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IGERT Trainee Completes Study of Indoor Ozone Chemistry
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). Generally, he studies particle formation that can occur indoors due to chemical reactions between ozone and terpenes, which are natural scenting agents that are commonly emitted from many types of consumer products.
The first part of his research determined the effectiveness and adverse consequences of operating ion generating “air purifiers” indoors. His work suggested that these types of portable air cleaners, which are extremely popular in the United States, can both remove and generate pollutants indoors, but may cause more harm than good for several reasons described below. Waring conducted a two-part investigation: the first part was in a stainless steel laboratory chamber the size of a small room, and the second part was in a small room in an actual residence.
The laboratory chamber work was largely focused on characterizing the effects of the ion generators. Initially, 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. For the studied range of particles, the two HEPA air cleaners had average CADRs of 188 and 324 m3/h; the electrostatic precipitator 284 m3/h; and the two ion generators 41 and 35 m3/h. Thus, the ion generators tested in this study (two of the most widely marketed and purchased brands) proved to be approximately 5 to 7 times less effective than comparably-priced HEPA filters and electrostatic precipitators. Ion generators generate ozone as a byproduct of their operation. Ozone is regulated as an outdoor pollutant by the EPA, and indoors it is the primary driver of indoor chemistry. Ozone is known to be a strong respiratory irritant that can lead to onset of asthma and has been linked to premature 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 mg/h, and the two ion generators emitted ozone at rates of 3.3 and 4.3 mg/h. 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 indoors by ion generators and electrostatic precipitators is relevant and should be of concern. As first stated above, ozone also initiates reactions with certain unsaturated organic compounds that yield airborne particles, other oxidized products, and free radicals. For the last part of the chamber research, Waring determined if these ion generators, which are devices marketed and intended for particle removal, could ever generate more particles than they removed. He studied five different ion generators that were operated separately in the presence of a plug-in liquid or solid air freshener, which acted as source of terpenes, which are a group of generally reactive, unsaturated organic compounds commonly found indoors. For air exchange rates of between 0.5 and 1.0 h-1, which are typical for many homes, all five ion generators acted as steady-state net particle generators within the measured particle range (4.61-157 nm). Given these results, the pollutant removal benefits of ozone generating air cleaners may be outweighed by the generation of indoor pollution, even for the very pollutants they are intended to remove. It is important to characterize effects in real environments, rather than only in a laboratory. Waring experimentally characterized the indoor air quality implications of operating 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 (< 500 nm) and ozone, as well as formaldehyde, a known human carcinogen. His research allows exposure and health professionals to make justified recommendations about the use of these ion generating air cleaners indoors.
For the second main part of his research, Waring has investigated more fundamental aspects related to indoor particle formation. He first noticed that indoor surfaces play a large role in the amount of particles formed indoors due to ozone reactions with terpenes, and he has investigated the effects of different surfaces in a set of small chamber experiments. Also, while on an IGERT funded internship at the National Institute of Occupational Safety and Health (NIOSH), he discovered that particular mixtures of reactive chemicals can substantially influence the amount of particles that are formed and plans to investigate this phenomenon further in the future.
Waring is completing his last year at the University of Texas at Austin as a Harrington Dissertation Fellow, the most prestigious fellowship for a current graduate student available at the university. He has accepted a faculty position as an Assistant Professor of Architectural Engineering at Drexel University, which is set to commence this fall.
Address Goals
Trainee Michael Waring’s dissertation research is the most robust research completed to date on the role of ozone-generating air purification systems on indoor air quality, particularly as related to both homogeneous and heterogeneous (surface) chemistry. His numerous and important findings go the heart of the strategic goal of discovery. It is relevant that Michael’s work on surface chemistry was spawned by accidental observations related to the importance of surface types during his experiments on homogeneous indoor ozone chemistry.
There is no doubt that Michael has developed great expertise in the area of indoor ozone chemistry (learning). This knowledge should serve him well given an American trend of continued weather proofing for energy conservation. Weather proofing to reduce air infiltration into buildings increases the time available for chemistry to occur indoors. In effect, our nation is making buildings more chemically reactive. Michael’s expertise should serve him well as he embarks on his own academic career at Drexel University, and passes on what he has learned to other aspiring indoor environmental scientists and engineers.
