If you haven’t read our article series on “Which Air Purifier Should I Choose for My Home?”, it’s a good place to start.  Part 1 is on Air Flow, and Part 2 is on Air Cleaning. Once you have a method of air circulation and ventilation in your home (these are necessary to make the most of an air cleaner), an air cleaner(s) can be installed to remove contaminants.  Air cleaners or purifiers work by different methods and it seems that new methods are being invented all the time.  Some use several technologies, so it can be a bit confusing!  We’ll sort through them here. 

Filtration permits fluids such as air to pass through but retains solid particles, and filter science is more complex than it appears on the surface.   Although we often think of filters as sieves that have holes that are smaller than the particles they capture, in actuality filters can “catch” particles in four different ways, and even catch particles that are smaller than the holes (which would not happen if the filter were just a sieve).  For more information on the different ways that filters capture particles, check out our article “Which particles can get captured by a filter or by my lungs?”  Here are some common filter types:

• HEPA filters, standing for High Efficiency Particulate Air system, were first invented in the early 1940’s by the Manhattan Project as a way to reduce the spread of radioactive contaminants (high-efficiency particulate air system).  Since then they’ve been used in many different industries from aircraft to hospitals, and now are very popular in homes.  They are designed to capture at least 99.97 percent of fine airborne particles larger than at least 0.3 microns (millionths of a meter).  In residential use, they can be installed in other appliances like vacuum cleaners.   Air must be pulled or pushed through the filter by fan(s) in order to be cleaned.  The filter portion of many HEPA cleaners is disposable, so that all contaminants removed are disposed of with the filter and a new filter is installed.  
• Electrostatic filters: These type of filters have a long life because they can be cleaned and reinstalled.  They consist of two parts: an ionizing section, which uses high-voltage wires to impart a charge to the incoming particles, and a collector section which uses plates with the opposite charge to attract and retain the particles until they are cleaned.  Electrostatic filters are not as efficient at capturing very small particles as HEPA filters.  
• VOC filters:  We have more information about VOCs, or volatile organic compounds, here.  Since VOCs are gasses, not particles, a VOC filter is actually removing chemicals from the air by binding them to the filter.  Activated carbon is a common material used to remove VOCs.  It is extremely porous and the compounds that make up the VOCs are attracted to the carbon atoms, adsorbing (sticking to the outside of the carbon atom) until all of the empty spots for adsorption are full. 

Germicidal elements

When I was growing up in a rural area with dogs and cats in the house, fleas were more than an occasional problem.  If you let the fleas remain on pets for too long, they multiplied and infested wherever they could live comfortably–in carpets, clothing, beds, etc., generally making life miserable for everyone in the house!  We had to have a strategy to keep them at bay so that they didn’t have a chance to multiply using flea collars, topical flea medicine, internal flea medicine or flea aerosol “bombs” (which in retrospect were pretty toxic to our home).  With microbes, it’s the same thing– although a microbe or mold spore here or there can still “bite” you, it’s the proliferation of them that can really make life miserable.  Therefore purifiers are equipped with various germicidal or anti-microbial elements that act as “poison” to microbes, but safe for humans, pets and food.  

Unlike HEPA filters, these types of technologies don’t depend on large fans to pull all the air in the house through the purifier.  Instead, they just pull in a small airflow and use it to send out a stream of elements that disperse and deactivate contaminants throughout the space.  Bipolar Ionization works in this way.  The ions are generated when high voltage emitters split water vapor in the air (humidity) into positively charged hydrogen ions (H+) and negatively charged oxygen molecules (superoxide, O2-) and negatively charged hydroxyl ions (OH-).  Because of their charge, the ions can do several things: 

• Cause dust particles to stick together (agglomerate), so that they become large enough to be trapped in a separate filter, or fall to the floor where they can be removed
• Remove hydrogen molecules from the surface of microbes, which damages the surface and other parts and renders them unable to infect.   
• Interact with VOCs to change them into less harmful compounds
• When the hydrogen and oxygen ions draw back together in an effort to reform into H20 molecules, they also attract airborne particles like dust, pet dander, allergens, mold spores and even droplets containing viruses.

Hydrogen peroxide is H2O2, and although it’s chemically similar to water and decomposes into water, it’s a much more powerful cleaner than water.  Hydrogen peroxide is a very reactive compound that oxidizes (steals electrons) from substances in which it comes in contact.  The good news is that it’s a safe disinfectant in small concentrations, and can last about 30 minutes in the air depending on temperature, humidity, and reactive contents in the room.  Although it’s expensive and energy-intensive to make and transport H2O2 in bulk, several air purifier manufacturers have devised methods to produce and distribute it from their purifiers to safely kill germs in the air and on surfaces.  Photocatalysts and small, high-voltage currents are two methods of generating the H2O2; for more information check out our article Hydrogen Peroxide as an Air Cleaner. 

Ozone (O3) is another powerful oxidizer.  In higher concentrations, it can be used without personnel present to sterilize medical equipment and deodorize furnishings like cars and hotel rooms.  Since oxygen is most stable when it exists in pairs, ozone transfers the extra oxygen molecule to “stinky” compounds such as cigarette VOCs, changing their chemical makeup and thus deodorizing them.  In this way it’s more effective than deodorants that mask or encapsulate the odor molecule.  However, ozone can damage certain materials and is a respiratory irritant and can’t be used in the presence of people with respiratory conditions or asthma.   Some ionizers that produce a corona discharge, as well as some hydrogen peroxide generators emit ozone as a by-product, so their use must be strictly regulated (HypoAir’s devices do not produce a harmful level of ozone).     Scientists have long been aware of its air-cleaning capabilities high in the atmosphere (10-30 miles above the earth’s surface), but it also has the capability of being a ground level health hazard when pollution from cars and industry react chemically in the presence of sunlight. (Ozone and Your Health)  For these reasons, the California Air Resources Board (CARB) has set a limit of 50 parts per billion for max air cleaner emission concentration. 

Ultraviolet light (UV light) is often used in air purifiers to deactivate microbes.  We explain more about it is in this article.  UV light’s efficiency in deactivating microbes is highly dependent on the wavelength of the light, intensity, distance and residence time (how long the contaminants are exposed).  You may know that looking directly at UV light is dangerous to your eyes and skin, but if you are out of “line of sight” of the light, it is not harmful.  Therefore in order for it to be effective, UV light needs direct exposure.  It is generally installed inside purifier chambers, inside air conditioning systems, or atop fans pointing toward the ceiling.  It can be used to sterilize medical equipment and surfaces when people are not present, but again, anything that is not exposed to the light is not affected.  A promising area of research is finding and applying “safe” wavelengths (such as 230 nm or less, and specifically 222 nm, which do not damage skin or eyes) so that germicidal lights can be employed in the presence of people.   Because of these limitations, UV is generally used with at least one other technology in order to overcome its inability to kill “around corners” and at farther distances and shorter residence times.

Photo by Nubelson Fernandes on Unsplash