ReviewThe health significance of gas- and particle-phase terpene oxidation products: A review
Introduction
In recent years, there has been increasing interest in the potential health impacts of secondary organic compounds formed via terpene oxidation reactions. Such compounds include both gas-phase materials such as formaldehyde as well as particle-phase components such as pinic acid (Grosjean et al., 1992, Jang and Kamens, 1999). Interestingly, research related to this issue is divided based on the medium of interest, with both indoor air and ambient air researchers involved, often without knowledge of or interaction with the other. For example, at a recent indoor air meeting (Indoor Air 2011), there were a total of six papers on terpenes and/or secondary organic aerosol; however, these were all presented by researchers primarily involved in the indoor air field and were focused on indoor air sources, formation, and health effects. Conversely, at the American Association of Aerosol Research's 2012 annual meeting, there were 14 presentations on secondary organic aerosol (SOA) specifically related to terpenes, but none took an indoor air perspective. It would be helpful if the two research communities could share information in a more systematic way, as each discipline has much to learn from the other.
Indoor air research on terpene oxidation products was prompted in large part by the investigation of health complaints in indoor environments, including eye, nose, and throat irritation, dry skin, headaches, and other nonspecific symptoms (Wolkoff et al., 1997). Despite extensive epidemiological research on potential causes of these complaints, the cause of these symptoms remains elusive. Some hypotheses that have been considered include low levels of multiple volatile organic compounds (VOCs), low relative humidity, thermal comfort effects, and elevated CO2 concentration indoors (Norback, 2009). More recently, the idea that reactions among indoor air pollutants can generate irritating compounds has been forwarded, and a number of studies have noted toxicological effects in animal models exposed to reaction products (e.g., Rohr et al., 2002, Sunil et al., 2007, Wolkoff et al., 1999, Wolkoff et al., 2012).
There is also much interest in terpene oxidation from an ambient atmospheric chemistry perspective, particularly with respect to secondary organic aerosol. Secondary aerosol forms from gas-to-particle conversion processes, including nucleation, condensation, and heterogeneous and multiphase chemical reactions (Hallquist et al., 2009). Much attention has been focused on understanding the mechanisms of formation of SOA, identifying the products formed, modeling SOA formation at local and regional scales, and evaluating the impact of these aerosols on climate. Less attention has been given to investigating potential health effects from SOA exposure, although interest appears to be increasing in this area as understanding the role played by various particulate matter (PM) components in adverse health effects becomes a higher priority research topic. Currently, PM is regulated in the United States with a mass-based National Ambient Air Quality Standard (NAAQS), despite accumulating evidence that different components of PM have different inherent toxicity (e.g., Kelly and Fussel, 2012, Rohr and Wyzga, 2012). Knowledge of the health effects of other major PM components such as sulfate, nitrate, and elemental carbon is reasonably well-developed; however, the effects attributable to the organic carbon component, comprised of both primary aerosol and SOA, is less well understood (Mauderly and Chow, 2008).
This review attempts to bring together the two distinct sets of literature related to indoor and ambient air and critically evaluate, based on existing evidence, the potential for human health impacts from exposure to terpene oxidation products. It should be noted that the focus is on monoterpenes (and isoprene, a hemiterpene); little is known about the health impacts of sesquiterpenes or their oxidation products and thus they are not included in the scope.
Section snippets
Methods
An extensive search was conducted for published information related to terpene oxidation product epidemiological, toxicological, and controlled human exposure studies on the PubMed database, using combinations of keywords “terpene”, “ozone”, “oxidation”, “pinene”, “limonene”, “isoprene”, “toxicology”, and “epidemiology”. In addition, to allow inclusion of up-to-date information on terpene oxidation product chemistry, formation, and related issues, the Science Direct database was searched using
Sources
Terpenes are unsaturated organic compounds of biogenic origin that contain one or more CC bonds. The monoterpenes (C10H16) and sesquiterpenes (C15H24) are volatile and are emitted from vegetation in large quantities. Monoterpenes such as α-pinene and d-limonene are commonly found in room fresheners and pine cleaners, wood products, and wood-based furniture coatings; these two terpenes are often detectable in the form of a pinene or lemon scent, respectively. They have also been measured in
Discussion
A broad spectrum of literature exists on the health impacts of terpene oxidation products; this literature spans the indoor air and ambient air areas, and includes epidemiological, toxicological, and controlled human exposure studies. In considering the literature as a whole, terpene/ozone reaction products can cause adverse effects in rodents at high concentrations. Most notably, sensory irritation and airflow limitation have been observed consistently in response to pinene, limonene, and
Knowledge gaps and future research
Despite substantial effort on the part of indoor and ambient air scientists, there are many unanswered questions with regard to the health effects of terpene oxidation products. The indoor community has appeared to be more united in an attempt to address the major scientific questions, and in fact a 2004 workshop entitled “Indoor Chemistry and Health” was aimed at promoting communications between experts in indoor chemistry and those examining health effects (Weschler et al., 2006). Workshop
Conclusion
Although terpene oxidation products have been studied for decades, the vast majority of research on this topic has focused on the chemistry and formation of these materials and not on their potential health effects. More recently, with the advancement of the “reactive chemistry” hypothesis to potentially explain complaints in indoor settings, efforts have been expended to test this hypothesis using a variety of animal and human exposure experimental designs. While results have varied and not
Acknowledgments
This work was funded by the Electric Power Research Institute. Dr. Rohr is employed by the Electric Power Research Institute, which is primarily supported by the electric industry in the U.S. and abroad. EPRI is an independent non-profit 501(c)(3) organization that funds external research at a number of universities and institutes worldwide.
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