarding the floor air, otherwise the filtration efficiency is only local near the air purifier and air stays polluted in the other parts of the space. If the doors are kept closed, each room has to have its own purifier.

Outdoor doors and windows have to be closed and well-sealed, otherwise the air purifier is not able to clean the air. However, this may lead to some other problems like increased carbon dioxide levels or humidity (insufficient ventilation). High carbon dioxide level creates drowsiness and headache for users and high moisture level increases the risk of mildew and mould growth.

Another option would be to install an ambient air purifier unit that takes sufficient amount of outdoor air, filters it and supplies it to the indoor spaces. This option reduces the filtration cost as only the minimum outdoor air requirement needs to be filtered and it prevents the infiltration of dirty outdoor air. Outdoor air supply also ensures low enough carbon dioxide level in the space.

As a summary, the good indoor air purification system consists of:

  • Correct air volume / size of room air purifier to clean the entire space;
  • Sufficient outdoor air intake to ensure low enough carbon dioxide and humidity levels indoors;
  • Coarse filters to protect the fine filter from dust;
  • Certified fine filter (HEPA not necessary) to remove fine and ultra-fine particulates;
  • Chemical filter in case the ambient air contains harmful gases.

It is also important to use low-emission building materials and cleaning products, remove internal pollution sources and prevent any moisture damages in the building structures.

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