Interior Air Quality
Monitoring, Filtering, and Purification
Alex Pluemer for Mouser Electronics
Maintaining healthy indoor air quality (IAQ) is essential to cultivating a happy, productive workplace. According to the U.S. Environmental Protection Agency (EPA), Americans spend approximately 90% of their time indoors, where airborne pollutants can be 2 to 5 times higher than outside. Indoor concentrations of certain pollutants have increased over the last few decades, and we can’t ignore the increased prevalence of certain synthetic building materials, household cleaning materials, and self-care products. Their widespread usage in homes, schools, and office buildings creates a more persistent presence of these pollutants in our indoor environments, leading to higher concentrations than ever before.
Environmental monitoring sensors can measure the concentrations of airborne pollutants like gases, volatile organic compounds (VOCs), and particulate matter using laser and light refraction technology, providing more accurate, timely data than ever before. Health and safety are the obvious priorities, but studies have shown that high concentrations of carbon dioxide (CO2) in an office or workplace can negatively affect productivity by making employees sleepy or sluggish. Adequate ventilation and air filtering and purification systems can help significantly reduce the spread of viruses or other airborne illnesses in a workplace, cutting down on absences and sick days.
At the time of this writing, there are no federally or internationally recognized guidelines for what constitutes “healthy” indoor air, although organizations like the EPA and the U.S. Centers For Disease Control (CDC) offer a list of recommendations for maximum allowable concentrations of certain airborne pollutants to ensure a safe environment. Facilities managers and heating, ventilation, and air conditioning (HVAC) system engineers are often left to use trial and error to determine IAQ's effects on health and productivity, but that's beginning to change as the importance of IAQ becomes more widely understood. In this article, we'll examine the importance of IAQ, the technologies currently being used to monitor and improve it, and how those technologies might evolve.
The Impact of Poor IAQ
The impact(s) of poor IAQ on human health can be challenging to quantify in the present; it often takes months or years for health problems relating to exposure to some airborne pollutants to develop. We do know that repeated exposure to high concentrations of certain synthetic building materials (e.g., asbestos) or VOCs—which are present in everything from household chemicals to mass-produced furniture—can result in severe health risks, often cancer. Mold spores are another common indoor pollutant that can lead to lasting and difficult-to-diagnose medical conditions.
The most important pollutants to monitor are those that present the most significant health risks and are commonly found indoors, as opposed to airborne pollutants like particulate matter that come in from the outside. Wildfires or other forms of combustion create tiny particles in the air that can lead to respiratory issues and, ultimately, heart and vascular disease; but people can effectively mitigate those risks by closing doors and windows and ensuring HVAC systems are functioning properly.
As previously stated, the issues associated with poor IAQ aren't limited to long-term health and wellness. Poor ventilation can cause germs to spread around a school or workplace more broadly and rapidly—a key reason why hospitals and medical facilities closely monitor internal airflow. Poor air quality can also adversely affect productivity: Studies have shown that high concentrations of CO2 in the air make people drowsy, less alert, and generally less productive. This can be a problem in smaller office spaces or conference room settings where many people share a small space. So much CO2 is being expelled into these small areas that it can saturate the environment without optimal ventilation. This phenomenon is so well understood that casinos worldwide pump oxygen into their indoor ventilation systems to keep gamblers awake and engaged for as long as possible.
IAQ Monitoring Technology
Air quality monitoring technology is still evolving; sensors are constantly being developed with smaller form factors and enhanced functionality. Sensors that can detect particulate matter and certain gases and VOCs are currently available, most of which implement lasers and optics to analyze their environments. Concentrations of particulate matter in the air are typically measured with light reflection techniques, through which a laser emitted from the sensor is reflected by a photodiode when it meets a particulate. The amount of light reflected at the diode tells the sensor the concentration of particulates in the atmosphere and the size of the particulates themselves, which is relevant given that more significant pieces of particulate matter (measured in micrometers, so still minimal) are more harmful to the lungs than smaller ones.
AQ monitors detect gas through a similar method called infrared light absorption. The infrared light that passes through a gas will be partially absorbed. By comparing the intensity of infrared light emitted from the source to the light hitting a receiver, sensors can quantify the level of absorption and the concentration of gases like CO2, carbon monoxide (CO), and nitrogen oxides (NOx) in the air.
As far as AQ sensor technology has come, some lab analysis is still required to effectively monitor certain pollutants that can't be separated from the air without lab processes. Specifying which VOCs are present in the atmosphere usually entails gas chromatography, a process that pushes air samples mixed with an inert gas through a column to measure the rate at which disparate chemical components pass.
While lab analysis can take time, the Internet of Things (IoT) makes IAQ monitoring a real-time endeavor. Sensors can be networked and connected via wireless protocols to handheld devices or computers that collect and record data at intervals of one second or less. All those data points would be difficult to evaluate without software and algorithms that detect patterns and changes in a localized environment (for example, recognizing that indoor humidity levels tend to go up in the summer months, thereby increasing the risk of mold or mildew forming). Networking sensors with HVAC systems, air purification systems, and handheld devices also allows facilities managers to adjust those systems remotely, which can save time and energy in larger office buildings or campuses.
IAQ Filtering and Purification
Air filtration and purification systems do the heavy lifting of maintaining healthy interior air quality. The effectiveness of various types of air filters is compared using the Minimum Efficiency Reporting Values (MERV) scale, which uses a range of 1–16. (To illustrate, a standard window air conditioner typically has a MERV of 1–3 while a cleanroom-quality air filter has a rating of 14–16, meaning it can capture over 90% of particulate matter passing through it.)
Most modern filtration systems implement high-efficiency particulate arresting (HEPA) filters to sift out particulate matter from the air. HEPA filters typically remove over 99% of particulates ≥0.3 micrometers in diameter and smaller particulates at a lower rate. Implementing HEPA filters with standard prefilters can help separate some more extensive particulate matter from the air, taking some of the strain off a HEPA filter and extending its lifetime.
HEPA filters are highly effective in laboratory and hospital settings. Still, they are often impractical in residential or manufacturing environments because air passes through HEPA filters much more slowly than less effective filters. Additionally, HEPA filters require airtight sealing to remain more effective than medium-efficiency filters. Studies have indicated that air filters rated 7–13 on the MERV scale can be just as effective as HEPA filters in larger commercial and residential buildings.
Eliminating and reducing the spread of airborne pathogens has become a larger interior air quality concern, especially since the beginning of the COVID-19 pandemic. Ultraviolet germicidal irradiation (UVGI) is a rapidly increasing technology designed to kill pathogens, bacteria, and microorganisms present in the air by pushing air past UV lamps that effectively sterilize it. UVGI is often implemented in conjunction with filtration systems to help sift any dead organic material out of the air after exposure to UV light. UVGI is also commonly implemented in potentially dark or damp areas to help prevent mold spores from growing.
UVGI can be expensive to implement and has some practical drawbacks: For example, passing enough air close enough to UV lamps to effectively kill pathogens can create logistical problems, and the UV bulbs require frequent replacement. Ionizer purifiers, a less costly alternative to UV-based systems, electrically charge air particles and then attract them to grounded conductors. Ionizer purifiers without built-in fans are quieter and draw less power than purifiers with fans, but they also work more slowly and are ultimately less effective.
Innovations in IAQ Technology
A variety of emerging technologies in the air purification space are already being implemented on a smaller scale than HEPA filtration and UVGI. One of the most promising is photocatalytic oxidation (PCO), a process that uses short-wave UV light to energize a catalyst (typically titanium dioxide) introduced into the air, and then oxidizing bacteria and VOCs present in the air. Like UVGI, PCO is generally used in conjunction with conventional filtration systems as it can't oxidize particulate matter. PCO systems are also among the most expensive purification systems currently on the market.
Another rapidly developing technology is thermodynamic sterilization (TSS). As the name implies, TSS uses thermal coils to heat the air to approximately 204°C (400°F), which reportedly kills 99.9% of all biological organisms in the air. TSS has some of the same logistical drawbacks as UV-based purification technologies and also must be implemented with a separate filtration system as it doesn't eliminate particulate matter. Still, unlike ionizer purifiers, TSS reduces the amount of ozone in the atmosphere.
Immobilized cell technology, a method of attracting charged particles in the air to a bioreactor that renders them inert, is also showing promise. Charged air particles (i.e., particulates, gases, VOCs) are attracted to water inside the system, which acts as a ground to separate and then oxidize them in a solution of enzymes and bacteria. Immobilized cell technology has been demonstrated to oxidize matter 12 times more effectively than natural oxidation.
The Future of IAQ Technology
UVGI, ionizers, and other air purification technologies face some obstacles before they can compete with advanced filtration systems on a practical and economical basis. Many modern HVAC systems (in drier climates) use air-side economizers, devices that use dampers to let in colder air from outside the building(s) to help take the pressure off mechanical cooling apparatus and reduce energy consumption. These dampers have prefilters and high MERV-level end filters (the CDC recommends 13 or above) to clean the incoming air. The rate at which the inside air is being completely replaced by outside air can be adjusted according to the outdoor temperature and the size of the facility. If the indoor air is being replaced often enough, then any ionized or otherwise purified air would simply be pushed outside in favor of cooler outdoor air. Under those conditions, ionizers or UVGI would be impractical and significantly more expensive in terms of initial outlay and ongoing energy costs. Alternative filtration methods are a better fit in warmer, more humid locales where indoor air is more likely to be recirculated than replaced.
The emergence of COVID-19 and other respiratory viruses has made indoor air quality—and specifically the elimination of airborne pathogens—a greater concern than ever before. The regulatory environment around indoor air quality, which is mostly theoretical at this point, is also certain to develop. Purification technologies that treat the air itself rather than just attempt to filter pollutants out of it have demonstrated promise in effectively reducing the contraction of airborne pathogens, and when the medical and scientific communities begin to use them in hospital and laboratory settings, then commercial office buildings, manufacturing facilities, and schools could quickly follow suit.