HEPA filtration plays an important role in improving indoor air quality in domestic, medical and industrial settings. Leading hospitals around the world trust HEPA air purifiers for their airborne hygiene and infection control needs. The Environmental Protection Agency confirms that HEPA filtration is the most effective and reliable way of dealing with airborne particle pollution.
HEPA stands for High-Efficiency Particulate Air and is a part of most types of high-performance mechanical air purifiers. HEPA filtration may be combined with other filtration, such as gas absorbing filters or microorganism destruction systems. A HEPA filter actually consists of a web of glass fibres capable of trapping close to all sizes of particle pollution. HEPA filters are found in both fitted installations that combine ventilation, heating and air conditioning, and in portable air purifiers. But of course, HEPA air purifiers are not the only type of air purifier – there are also electrostatic systems and ionisers as well, which work in a different way.
A true HEPA filter is defined by meeting the specifications of the United States Department of Energy for DOE regulated applications. A true HEPA filter has to be able to remove 99.97% of airborne particles of size 0.3 micrometres diameter passing through it. A HEPA filter can, of course, remove particles bigger as well as smaller than this, but it is the 0.3-micrometre particles that are hardest to trap in the filter, so that is why these are the particles used to judge performance. Air purifiers labelled ‘HEPA-type’, ‘HEPA-like’, ‘HEPASilent’ or even ‘99%-HEPA’ will most likely not meet the same performance standards as a proper HEPA air purifier, but are most of the time just misleading terms of art that marketing departments come up with to increase sales.
During the late 1940s, the United States Army Chemical Corps and the United States Atomic Energy Commission developed the first HEPA filters. The main purpose was to protect against the spread of radioactive dust during the development of the atom bomb. These early HEPA air cleaners were rigid and unwieldy machines, quite unlike their modern counterparts. They actually contained asbestos imported from Africa or Bolivia. Concern over the health impact of asbestos led to the development of alternative materials. There were also concerns about leaks and defects in the filter, so again research was needed into a better HEPA filter material.
During the development of the atom bomb, HEPA technology was considered ‘top secret’ but, after the war, the government declassified the technology so it could be used in commercial and residential settings, as it is today.
The HEPA filter media is a specialised filter consisting of a folded mat of microscopic borosilicate glass fibres. It has three different modes of action, each of which acts best upon a different size of a particle. Interception deals with particles above 0.4 microns in size and involves the particle sticking to a fibre in the HEPA filter when it comes close to it. In impaction, which also mainly affects particles above 0.4 microns, the particle is blown towards the fibres by the airstream in which it is travelling. It then collides with the fibres and sticks fast to them. Finally, there is diffusion, which mainly affects smaller particles, of size below 0.1 microns. The particle collides with air molecules that throw it off course. Diffusion keeps the particle within the filter until it is captured by impaction or interception.
What about particles at around 0.3 microns? They are captured, but not as efficiently. The 0.3-micron 'window' is known as the most penetrating particle size (MPPS), because particles in this size range penetrate further into the mesh before being trapped than smaller and larger particles do. In fact, the ability of HEPA filters to capture particles of size 0.3 microns is actually used as a measure of its effectiveness. The DOP penetration test involves sending a test stream of particles of this size through the filter and using a particle counter to measure the particles in the ongoing stream and outgoing stream to see what proportion is removed by the filter.
A HEPA air purifier basically uses the same principle as a spaghetti strainer - it holds back everything that is too big to fit through. The benefit of this kind of filtration is that it does not lose its filtration efficiency over time; a true HEPA filter becomes more efficient in filtering air pollution. The airflow delivery rate of an air purifier using mechanical filtration might decrease, but the filtration efficiency will increase. This makes mechanical HEPA filtration the most reliable filtration technology because you know that the air that goes through the filter is always cleaned at the same high rate, whereas technologies such as ionisation generally suffer from a significant loss in filtration efficiency over time.
As mentioned before, the term “HEPA” filtration is unfortunately not properly regulated. Every air purifier manufacturer can use the term "HEPA filtration" as they please. So look for test results and product reviews from an independent air purification expert before purchasing it.
A HEPA air purifier will remove airborne particles over a wide size range. Examples include:
First, be aware that efficiency and effectiveness, although they sound similar, are not the same thing when it comes to choosing a HEPA air purifier. Efficiency is usually given as a percentage and it is a measure of the ability of the air purifier to remove particles from the air passing through it. The effectiveness is a measure of the air purifier's ability to reduce airborne particles in an occupied room.
Efficiency depends on both airflow rate through the purifier and on the particulate load.
Effectiveness depends upon efficiency, the amount of air being filtered, and the path the clean air follows after leaving the filter.
So a highly efficient filter could actually be quite ineffective if the amount of air it has to filter is large and the airflow rate low.
A number of methods have been developed to determine the efficiency of HEPA filters installed in heating, ventilation, and air conditioning (HVAC) but these are not necessarily applicable to HEPA filters in portable air cleaners. Tests can also be done using a particle counter, which is a laser-based device that can detect the density of particles in a room. Such tests are a very accurate way of measuring the impact that an air purifier has on a room's air quality.
There is also a measure that has been developed by the Association of Home Appliance Manufacturers called the Clean Air Delivery Rate (CADR). Important to note here is that this is the Association of Manufacturers that have come together to develop a standard to test their own products - so not an independent standard as we would like to see. Nevertheless, CADR is used by the Environmental Protection Agency and the Consumers Union to compare air purifiers. It gives measures only for dust, tobacco smoke, and pollen removal. CADR does not give you any information about the long-term performance of the air purifier (units are only tested for 120 seconds – yes, seconds. Not days or weeks as it should be). That means that some technologies - such as ionisation - can have a great CADR rating, even if the ioniser's filtration efficiency would decreases by 80% within a couple of days or hours for example.
If you want to find out more about CADR and its problems, visit our blog post CADR – Clean Air Delivery Rate.
What are the possible limitations of a HEPA filter?
An air purifier fitted with a HEPA filter will primarily remove airborne particle pollution. It will only remove some of the gaseous pollutions, like Volatile Organic Compounds. To effectively remove gaseous pollution you will need a system that also incorporates high quality activated carbon which will absorb gases.
A drawback of true HEPA filtration is that the air purifiers tend to be larger than other air cleaners. The larger size of a true HEPA air cleaner is required in order to be able to provide the desired airflow rate. You should be sceptical of an air purifier that is very small because it is doubtful that a small unit will move enough air to make a significant difference in air quality – even in a very small room. We recommend that a unit should clean the given room at least twice an hour. If the air purifier’s filtration efficiency is low, several more air exchanges will be needed.
No air purifier removes settled particles or dust unless they become airborne again. That is why they are called ‘air purifiers’ and not ‘dust collectors’. So while an air purifier can effectively take contamination out of the air, it might not necessarily create a dust free environment (depending on the particle size that the dust consists of).
Some of the most recent clinical studies are:
What are the main applications of a HEPA filter?
Modern HEPA air purifiers can be found in any industry or sector where airborne contamination can be an issue, such as healthcare, hospitality, office environments and manufacturing. In the home, portable air purifiers and vacuum cleaners fitted with HEPA filters can be used to remove dust containing house dust mite allergens, pet dander, pollen, mould, and traffic pollution which might otherwise provoke asthma or allergy symptoms.
The IQAir HyperHEPA filter can remove particles down to 0.003 microns in size with a guaranteed efficiency of more than 99.5%. This is 100 times smaller than the lower limit of other HEPA air purifiers and 10 times smaller than a virus. The significance is that the HyperHEPA filter operates on the nanoscale, dealing with the ultrafine particles which other air purifiers cannot remove. The smaller the particle, the further it can penetrate into the body and therefore the greater the potential health risk. The performance of the HyperHEPA can also help create the ultra-clean environments required in certain industry sectors.