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IGERT Trainee Shows that Ion Generating Air "Purifiers" Actually Generate Indoor Air Pollution
Achievement/Results
Michael Waring, a doctoral candidate in Environmental Engineering, is funded through the National Science Foundation’s (NSF) Integrative Graduate Education and Research Traineeship (IGERT) program in Indoor Environmental Science and Engineering at The University of Texas at Austin (UT), and he is studying the effectiveness and adverse consequences of ion generating air purifiers.
His work suggests that these types of portable air cleaners, which are popular in the United States, can both remove and generate pollutants indoors, but may cause more harm than good for several reasons. Waring conducted a two-phase investigation in a 14.75 m3 stainless steel chamber. In the first phase, particle size-resolved (12.6-514 nm diameter) clean air delivery rates (CADR = product of particle removal efficiency for what flows through the device and air flow through the device) were determined, as were ozone emission rates, for two high efficiency particulate air (HEPA) filters, one electrostatic precipitator with a fan, and two ion generators without fans. The two HEPA air cleaners had count average CADR (± one standard deviation) of 188 (30) and 324 (44) m3 h-1; the electrostatic precipitator 284 (62) m3 h-1; and the two ion generators 41 (11) and 35 (13) m3 h-1. Thus, the ion generators tested in this study (two of the most widely purchased brands) proved to be approximately 5 to 7 times less effective than comparably-priced HEPA filters and electrostatic precipitators. Ozone is known to be a strong respiratory irritant that can lead to on-set of asthma and has been linked to pre-mature mortality. As such, sources of indoor ozone should be of concern in homes, the environment where most Americans spend the greatest fraction of their time. The electrostatic precipitator emitted ozone at a rate of 3.8 ± 0.2 mg h-1, and the two ion generators emitted ozone at a rate of 3.3 ± 0.2 and 4.3 ± 0.2 mg h-1. To put these numbers in perspective, for newer housing stock they are comparable to the ozone that enters a home from outdoors during mid-day summertime conditions in an urban area that fails to meet National Ambient Air Quality Standards for ozone.
In summary, the ozone generated by ion generators and electrostatic precipitators is relevant and should be of concern. Ozone also initiates reactions with certain unsaturated organic compounds that produce ultrafine and fine particles, carbonyls, other oxidized products, and free radicals. During the second phase of his research, Waring studied five different ion generators that were operated separately in the presence of a plug-in liquid or solid air freshener, representing a strong source of monoterpenes, a group of generally reactive, unsaturated organic compounds. For air exchange rates of between 0.49 and 0.96 h-1, typical for many homes, three ion generators acted as steady-state net particle generators in the entire measured diameter range of 4.61-157 nm, and two generated particles in the range of approximately 10 to 39-55 nm. Aldehyde and terpene concentrations were also sampled for one ion generator, and concentrations of terpenes decreased and formaldehyde increased. Given these results, the pollutant removal benefits of ozone generating air cleaners may be outweighed by the generation of indoor pollution.
Finally, Waring experimentally characterized the indoor air quality implications of an ion generator in an actual 27 m3 residential room. He found that the use of an ion generator in the presence of a common air freshener can lead to a net increase in fine and ultrafine particulate matter (<0.5 µm) and ozone, as well as formaldehyde and nonanal. His research allows exposure and health professionals to make justified recommendations about the use of these ion generating air cleaners. Waring’s studies on ozone chemistry in the near vicinity of ion generating air purifiers will continue through the summer of 2008, with future experiments focused on the role of heterogeneous (surface) reactions on the production of oxidized gases and ultrafine particles.
Address Goals
Sales of indoor air purifiers have sky-rocketed in the United States over the past decade and generally increased in developed countries around the world. Waring’s research on ion generating air purifiers has lead to several important discoveries that shed important insight on the adverse effects of the most commonly-purchased types of these purifiers. Specifically, they emit ozone that reacts with unsaturated organic compound to form ultrafine particles in indoor air. In some scenarios, ion generating air purifiers can be net particle generators. A good metric for the importance of Waring’s discoveries is that his work has already been published or accepted for publication in three peer-reviewed journals with high impact factors. The results should provide important information to policy makers and the public regarding the effectiveness and risks of home air purifiers.
Waring’s work, albeit highly involved in laboratory chambers and field, has been boiled down to a more simplistic experiment that is demonstrated to the public and goes to the goal of learning. An ion generator is used in the presence of a terpene source such as a solid air freshener or cut lemon. An ultrafine particle detector is used to show how rapid the kinetics of ozone/terpene reactions can be, and to also demonstrate the rather significant increase in airborne ultrafine particles that form via self-nucleation and condensation of low vapor pressure reaction products. In addition to providing a learning experience for the public, IGERT trainee Michael Waring also engaged in the learning process by virtue of the need to boil down rather complex chemistry into a simple demonstration and explanation for the public.
Finally, Michael Waring’s knowledge of ozone-initiated indoor air chemistry and the experimental tools needed to complete such research has been translated to several other IGERT trainees and affiliates. In one example, IGERT affiliate Donna Kunkel is making use of a new NSF-funded test house facility (research infrastructure) to study the selective use of activated carbon panels and sleeves placed over fan blades to remove ozone and oxidized products from indoor air without additional energy penalty to buildings.
