There are currently 5 different air purification technologies used in air purifiers. HEPA/mechanical filtration, activated carbon, UV light, ionization, and ozone generation.
1. HEPA / Mechanical Filtration
HEPA (High Efficiency Particulate Air) filtration is a type of mechanical filtration that meets a high standard of the particulates it removes from the air. In order for a filter to be classified as HEPA it must be tested and confirmed to remove at least 99.97% of particles down to 0.3 microns - no easy task. Some air purifiers have misleading claims about using HEPA filtration although the actual performance is significantly less. They often call these filters "HEPA like" or "HEPA style" despite not even being close to meeting the HEPA standard.
Mechanical filters, both HEPA and non-HEPA, are made of fabric-like materials and operate by removing particulates from the air by simply catching them in the fibers. The air molecules are allowed to pass through the filter material but the larger particles get trapped in the filter's fibers much like pasta gets caught in a strainer.
Depending on the efficiency rating, mechanical filters can remove particles such as pollen, dust, mold spores, fiberglass dust, asbestos, smoke, radioactive fallout, pet hair and dander, dust mite allergens, bacteria, and even viruses. These filters are rated based on standards for the size and percentage of particles they remove from the air. In the United States, it's common to see filters rated on the MERV scale, which is derived from the ASHRAE 52.2 test standard. The higher the MERV rating, the higher the smaller the particles the filter removes. MERV 8 filters are sufficient for capturing pollen, cement dust, and mold spores, whereas MERV 16 filters are used for capturing tiny particles such as bacteria and smoke and can be used for keeping hospitals and surgery rooms clean. HEPA filters are tested according to IEST-RP-CC001.6 standards.
Pros: Eliminates the most and the smallest particles from the air 100% safely.
Cons: Restricts airflow.
2. Activated Carbon
Activated carbon, or activated charcoal filters, have been used for about a hundred years for purifying both air and water. It was found to be useful in gas masks to protect soldiers during WWI. Activated carbon is carbon that has been processed to give it a large surface area of microscopic pores for adsorbing pollutants.
It works by capturing microscopic contaminants in the tiny pores throughout the carbon in a process called adsorption that are just too small for mechanical filters to capture. The primary use of activated carbon filters is to remove volatile organic compounds (VOCs), odors, and smoke. VOCs such as formaldehyde are common inside homes and can cause breathing issues. Sources of VOCs include building materials, cleaning products, fabrics, and furniture.
Some mechanical filters have a slight dusting of carbon added to them, which offers little surface area to catch pollutants. On the other hand some air purifiers, such as Aspen, use several pounds of activated carbon granules for maximum effectiveness at removing odors and VOCs.
Pros: Removes extremely small pollutants and chemicals such as VOCs and odors, and is 100% safe.
Cons: Restricts airflow.
3. UV Light
UV light is a high energy form of electromagnetic radiation that is not visible to the eye. The idea is that it can be used in air purifiers to kill bacteria and viruses in the air. However in reality there are some challenges to this.
Not all UV light is even capable of killing bacteria and viruses. It has to be the highest energy wavelength of UV called UV-C. For reference, UV-A is what's used in blacklights and makes up 95% of the UV in sunlight and has a wavelength of 400-315 nanometers. UV-B makes up 5% of sunlight and is what gives you sunburn and has a wavelength of 315-280 nanometers. UV-C has a wavelength of 280-100 nanometers and is not found in sunlight, which is good since it would cause serious burns and cancer.
In order for UV to damage bacteria and viruses, it must be UV-C. That poses a few issues. First, UV-C is dangerous to humans, causing serious skin burns and eye damage. Second, a byproduct of UV-C is ozone production, which as we'll see below, is harmful to human health. And most importantly, in order for UV-C to actually kill or damage bacteria and viruses, the microorganisms must be exposed to the UV for a long period of time, not just the fraction of a second the air is exposed to the UV while travelling through an air purifier. The exposure time is simply too short in order to kill germs.
Pros: Would be able to damage bacteria and viruses if they were exposed to the UV for a longer period of time.
Cons: In practice does not kill bacteria and viruses, can produce harmful ozone, can burn skin and eyes if exposed to humans.
4. Ionizers
Air ionizers or electrostatic air cleaners use electricity to electrically charge molecules in the air. These charged particles, ions, collect particles in the air and eventually find a surface in the room to stick to. Basically ionizers take the pollutants out of the air and put them on your walls and furniture. Of course when the charge neutralizes the pollutants are released back into the air.
In practice most residential ionizers are not even powerful enough to get that many particles to stick to the walls and have been proven to have no effect on airborne particulate levels.
Pros: May take some particles out of the air and put them on the walls.
Cons: May take some particles out of the air and put them on the walls, causing a buildup of dirt that is nearly impossible to clean.
5. Ozone
Ozone generators are touted as being able to clean the air, and there are many of them on the market. Despite these claims by the sellers of these devices, both health professionals and government agencies agree that they are unsafe.
Ozone can neutralize some chemicals, however it can take months for this process to occur. The negative health effects of breathing ozone can include chest pain, coughing, airway inflammation, reduced lung function, and even lung tissue damage. The effects of ozone are bad enough that states like California have banned the sale of units that produce it either intentionally or as a byproduct of UV light.
In addition, ozone does not reduce the amount of particulates in the air.
Pros: Can neutralize some chemicals given enough time.
Cons: Extremely bad for human health, causing respiratory issues.
Conclusion
Since UV lights, ionizers, and ozone generators all are not very effective at cleaning the air and pose health risks to humans, we design our air purifiers without those. We use the only two methods that are proven to be the most effective at cleaning the air and 100% safe - HEPA filtration and activated carbon. Sometimes the most elemental technologies are the best.
