Air Purifier Guide

How to Choose an Air Purifier for Your Needs

A comprehensive review of the best air purifier technologies available in Singapore today

General Questions

There are many situations where an air purifier will help.

People with allergies (e.g. a constant runny nose and allergic asthma) will see an immediate difference if they use a medical-grade air purifier, because these air purifiers can dramatically reduce the allergens in the air. Those having a cold might also feel better for the same reason.

Because medical-grade air purifiers can trap germs, they can reduce the airborne germ-count when someone is unwell, reducing the likelihood of someone else catching the cold or flu.
Simply put, no.

Even if the air purifier is using True HEPA filters (the same grade as cleanroom and hospital critical-area filters), your room will never even come close to a cleanroom. Why? Because those areas use large customised systems that flow the air in a very specific way (laminar airflow), and they need to pressurise those areas relative to adjacent rooms. They also require control measures like protective clothing that reduce normal contaminants entering the room.

All that is impractical and unnecessary for a normal home or office environment. Buying a good air purifier can reduce airborne contaminants in these areas, but they will never even come close to a cleanroom or hospital operating theatre.
People with allergies (for example, allergic rhinitis and allergic asthma), can tell very quickly whether an air purifier works for them. It’s a lot harder for others to tell whether the unit is performing as expected.

Generally, if someone has a medical condition, you would want a medical-grade air purifier. Otherwise, for general health, it’s fine to get something that works for them. This guide will review the various technologies and their pros and cons.
The key things to look out for in a good medical-grade air purifier:
  • True HEPA. It should use True HEPA filters, because this is the standard used in critical areas today, like cleanrooms or hospital operating rooms. A True HEPA filter will remove everything (germs, dust particulates etc.) with at least 99.97% efficiency. See below for more.
  • High airflow. A powerful (yet quiet) motor is required to move air in the room into the air purifier and through the dense media filter and send it back out. The more volume it can purify, the quicker and better the results. Unfortunately, this often means the unit has to be larger to be effective. It’s very important to select a model that’s suitable for the room size.
  • Safe from ozone and OH Radicals. Sensitive individuals and children with an existing medical condition would be more vulnerable to even minute traces of ozone or OH Radicals (and hydrogen peroxide) that are added into the air by some technologies. A good medical-grade air purifier will avoid any technology that produces any of these harmful contaminants.
Consumers need to be aware of the products with ionizing features, as these may release potentially harmful ozone in the indoor environment. They shoud also invest in an air purifier that has a HEPA and activated carbon filter.
Dr Ang Keng Been
President, Indoor Air Quality Society Singapore

Review of Air Purifier Technologies

Just like a bicycle is very different from a sports car, a simple ion-generator is very different from a True HEPA air purifier. The best air purifier filters available today use True HEPA filters (the same grade found in hospital critical areas and cleanrooms).

These days, most air purifiers will combine different technologies. For example, medical-grade air purifiers will often use both True HEPA filters for germs and sub-micron dust particles, and Activated Carbon for odour and VOCs (Volatile Organic Compounds).

The two broad groups of technologies today are:

Group 1: HEPA (and other Media Filters)

Group 2: Ion-Generators (and Ozone Producers)


Group 1: HEPA (and other Media Filters)

Media filters use a filter material to physically trap airborne contaminants, using various natural physical forces. They do not produce any by-products like ozone or OH (hydroxyl) radicals. The media needs to be changed when spent. These are the related technologies:

This is the highest-end technology in wide use today. This is the same grade of filters used in hospital operating theatres and ICU wards, and in industrial cleanrooms. True HEPA filters are able to trap germs, drying them out and eventually killing them within. These filters were first created by the US Military to remove sub-micron radioactive particles in the air.

To be classified as True HEPA, it has to be rated at least 99.97% efficient at filtering out particles at 0.3 micron using the Mil-Std 282 D.O.P. test.

Pros: Medical-grade filtration, 100% natural and safe. Highest grade of filtration.

Cons: Can be a bit noisy when operating at full-strength because air needs to move through the very dense filter. Once the filter is full, it needs to be replaced.
HEPA or HEPA-type filters use the same borosilicate material as True HEPA filters, but are lower-grade becasue they do not pass the 99.97% D.O.P. test at 0.3 micron. Their efficiency can range between 25% to 95% of that of a True HEPA

If a manufacturer’s packaging or manual does not state “True HEPA”, it means it is a HEPA-type.

Pros: Some could still be quite efficient (e.g. 95% by D.O.P. test). Uses 100%-natural means of filtration. Slightly quieter than True HEPA, less-powerful motors required.

Cons: not as efficient as True HEPA air purifiers. Once the filter is full, it needs to be replaced.
The filter fibres are given a permanent electrostatic charge which traps dust particles. The most common of this are 3M filters. Their maximum efficiency is normally about 60% of a True HEPA filter, with actual operating efficiency lower than that.

Pros: Cheaper to maintain. Smaller footprint, doesn’t require strong motor.

Cons: Efficiency is lower than True HEPA and HEPA filters. Once full, the filter needs to be replaced.
Activated Carbon naturally absorbs odours (haze or tobacco smoke) and VOCs (Volatile Organic Compounds, like formaldehyde). They act like sponges, with plentiful micro-tunnels that trap gas molecules. Activated carbon is a standard that’s used for this purpose in many applications today (e.g. car filters). Performance will vary with the amount of the actual carbon provided.

Pros: Very effective at removing odours and VOCs naturally.

Cons: Once the carbon is full, it will stop absorbing and has to be replaced.
When inhaled, ozone can damage the lungs. Relatively low amounts of ozone can cause chest pain, coughing, shortness of breath and, throat irritation. It may also worsen chronic respiratory diseases such as asthma as well as compromise the ability of the body to fight respiratory infections..
US Environmental Protection Agency

Group 2: Ion-Generators (and Ozone producers)

Ion-generators charge the air to produce negative (and positive) ions. They are generally good for killing germs and de-activating VOCs, but less efficient with removing  contaminants. The ionizing feature is economical as there are no filters to change (unless the units also use media filters). However, ozone is always continuously produced in the process, and some technologies also produce OH (hydroxyl) radicals; both of these are harmful contaminants in themselves. These are the related technologies:

Generally, these machines use needles or wires to charge the air with negative ions. The negatively-charged ions will be attracted to surrounding surfaces like walls and tables. Negative ions are associated with waterfalls and beaches, hence some people feel a bit more refreshed in the presence of an ioniser.

The simplest machines will just ionise the air around them statically without having a blower to move the air like most air purifiers. Others might use a blower and a main filter, and ionise the air that comes out of the machine.

Pros: Feel refreshed. No replacement filter required.

Cons: All ion-generators produce tiny amounts of ozone as a by-product, which is harmful to health. Actual air-cleaning efficiency is very low. Dirty patches gather around the ioniser.
Ozonizers produce ozone, primarily to de-activate odours and VOCs (Volatile Organic Compounds) in the room. For example, painters might use an ozonizer to neutralise the odour after the room is painted.

Because ozone also kills germs, ozonizers have also been marketed as air purifiers for home or office use. This is not the correct use of an ozonizer, as ozone is itself a contaminant. What makes it such an effective germ-killer also makes it dangerous to humans – causing respiratory symptoms, changes in lung function and airway inflammation. Ozone is listed by various government authorities, including Singapore’s National Environmental Agency, as a hazardous pollutant. Ozone in the upper layers of the sky (the stratosphere) is a good thing, blocking out harmful solar radiation. But at ground level, where people gather, ozone is harmful to health.

Pros: Effective for neutralising germs, odour and VOCs. No filter change required.

Cons: Ozone is harmful to humans (ozonizers can be used, just not with people around). Sometimes gases are broken down into more harmful components (e.g. tobacco smoke disintegrates into finer and more toxic particles. See Berkeley study shows ozone and nicotine a bad combination for asthma).
Plasma-based ion technology produces positive ions (H+) and negative ions (O2+ superoxides). These ion clusters are blown into the air where they become attracted to various airborne particulates and collide with them. The ion clusters also collide with each other. The collision of 2H+ and O2+ions produce very reactive hydroxyl radicals (OH radicals). Hydrogen peroxide (H202) may also be formed, but these will finally turn into OH radicals.

Pros: The unstable OH radicals easily oxidise germs in the air, killing them. Will also de-activate odours and VOCs (Volatile Organic Compounds). Like ionisers, this technology doesn’t require filter change (unless the unit incorporates filters).

Cons: Unlike the photocatalytic technology (see below) that produces OH radicals within the unit, the plasma-based ion system produces them directly around the room. It is well known that OH radicals are destructive (H202 are even more destructive), and have been linked to various ailments including cancer.* The long term use of this technology is still unclear. Also, tiny amounts of ozone are continuously generated when producing ions – ozone is itself a known harmful contaminant. As with ozonizers, sometimes when gases or odour are broken down, they can become even more toxic than before. If you feel unwell or feel irritation in the sinus, throat or lungs, consider switching off the ionising function.

* For example, Wikipedia: “The hydroxyl radical is extremely reactive and immediately removes electrons from any molecule in its path, turning that molecule into a free radical and thus propagating a chain reaction. However, hydrogen peroxide is actually more damaging to DNA than the hydroxyl radical, since the lower reactivity of hydrogen peroxide provides enough time for the molecule to travel into the nucleus of the cell, subsequently wreaking havoc on macromolecules such as DNA.”
Photocatalytic technology uses UV light acting against a semiconductor material like Titanium Dioxide (TiO2) to produce OH Radicals (hydroxyl radicals). These OH Radicals are very reactive and will quickly kill germs and other bio-aerosols. Whereas the Plasma Cluster technology produces these OH Radicals directly around the room, the Photocatalytic method only produces them within the air purifier, so that they react with germs and VOCs and break them down within the unit.

Pros: Effective in killing germs and other bio-aerosols. Also de-activates odours and VOCs. No filter changes required (unless unit uses media filters).

Cons: Unspent OH Radicals might be released into the air (if they do not have a chance to react with anything), though they are short-lived. OH Radicals are harmful to health. Also, tiny amounts of ozone are continuously generated to produce OH Radicals – ozone is also a known hazardous contaminant. Like ozonizer, be aware that sometimes contaminants may be broken down into smaller parts that are more toxic than before.


UV light is used to kill germs and other living organisms when air moves through the air purifier. This technology is primarily concerned with germs and does not address dust particulates or odours and VOCs. The UV tube needs to be replaced after a period of use. As UV light is harmful to health, care must be taken when handling the air purifier to ensure there is no leakage.

Pros: Very effective in killing germs and other bio-aerosols. No filter change required (unless unit uses additional filters)

Cons: Does not remove dust particulates nor address odour and VOCs (require additional filters to do so). Dead bio-aerosols are circulated back into the air.

True HEPA technology

True HEPA stands for High-Efficiency Particulate Arrestance or High-Efficiency Particulate Air. A True HEPA filter is made of borosilicate material, which is densely-packed glass fibres. A True HEPA filter cannot be called True HEPA unless it achieves 99.97% efficiency on the cleanroom-grade dioctyl phthalate (D.O.P.) or equivalent Sodium Chloride (NaCl) test. This is the definition of the filter.

In either test, neutrally-charged dioctyl phthalate or Sodium Chloride particles with a constant mean diameter of 0.3 micron (i.e. 3/10,000th of 1 mm), are passed through the filter. The capturing efficiency is then calculated. The diameter of 0.3 microns is used because this is the particle size that is most difficult for any filter to capture. Technically, 0.3 micron is the most penetrating particulate size. So measuring at 0.3 micron would give the minimum efficiency of the filter, as measuring at all other sizes would give a higher efficiency. This is the most conservative and stringent test.

Measuring at other particle sizes (e.g. 0.1 micron) would always yield a higher efficiency and does not give a true picture of the filter’s performance. A True HEPA will always be nearly 100.00% efficient at particle-size of 0.01 or 0.001 micron.
This would be true if the True HEPA filter was a sieve – like paper with tiny holes with the holes trapping particles But it’s not. True HEPA material is made of unwoven borosilicate material such that a sub-micron dust particulate entering the filter would be crashing into a complex system of tunnels intertwined throughout the depth of the filter.

The “tunnel size” is much larger than the sub-micron particles, so they do not actually get clogged that way (good pre-filters are necessary to filter out the larger-sized particles that could clog the True HEPA). Instead, the sub-micron particles get trapped when they collide with the “walls” of these tunnel-like systems, by three physical processes: impaction, interception and Brownian diffusion.

Also, because True HEPA filters come pleated, the actual surface area is many times larger than the cross-sectional area of the air intake. This greatly increases the filter capacity.

In this way, if prefilters are maintained regularly, the True HEPA filter can last a long time with normal use – 1 to 2 years, or more, depending on the filter size.
No, they wouldn’t.

Dust mites are generally 100-200 microns (1 micron = 1/1,000th of 1mm). They are generally too large to go airborne and be trapped inside any filter.

Germs do get trapped by True HEPA filters with a 99.97% efficiency, where they eventually die when deprived of moisture. This is why in critical-use areas like cleanrooms and Hospital operating rooms, True HEPA filters remain the primary filtration method today.