Myth vs Fact: Managing Air Quality
More than ever, in the past year we really appreciated the impact that the air we breathe can have on our daily lives. As we move forward, indoor air quality management should be at the forefront of any plan to re-open indoor gathering spaces.
Heating, ventilation, and air-conditioning (HVAC) related technologies play a key role in reducing exposures to airborne contaminants harmful for humans. Yet, these technologies greatly vary in their effectiveness and ability to minimize harmful effects on health and comfort. With an increasing number and variety of filtration and air-cleaning alternatives available on the market, it is important to understand the differences in these technologies.
Mechanical & Electronic Air Filters
Mechanical filters use media with porous structures that contain fibers or stretched membrane material. A portion of the particles in the air entering a filter attaches to the filter and is removed from the air as it passes through.
Electronic filters include a wide variety of electrically connected air-cleaning devices that are designed to remove particles from airstreams. Removal typically occurs by electrically charging the particles and then collecting them on oppositely charged deposition plate. These filters have been shown to range from being relatively ineffective to very effective at removing indoor airborne particles.
The overall effectiveness of reducing indoor particle concentrations depends on several factors that include: single-pass particle removal efficiency of the filter, the rate of airflow through the filter, location of the filter, and size of the particles.
Air filter system effectiveness can be maximized by recirculation of indoor air through filters and re-filtering blended outdoor air with return air. Filtering the incoming outdoor air before this air enters the occupied space is effective in reducing indoor air concentrations of outdoor air particles, especially in airtight buildings.
These types of filtration systems can reduce indoor airborne concentrations of harmful particles by removing them from the airstream but not by inactivating or killing infectious species. Research suggests that having filters in HVAC systems, relative to having no filters, will substantially decrease the portion of disease transmission caused by infectious particles.
Sorbent Air Cleaners
Sorbent air cleaners involve physical and chemical adsorption to remove gaseous contaminants from airstreams. A solid porous material, typically activated carbon, attracts and adsorbs the contaminant. However, due to relatively weak forces, this is a reversible process and gases once adsorbed can later find their way back into the air stream.
These systems can remove a broad range of contaminants with moderate to high efficiency. The net rate of adsorption depends on the rate at which contaminant molecules reach the surface of the media, the percentage of those making contact, which are adsorbed, and the rate of desorption.
While there is no empirical data available to draw conclusions about the health benefits of using sorbents in typical buildings, studies show significantly improved ratings of acceptability or satisfaction with air quality and odor intensities. Although perception of air quality comfort is not a health outcome, it may be considered an indicator of a potential subsequent effects of exposures on health.
UV-C
Ultraviolet (UV-C) disinfection is used to degrade organic material and inactivate microorganisms. The system is not a filter; thus, inactive particles remain in the airstream, which, in the case of dead fungal spores, may still cause a negative human response to their integral mycotoxins. The typical source of UV-C in commercial and residential air and water systems is low-pressure mercury vapor lamps, which emit wavelengths at a near optimal level. UV-C systems may be installed inside HVAC systems, irradiate air near the ceiling, or be incorporated in a stand-alone (portable) air cleaner.
The effectiveness of a UV-C system to inactivate microorganisms in the air and/or on surfaces has been amply demonstrated. Experience suggests that control of a moving airstream does not provide favorable killing rates because of the short exposure time. Under ideal conditions, inactivation and/or killing rates of 90% or higher can be achieved but depend on the following: the type of microbial contaminant; specific species; physical or mechanical factors such as UV-C intensity, exposure time, lamp distance and placement, and lamp life cycle and cleanliness; air movement and patterns; temperature; relative humidity; and air mixing. Airborne removal is best applied in conjunction with filtration of particles with pre-filtration in order to protect lamps and mechanical filtration downstream for microbial particles.
Ozone Generating Air Cleaning Devices
Some air cleaners produce ozone by design to achieve air-cleaning effects and the removal of contaminants, while others simply produce ozone as a by-product of the air-cleaning processes. Any device that uses electricity during the air cleaning process has the potential to generate ozone.
The current state of the science regarding the health effects of ozone, and the EPA, strongly suggest that the use of air cleaners that emit ozone by design should not be permitted. There is more uncertainty about recommendations for air cleaners that do not use ozone by design but still unintentionally produce ozone as a by-product.
Though there are devices that emit ozone but at the same time reduce concentrations of other harmful contaminants, the risks and benefits of such a trade-off are highly uncertain. In the absence of definite information regarding safe levels of ozone, use of an any ozone-emitting air cleaner, may be a net negative impact on indoor air quality and thus should be used with extreme caution. If possible, non-ozone-emitting alternatives should be used.
No matter which of these filtration and air-cleaning devices is best suited to your specific needs, active maintenance and monitoring is needed to ensure optimal performance.