Indoor Air Quality is a convoluted topic on the best of days. The IAQ segment of the HVAC industry has grown tremendously overall and sees spikes of urgency during certain health crises like the annual flu season.
There are many types of IAQ technology available, and each possesses its own advantages and disadvantages. It is important to have knowledge of these technologies and be able to discuss opportune applications for them.
There are two main categories of air quality that are of concern: particulates and germicidal. Each must be treated in a different way.
Prior to 2015 the accepted method of managing indoor air quality per the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) was centered around the concepts of dilution and filtration. With dilution, fresh outside air was introduced into a space in order to dilute possible stale or poor-quality air in a conditioned space. This method is very useful in maintaining higher air quality but has its own drawbacks in certain climates due to the introduction of humid fresh air. For this method to work, there must be a means of treating the incoming air prior to allowing it into the conditioned space. Filtration is still a crucial element in both the previous standards of addressing air quality, as well as newer methods being explored today. Filtration is centered on the reduction of airborne particles called particulate matter.
Particulate matter is the mixture of solid particles and liquid droplets found in the air. Common examples of particulates are dust, dirt, soot, and smoke. Other types of particulate matter may be formed in the atmosphere and are the result of chemical reactions from pollutants like sulfur dioxide and nitrogen oxides; these are common byproducts of manufacturing, power generation, and everyday motor vehicle operation. The particulates are very small, which allows for them to easily be inhaled during normal breathing.
To illustrate the size of some particulate matter let’s consider the size of a single human hair … it’s diameter on average is about 60 microns. Particulate matter is considered inhalable at sizes of 10 microns and smaller. Many particulates may have sizes of 2.5 microns and smaller. These microscopic particles can easily work their way into our lungs and in some cases even our bloodstream. The best way to combat particulates in a space is through filtration.
Filtration allows us to remove particulates from the air. Commonly, filters use a disposable media that catches particulates until full at which time they are replaced.
Another method is to utilize an electronic air cleaner. Electronic air cleaners use a pre-filter for removing large particles from the air, then the air passes through ionizing wires that charge particulate matter with a positive or negative charge. The particulate matter is then collected on metal collecting plates charged with the opposite charge from the ionizing wires. Lastly, there is a post-filter designed to catch any remaining particles not caught on the collector plates.
Each of these methods has its positives and negatives. The disposable media requires no maintenance but does have a replacement cost and will have to be performed at regular intervals (each home is different … change your filter when it’s dirty). The electronic air cleaner has no replacement cost but does require regular maintenance to clean the filters and plates. Both methods are very good for controlling particulate matter in the airflow while neither do anything, on a germicidal spectrum, to purify the air.
The filtration ability of a device is rated in a “MERV” rating. Minimum Efficiency Reporting Values, or MERV, reports a filter’s ability to capture larger particles between 0.3 and 10 microns (µm). MERV ratings are generally a lot like SEER ratings in that they hold little actual value to the average person, but they do allow for multiple products to be compared on a standardized scale. This can be extremely useful when deciding which filtration device, you would like to invest in. The below scale shows the technical criteria that must be met for the various MERV ratings:
|Average Particle Size Efficiency in Microns
|Catches less than 20% of particles 3.0 – 10.0 Microns
|Catches 49.9% of particles 3.0 – 10.0 Microns
|Catches 84.9% of particles 3.0 – 10.0 Microns
|Catches 85% or greater of particles 3.0 – 10.0 Microns
Catches 50% – 64.9% of particles 1.0 – 3.0 Microns
|Catches 90% or greater of particles 3.0 – 10.0 Microns
Catches 80% – 89.9% of particles 1.0 – 3.0 Microns
|Catches 90% or greater of particles 1.0 – 3.0 Microns
Catches 75% – 84% of particles 0.3 – 1.0 Microns
Catches 75% or greater of particles 0.3 – 1.0 Microns
The area of germicidal air quality is the niche of the industry that has seen lots of attention and innovation in the past decade or two. Companies have developed a plethora of products designed to clean the air in a home. The immediate front runner when discussing air cleaning is Ultraviolet (UV) Lights. UV lights can be used as a disinfection method for air. The term is Ultraviolet Germicidal Irradiation and it utilizes UV-C light to kill or inactivate microorganisms (bacteria, viruses, mold, and other pathogens). Exposure to UV-C light destroys the nucleic acids needed for microorganisms to survive.
While there is a strong need for UV lamps inside of HVAC equipment, they are often marketed as air cleaners when the technology will have limited positive impact on fast-moving air in a duct system. The effectiveness of germicidal UV depends on the length of time a microorganism is exposed to UV, the intensity and wavelength of the UV radiation, the presence of particles that can protect the microorganisms from UV, and a microorganism’s ability to withstand UV during its exposure. For these reasons, UV is not as effective on moving air. However, a UV lamp is an excellent stationary cleaner that can be installed on an evaporator coil, drain pan, or blower compartment in order to provide constant UVGI exposure to these components. This constant exposure will provide a much higher rate of disinfection on areas of HVAC equipment that are often exposed to damp conditions proven to allow for microorganism growth.
While UV-C Light on its own is not an ideal air cleaner, it can be paired with a catalyst in order to form a very effective air cleaner known as a Photo Catalytic Oxidizer (PCO). Various materials may be used as a catalyst, one of the more common is Titanium Dioxide. When UV-C light hits Titanium Dioxide it creates a radical hydroxyl that attacks larger pollutant molecules and effectively breaks them apart into harmless substances like carbon dioxide and water vapor. The advantage of this type of air cleaner is that it actively destroys pollutants as opposed to even the very best of filtration devices which can only collect and hold these pollutants. The maintenance is relatively low. The titanium dioxide component generally does not need to be replaced and the UV lamp lasts 2 years with normal use. This air cleaning technology is very appealing as an active germicidal cleaner that provides a noticeable effect in conditioned spaces.
However, there are two potential disadvantages of PCO devices. The first is that there is a small amount of ozone produced during the process. Ozone is a carcinogen and is deemed harmful by the EPA. PCO devices only produce ozone as a byproduct and not as a means of air cleaning. Manufacturers are required to keep their products ozone emission below FDA mandated levels of .05 parts per million by volume. Regardless, some people are very sensitive to ozone and may even find these low levels to be bothersome.
The second is that there is concern over what the long-term effects of being regularly exposed to hydroxyl radicals will be. A hydroxyl radical is a Reactive Oxygen Species (ROS) which is an unstable molecule with an unpaired electron. As this highly reactive molecule searches for an electron to “steal” it converts whatever it steals an electron from into a “free radical” molecule that then continues the process; the cycle continues until it is ultimately quenched by the ROS coming into contact with a molecule that serves as an antioxidant and stabilizes the ROS.
There is concern among many health experts and scientists over potential DNA damage to humans caused by a high concentration of hydroxyl radicals as well as the unknown byproducts of the reactions created by these ROS molecules. Some studies have suggested the byproducts could indeed be more harmful than the original pollutants. These are clearly considerations that should be noted by consumers when selecting an air cleaning product for their home.
Another type of air cleaner that has come into the HVAC market in recent years is an ionizer. These products are designed to recreate the freshness of outdoor air inside a conditioned space. The initial description of the product is very appealing: no moving parts, minimal maintenance, and no ozone byproducts.
The intent of the product is to produce a large number of ions that when mixed with air in the conditioned space will create a sort of invisible plasma field; some of the pollutants in the air will be destroyed by reacting to this plasma field while others will become negatively charged by the ions which will make them cluster and fall to the ground for easier collection during normal cleaning. This same technology is being looked at heavily by the food industry for the sterilization of food; thus far initial studies have shown a lot of promise … but the technology will need to be scaled for commercial use.
There is a lot of potential with ionizers and plasma fields for air cleaning effect, however, there are still some downsides: ionizers produce ozone despite much of the marketing material advertising otherwise. There are also many concerns with potential health effects caused by these devices that mimic the concerns of PCO device use. These devices possess the potential in some cases to produce unwanted byproducts such as formaldehyde. They also have the potential to create ROS molecules which again can create concern seen in PCO devices. Finally, while the introduction of a plasma field created by large numbers of ions has been shown to effectively sterilize treated areas … in practical use there is a limit to the amount of ions or plasma field that can be generated within a single device intended for whole-house air cleaning. There is not enough information on the performance of these devices in real indoor settings.
The available options for cleaning the air in a home or office are numerous. There is no clear winner when it comes to choosing a method or a specific device. Each method, when used properly, will provide benefits while none will address all needs for air cleanliness. Some of these methods may include negative effects that outweigh their benefits to specific consumers.
However, the IAQ segment of the HVAC industry is growing rapidly. Consumers are realizing the negative effects of too much time inside of buildings with poor air quality and as solutions work their way more to the mainstream they are being sought out by our everyday consumers. More frequently the term “air conditioning” is focusing less on the cold air aspect and more on the overall “condition” of the air (humidity, quality, temperature, smell, etc.). As industry professionals, it helps us to continually learn about emerging and rapidly changing technologies.
- ASHRAE Journal, ed. September 2019. Technical Feature: New Guidance for Residential Air Cleaners. Atlanta: ASHRAE.
- EPA. 2018. Guide to Air Cleaners in the Home and Residential Air Cleaners: A Technical Summary. Washington, D.C.: U.S. Environmental Protection Agency.
- National Center for Biotechnology Information. Free Radicals, Antioxidants in Disease and Health
- National Center for Biotechnology Information. DNA Base Damage by Reactive Oxygen Species, Oxidizing Agents, and UV Radiation
- EPA. What is a MERV Rating