Testing air purifiers – Methods & Standards

This page is meant to give a background in practical terms of the difficulties for consumers and other end users as well as for manufacturers of air purification equipment as the methods and standards of testing are insufficient to give relevant information. Our aim is to contribute to highlight the need of an adequate universal test standard for portable air purifiers and consequently to inform of the deficiencies and sources of error of existing test methods used to rate such devices today. Our objective is to initiate and support the development of a new test standard, fair and trusted by the industry and consumers alike. We will encourage organizations, industrial participants, testing & certification institutions and media to become engaged in this project.


The objective of air purifiers in general

The objective of air purifiers is to minimize the hazardous health effect of polluted indoor air in a user friendly manner by continuously reducing the concentration of particles in the air to a minimum, especially the smallest particles (nano particles, below 0,1micron) as these are considered causing the most severe health problems.


The air purification solution

Source control and well functioning ventilation systems create the base of a healthy indoor air environment. However even when such systems are well maintained, they drive enormous volumes of particles through their filter banks in the supply air right into the building. Since people and their indoor activities generate millions of particles, normal ventilation is not sufficient. Indoor air purification is complementary and mostly necessary to reach an acceptable indoor air quality.


As the indoor air quality is very volatile and varies rapidly depending on a great number of factors, by principle any and all air purifiers must continuously be in operation to be able to keep the concentration of particles down. As soon as any such device has been turned off the indoor air quality deteriorates immediately.


A few air purification technologies dominate the market

• Conventional electrostatic precipitators also use a fan to produce an artificial airflow passing through a chamber where most particles get electrically charged before they continue into a collector cassette made to lead the particles to get caught by a counter pole surface. Such a cassette is mostly made of aluminium plates whereby every other plate is positive and the others negative. When the cassette is dirty, you can remove it, clean and use it again. This construction has a very low airflow resistance, hence minor noise or energy consumption problems. However the filtration effect reduces drastically as soon as the cassette is getting slightly dirty so in order to uphold a reasonable effect it needs continuous maintenance. And this construction generates ozone.


• Mechanical filter air purifiers use a fan to create an airflow through the filter. They often use filters similar to HEPA (High Efficiency Particulate Air Filters). True HEPA (or ULPA) filters make a lot of resistance in the airflow which consumes a lot of energy and causes noise problems. As they are very tight they are expensive and do not last long. But also other combinations of different mechanical filters are used, sometimes in combination with pre ionizing. All such filters get clogged over time which reduces the airflow so they must be replaced regularly.

• Air scrubbing means rinsing/cleaning air by pushing it through water, which means that it works as a humidifier at the same time. A major problem is to control or sterilize the water to avoid distribution of microbes. Mostly chemicals are added to the water for this purpose which generates distribution of such chemicals instead.


• Conventional ionizing air purifiers mostly produce volumes of electrons per second which generate negative ions that charge the particles in the air. Often it has a positive collector of some kind to attract the charged particles but they are also attracted to walls, floors, tables, draperies, electronic equipment, occupants, etc. This technique is silent and energy efficient. But it has been almost impossible to generate the necessary volume of ions per second to be able to reach out into a reasonable size room without generating a lot of ozone and without having serious problems with particles getting caught on different room surfaces making them really dirty.


• Ozone air purifiers release small amounts of ozone into the air to reduce airborne pollutants. They are especially efficient removing odours which is why ozone generators are frequently used in sanitation after fires. Sometimes a collector is used to attract particles. The ozone produced mostly exceeds the threshold value, which is considered hazardous to our health.


• Hybrid air purifiers use two or several technologies, mostly one of the above for basic particle reduction in combination with others.


• UV radiation is a well known technique to sterilize air e.g. over fresh fish, meat, vegetables etc. The technique is used with the same purpose inside air cleaners to kill microbes in the air flow.


• Photo catalytic oxidation uses UV radiation to activate titanium dioxide. The chemical process is supposed to remove odours from a number of gases as well as particles to a certain extent.


• Active carbon or similar reactive materials and/or adsorbents chemically react with certain gases, thereby removing odours e.g. from tobacco smoke. The effect is directly related to the mass (weight) of active carbon in the air flow. Air purifiers using this technology rarely have more than a small fraction of the mass necessary to have any substantial effect.


• Heating air until burning the particles in the air is in fact also in some sense an air cleaning method, however depending on what kind of particles are being burned and the corresponding consequences.


• Vaporization of water is actually the only hygienically acceptable method for humidification, being the natural generation of humidity over our lakes, rivers and oceans. Provided the vaporization is generated though a medium (filter) which is continuously fed by water to the extent it can vaporize and the filter never stands in water, this method also cleans the air as in nature. Water vaporizes in molecular form, which means that it cannot carry any microbes.

The process also requires energy which is taken from heat of the air which lowers the temperature by 5-6 ºC making it a cooling (air conditioning) method simultaneously.


Existing test methods & standards

The world market of portable air purifiers are dominated by US and Asia. In the USA the comparison tests based on more or less standardized methods have enormous influence and are thus important for both the industry and consumers. Below is a short documentation of existing tests mostly from the USA. Several of these are also internationally accepted.


CADR – Clean Air Delivery Rate

CADR is a method for testing the capacity to reduce smoke, dust and pollen particles in the 0.10 to 11 micron (µm) size range from the air. In ANSI/AHAM AC-1-2006, CADR is defined as “the rate of contaminant reduction in the test chamber when the unit is turned on, minus the rate of natural decay when the unit is not running, multiplied by the volume of the test chamber as measured in cubic feet.” The CADR uses cfm (airflow of cubic feet per minute) of air it cleans of a specific particle size, which necessitates that the device creates airflow mechanically.


Improved CADR – Clean Air Delivery Rate

This method is in principle as above but with a much more precise definition of size distribution of generated particles down to the particle size of 0.01 micron (µm) using particle counters able to measure down to this size and more sophisticated control measuring the air exchange. Certain aspects, however, are not well controlled e.g. the density of generated particles in comparison with “normal” particles.


Military Standard 2823 – DOP

It measures the percentage removal of 0.3 micron (µm) particles of dioctylphthalate (DOP). This test is used to rate high efficiency air filters, those with efficiencies above 98 percent, also referred to as “HEPA” (High Efficiency Particulate Air filters). True HEPA filters are defined to have a minimum particle collection efficiency of 99.97% DOP.


ASHRAE 52.1 & ASHRAE 52.2

52.1 test provided an efficiency rating such as a 30%, 65%, 90%. This was not considered to be sufficient since it did not specify the efficiency of filtrating different particle sizes. 52.2  thus was developed and shows the level of efficiency of the filter on specified particle sizes. MERV – Minimum Efficiency Reporting Value.


ASHRAE Standard 52-76 – Atmospheric dust spot test

This method is usually used to rate medium efficiency air cleaners (both filters and electronic air cleaners). The removal rate is based on the cleaner’s ability to reduce soiling of a clean paper target, a capacity dependent on the cleaner removing very fine particles from the air. The test shows the dust-holding capacity and its resistance to air flow. It should be noted that ASHRAE Standard 52-76 shows overall efficiency of removal of a complex mixture of dust, not specific particle sizes. A filter with an ASHRAE dust spot rating of 95 percent only removed 50-60 percent of particles in the 0.1 to 1 µm size range.


Air Purifiers America

Air Purifiers America tested models from several air purifier manufacturers with a high performance particle counter to measure the capacity to remove different size particles from the air. These particles are 0.3 microns and larger. Each unit was tested in a 132 sq foot room in their office with each model located in the same place in the room. The test lasted for 20 minutes with the device on high speed and the particle counter placed in the centre of the room with the door and windows closed. The particle count was taken after the period of 20 minutes. The air ventilation system was shut off during the testing so as not to influence the results. Before each unit was tested, the particle concentration in the room returned to the baseline particle level so each unit would start from the same particle concentration.


EN 1822 (European Norm)

Is a two part test, which identifies the particle size that penetrates the HEPA filter most easily, hence the name MPPS (Most Penetrating Particle Size).

EN 1822, which was released in the year 2000, is the most advanced and stringent air filter testing standard for particulate filters.


Consumer Reports

Methods and standards are in detail not known to us.


Why improve these methods & standards?

The objective is of course to supply consumers with more valid evaluation data to be able to compare different air purifiers using any combination of technologies i.e. to compare apples with apricots, oranges, bananas etc, and not only with other apples. Such testing standards are also fair and constructive for serious manufacturers. The different air purification technologies described above function differently and they all have advantages and limitations. The main objective is to effectively reduce the particle concentration in the air and keep it at a low level.


In testing we should focus on the sizes of particles which are the most difficult for our defence system. During the last few years it has become scientifically established that almost all serious health problems are caused by nano particles below 0.1 micron.

However we also should bear in mind that it is even more important keeping the concentration of such particles low, continuously and around the clock. Time is obviously a very important factor.

So what should be evaluated is performance over time with recommended maintenance as well as other features like removal of odours, gases (VOC/CAC) biomedical and physical/chemical effects. Testing should be carried out on maximum capacity provided however the noise is below an established maximum level which is acceptable for continuous 24 hour operation at work and into the bedroom at home. The corresponding cost should of course be properly established.



Some deficiencies in testmethods and standards

The performance of air cleaners in removing particles from indoor air depends not only on the air flow rate through the purifier and the efficiency of its particle capture mechanism, but also on other vital factors. Among these are technological factors and other more user friendly factors, all crucial to obtain the long term objective of the air purifier and enable a fair comparison between air purifiers on the market.

Most tests, like for example CADR, are developed in a way that does not apply to all air purification technologies and products and they do not provide the most relevant consumer and user oriented evaluation. Many actually give misleading results in both excessive positive and negative directions. Since CADR and other test have become some sort of standard this is not sufficient neither for consumers nor manufacturers. Further, most tests give consumers an idea of the cleaning capacity when the purifier is new, clean, operating at its highest level and equipped perfectly with a new filter if it is a mechanical filter device or a new clean collector cassette if it is an electrostatic precipitator. Such test results are not relevant for performance over time.



Examples of other important aspects to consider are:


• What is the degradation rate of efficiency over time provided the maintenance and filter exchange is handled according to recommendations (i.e. how well does the device work over time)?


• What is the corresponding cost over time for recommended filter replacement and maintenance?


• How much maintenance is required and how complicated is it?


• What is the noise level of the air purifier have when it is working at top capacity (the one that is evaluated during tests) and is this noise level to be considered acceptable or is it preventing activity and causing interference with sleep and work?


• What is the performance when the purifier is working at an acceptable noise level for continuous around the clock operation?


• Is the test performed at a long enough time period to ensure that the product has had the possibility to attain its best results and can the required time that the air purifier needs to be able to perform at its best be considered acceptable for the consumer when using the product?


• How are particles for testing purposes being generated and from which chemical substances?


• What is the distribution of sizes of such generated particles?


• Are such particles similar to typical real life indoor air situation?


• Do they get electrically charged like typical real life indoor air particles?


• What is the density of such particles?


• How much ozone is generated by the air purifier?


• How effectively does the air purifier collect different sizes of particles and should the air purifier particle reduction be rated according to the accumulated mass of particles, the accumulated surface of particles or the number of particles collected or a combination?


• What is the characteristics of the particles used during the test and can these be comparable to real life particles of the same size i.e. same density = mass, similar capacity to be electrically charged etc.?


• Do organic particles like virus, bacteria and fungi stop growing once collected by the air purifier? Or does the device stop the growth of such microbes while in operation?


• Is there any proven biomedical and/or physical/chemical effect?


• What is the energy consumption and the cost related to it?


• What is the airflow rate generated by the air purifier?

• How well does the air purifier clean the air in an entire room rather than just the air closest to the unit and how well is the air leaving the device mixed with air in the room before entering the device again (recirculation problem)?


• Is any air entering the unit bypassing the internal capture mechanism?


• What is the air circulation in the test room and the air exchange per hour?


• Size of the test room?


• How is the air purifier placed in the test room?


• How well if at all does the air purifier remove gaseous pollutants?


• What is the size of the air purifier and how well does it fit in to peoples home environments?


There are more aspects to ad to this list and we are working on this in cooperation with authorized test institutes and key professionals. We hope and believe that media, organizations, government agencies and others also will become engaged in this objective.


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