Nanocluster Aerosols from Ozone-Human Chemistry Are Dominated by Squalene-Ozone Reactions

Nanocluster aerosols (NCAs, <3 nm particles) are associated with climate feedbacks and potentially with human health. Our recent study revealed NCA formation owing to the reaction of ozone with human surfaces. However, the underlying mechanisms driving NCA emissions remain unexplored. Squalene is the most abundant compound in human skin lipids that reacts with ozone, followed by unsaturated fatty acids. This study aims to examine the contribution of the squalene-ozone reaction to NCA formation and the influence of ozone and ammonia (NH3) levels. In a climate-controlled chamber, we painted squalene and 6-hexadecenoic acid (C16:1n6) on glass plates to facilitate their reactions with ozone. The squalene-ozone reaction was further investigated at different ozone levels (15 and 90 ppb) and NH3 levels (0 and 375 ppb). The results demonstrate that the ozonolysis of human skin lipid compounds contributes to NCA formation. With a typical squalene-C16:1n6 ratio found in human skin lipids (4:1), squalene generated 40 times more NCAs than did C16:1n6 and, thus, dominated NCA formation. More NCAs were generated with increased ozone levels, whereas increased NH3 levels were associated with the stronger generation of larger NCAs but fewer of the smallest ones. This study experimentally confirms that NCAs are primarily formed from squalene-ozone reactions in ozone-human chemistry.

Squalene Depletion in Skin Following Human Exposure to Ozone under Controlled Chamber Conditions

A major component of human skin oil is squalene, a highly unsaturated hydrocarbon that protects the skin from atmospheric oxidants. Skin oil, and thus squalene, is continuously replenished on the skin surface. Squalene is also quickly consumed through reactions with ozone and other oxidants. This study examined the extent of squalene depletion in the skin oils of the forearm of human volunteers after exposure to ozone in a climate chamber. Temperature, relative humidity (RH), skin coverage by clothing, and participants' age were varied in a controlled manner. Concentrations of squalene were determined in skin wipe samples collected before and after ozone exposure. Exposures to ozone resulted in statistically significant decreases in post-exposure squalene concentrations compared to pre-exposure squalene concentrations in the skin wipes when squalene concentrations were normalized by concentrations of co-occurring cholesterol but not by co-occurring pyroglutamic acid (PGA). The rate of squalene loss due to ozonolysis was lower than its replenishment on the skin surface. Within the ranges examined, temperature and RH did not significantly affect the difference between normalized squalene levels in post-samples versus pre-samples. Although not statistically significant, skin coverage and age of the volunteers (three young adults, three seniors, and three teenagers) did appear to impact squalene depletion on the skin surfaces.

Reactions and Products of Squalene and Ozone: A Review

This critical review describes the squalene-ozone (SqOz) reaction, or squalene ozonolysis. Ambient ozone penetrates indoors and drives indoor air chemistry. Squalene, a component of human skin oil, contains six carbon-carbon double bonds and is very reactive with ozone. Bioeffluents from people contribute to indoor air chemistry and affect the indoor air quality, resulting in exposures because people spend the majority of their time indoors. The SqOz reaction proceeds through various formation pathways and produces compounds that include aldehydes, ketones, carboxylic acids, and dicarbonyl species, which have a range of volatilities. In this critical review of SqOz chemistry, information on the mechanism of reaction, reaction probability, rate constants, and reaction kinetics are compiled. Characterizations of SqOz reaction products have been done in laboratory experiments and real-world settings. The effect of multiple environmental parameters (ozone concentration, air exchange rate (AER), temperature, and relative humidity (RH)) in indoor settings are summarized. This critical review concludes by identifying the paucity of available exposure, health, and toxicological data for known reaction products. Key knowledge gaps about SqOz reactions leading to indoor exposures and adverse health outcomes are provided as well as an outlook on where the field is headed.

Characterization of the off-body squalene ozonolysis on indoor surfaces

Chemical reaction and physical transport characteristics of indoor surfaces play an important role in indoor air quality. This study presents a kinetic model to describe the reaction of ozone with squalene on indoor surfaces in a family house, by incorporating external and internal mass transfer, surface partitioning, and chemical reaction on indoor surfaces. Field experiments were performed in the family house. The first 3-days of data, collected when the house was unoccupied, are used to derive the key parameters in the model, which are then used for predicting the concentrations in other unoccupied days. Comparison of squalene oxidation products during the occupied and unoccupied periods shows that even if the house is unoccupied for several days, the indoor concentrations of 6-methyl-5-hepten-2-one (6-MHO) and 4-oxopentanal (4-OPA) remain substantial, demonstrating that surface reaction of ozone with off-body squalene can significantly impact the composition of indoor air. Model predictions of the three compounds (ozone, 6-MHO, and 4-OPA) agree well with the experimental observations for all test days. Furthermore, we make the first attempt to estimate the duration of typical polyunsaturated aldehydes (TOP, TOT, and TTT), which indicated that these compounds, as well as off-body squalene, can persist on indoor surfaces for a relatively long period in the examined residence.

Planar chromatography and immunodetection of hydrocarbons on polyvinylidene difluoride membranes

Fast, cheap, and simple separation of lipids and hydrocarbons can currently be achieved using thin-layer chromatography. Here, we describe an alternative planar chromatographic method using polyvinylidene difluoride membranes as the stationary phase. The procedure has the same advantages of thin-layer chromatography over other expensive and time-consuming techniques, such as high-performance liquid chromatography or gas chromatography. Polyvinylidene difluoride membranes, however, also provide an immediate support for analyte development via immunodetection, are easy to manipulate, and potentially increase the performance of other detection methods. We show that polyvinylidene difluoride membranes are compatible with a variety of solvents that can migrate by capillarity and redistribute analytes between the membrane and the solvent according to their relative affinities, providing a chromatographic separation. We directly test the developed membranes by immunoblotting using anti-squalene antibodies that cross-react with acyclic isoprenoids. Separations of crude oils and plant extracts under different solvent conditions show the potential to resolve hydrocarbon group types and also to provide characteristic fingerprints of plant pigments and squalene degradation products. Polyvinylidene difluoride membranes prove useful as a stationary phase for planar chromatography and for the subsequent immunodetection of the separated compounds, providing a new and simple chromatographic technique to analyze lipids and hydrocarbons.

Physical-Chemical Coupling Model for Characterizing the Reaction of Ozone with Squalene in Realistic Indoor Environments

Squalene can react with indoor ozone to generate a series of volatile and semi-volatile organic compounds, some of which may be skin or respiratory irritants, causing adverse health effects. Better understanding of the ozone/squalene reaction and product transport characteristics is thus important. In this study, we developed a physical-chemical coupling model to describe the behavior of ozone/squalene reaction products, that is, 6-methyl-5-hepten-2-one (6-MHO) and 4-oxopentanal (4-OPA) in the gas phase and skin, by considering the chemical reaction and physical transport processes (external convection, internal diffusion, and surface uptake). Experiments without intervention were performed in a single-family house in California utilizing time- and space-resolved measurements. The key parameters in the model were extracted from 5 day data and then used to predict the behaviors in some other days. Predictions from the present model can reproduce the concentration profiles of the three compounds (ozone, 6-MHO, and 4-OPA) well (R² = 0.82-0.89), indicating high accuracy of the model. Exposure analysis shows that the total amount of 6-MHO and 4-OPA entering the blood capillaries in 4 days can reach 14.6 and 30.1 μg, respectively. The contribution of different sinks to ozone removal in the tested realistic indoor environment was also analyzed.

A Novel Insight into the Ozone-Skin Lipid Oxidation Products Observed by Secondary Electrospray Ionization High-Resolution Mass Spectrometry

Emissions of secondary products through reactions of oxidants, ozone (O₃), and hydroxyl radical (·OH) with human skin lipids have become increasingly important in indoor environments. Here, we evaluate the secondary organic compounds formed through heterogeneous reactions of gaseous O₃ with hand skin lipids by using a high-resolution quadrupole Orbitrap mass spectrometer coupled to a commercial secondary electrospray ionization (SESI) source. More than 600 ions were detected over a period of less than 40 min real-time measurements, among which 53 ions were characterized with a significant increasing trend in signal intensity at the presence of O₃. Based on the detected ions, we suggest detailed reaction pathways initiated by ozone oxidation of squalene that results in primary and secondary ozonides; we noticed for the first time that these products may be further cleaved by direct reaction of nucleophilic ammonia (NH₃), emitted from human skin. Finally, we estimate the fate of secondarily formed carbonyl compounds with respect to their gas-phase reactions with ·OH, O₃, and NO₃ and compared with their removal by air exchange rate (AER) with outdoors. The obtained results suggest that human presence is a source of an important number of organic compounds, which can significantly influence the air quality in indoor environments.

Distribution of ozone and its volatiles in indoor environment: a numerical simulation with CFD for the aircraft cabin

Ozone and its reactants are the main factors of air pollution in cabin environment, their distribution and propagation were influenced by different air supply modes in the cabin. Based the B767 aircraft cabin mode, a computational fluid dynamics (CFD) model of aircraft cabin had been built to study the distribution of ozone and the reaction product in the breathing zone, and the ozone risk under different air supply modes was evaluated. By comparing the three air supply modes, the simulation results show that the aisle air supply mode and personalized air supply mode made a high concentration of ozone and volatile organic compounds and increased absorbing risk in the breathing zone. Mixed air-supply mode made lower ozone risk in the breathing zone. But mixed air-supply mode can make a bad air mixture and makes a high ozone concentration near the cabin walls, which is needed to be improved. Ozone distribution for Y-Z section under the corridor air supply mode.

Efficient Coupling of Reaction Pathways of Criegee Intermediates and Free Radicals in the Heterogeneous Ozonolysis of Alkenes

In the gas phase, ozonolysis of olefins is known to be a significant source of free radicals. However, for heterogeneous and condensed phase ozone reactions, the importance of reaction pathways that couple Criegee intermediates (CI) with hydroxyl (OH), alkoxy, and peroxy free radicals remains uncertain. Here we report experimental evidence for substantial free radical oxidation during the heterogeneous reaction of O₃ with cis-9-tricosene (Tri) aerosol. A kinetic model with three coupled submechanisms that include O₃, CI, and free radical reactions is used to explain how the observed Tri reactivity and its product distributions depend upon [O₃], [OH], and the presence of CI scavengers. During multiphase ozonolysis, the kinetic model predicts that only ∼30% of the alkene is actually consumed by O₃, while the remaining ∼70% is consumed by free radicals that cycle through pathways involving CI. These results reveal the importance of free radical oxidation during heterogeneous ozonolysis, which has been previously difficult to isolate due to the complex coupling of CI and OH reaction pathways.

Breathing-rate adjusted population exposure to ozone and its oxidation products in 333 cities in China

While PM_2.5 (particles with aerodynamic diameter less than 2.5 µm) concentrations in China are beginning to decline because of pollution abatement measures, ozone (O₃) concentrations continue to rise. In this study, we have used a Monte Carlo approach to estimate breathing-rate adjusted (BRA) population exposure to ozone and its oxidation products based on hourly O₃ measurements collected in 2017 from monitoring stations in 333 Chinese cities. The median measured outdoor O₃ concentration in these cities was 31 ppb, while the median calculated indoor concentrations of ozone and ozone-derived oxidation products were 7.5 ppb and 21 ppb, respectively. The median BRA O₃ exposure concentration was 12 ppb, ranging from 2.2 ppb to 18 ppb among the cities. Eastern and central cities had higher exposure concentrations, while northeastern and western cities had lower. On average, the residents of these cities spent 88% of their time indoors. Consequently, even with breathing rate adjustments, indoor O₃ exposure averaged 50% of the total O₃ exposure nationwide. The median BRA exposure concentration for ozone-derived products was 18 ppb, ranging from 4.5 ppb to 32 ppb among the cities. On average, BRA exposure concentrations were 1.6 times larger for oxidation products than for ozone, while seasonal variations of exposure concentrations were smaller for oxidation products than for ozone. As many of the products of indoor ozone chemistry are toxic, the health consequences of exposure to such products should be further investigated.

The atmospheric chemistry of indoor environments

Through air inhalation, dust ingestion and dermal exposure, the indoor environment plays an important role in controlling human chemical exposure. Indoor emissions and chemistry can also have direct impacts on the quality of outdoor air. And so, it is important to have a strong fundamental knowledge of the chemical processes that occur in indoor environments. This review article summarizes our understanding of the indoor chemistry field. Using a molecular perspective, it addresses primarily the new advances that have occurred in the past decade or so and upon developments in our understanding of multiphase partitioning and reactions. A primary goal of the article is to contrast indoor chemistry to that which occurs outdoors, which we know to be a strongly gas-phase, oxidant-driven system in which substantial oxidative aging of gases and aerosol particles occurs. By contrast, indoor environments are dark, gas-phase oxidant concentrations are relatively low, and due to air exchange, only short times are available for reactive processing of gaseous and particle constituents. However, important gas-surface partitioning and reactive multiphase chemistry occur in the large surface reservoirs that prevail in all indoor environments. These interactions not only play a crucial role in controlling the composition of indoor surfaces but also the surrounding gases and aerosol particles, thus affecting human chemical exposure. There are rich research opportunities available if the advanced measurement and modeling tools of the outdoor atmospheric chemistry community continue to be brought indoors.

Heterogeneous Ozonolysis of Squalene: Gas-Phase Products Depend on Water Vapor Concentration

Previous work examining the condensed-phase products of squalene particle ozonolysis found that an increase in water vapor concentration led to lower concentrations of secondary ozonides, increased concentrations of carbonyls, and smaller particle diameter, suggesting that water changes the fate of the Criegee intermediate. To determine if this volume loss corresponds to an increase in gas-phase products, we measured gas-phase volatile organic compound (VOC) concentrations via proton-transfer-reaction time-of-flight mass spectrometry. Studies were conducted in a flow-tube reactor at atmospherically relevant ozone (O₃) exposure levels (5-30 ppb h) with pure squalene particles. An increase in water vapor concentration led to strong enhancement of gas-phase oxidation products at all tested O₃ exposures. An increase in water vapor from near zero to 70% relative humidity (RH) at high O₃ exposure increased the total mass concentration of gas-phase VOCs by a factor of 3. The observed fraction of carbon in the gas-phase correlates with the fraction of particle volume lost. Experiments involving O₃ oxidation of shirts soiled with skin oil confirms that the RH dependence of gas-phase reaction product generation occurs similarly on surfaces containing skin oil under realistic conditions. Similar behavior is expected for O₃ reactions with other surface-bound organics containing unsaturated carbon bonds.

Modeling the Time-Dependent Concentrations of Primary and Secondary Reaction Products of Ozone with Squalene in a University Classroom

Volatile organic chemicals are produced from reactions of ozone with squalene in human skin oil. Both primary and secondary reaction products, i.e., 6-methyl-5-hepten-2-one (6-MHO) and 4-oxopentanal (4-OPA), have been reported in indoor occupied spaces. However, the abundance of these products indoors is a function of many variables, including the amount of ozone and occupants present as well as indoor removal processes. In this study, we develop a time-dependent kinetic model describing the behavior of ozone/squalene reaction products indoors, including the reaction process and physical adsorption process of products on indoor surfaces. The key parameters in the model were obtained by fitting time-resolved concentrations of 6-MHO, 4-OPA, and ozone in a university classroom on 1 day with multiple class sessions. The model predictions were subsequently tested against observations from four additional measurement days in the same classroom. Model predictions and experimental data agreed well (R² = 0.87-0.92) for all test days, including ∼7 class sessions covering a range of occupants (10-70) and ozone concentrations (0.09-32 ppb), demonstrating the effectiveness of the model. Accounting for surface uptake of 6-MHO and 4-OPA significantly improved model predictions (R² = 0.52-0.76 without surface uptake), reflecting the importance of including surface interactions to quantitatively represent product behavior in indoor environments.

How do breath and skin emissions impact indoor air chemistry?

People are an important source of pollution indoors, through activities such as cleaning, and also from "natural" emissions from breath and skin. This paper investigates natural emissions in high-occupancy environments. Model simulations are performed for a school classroom during a typical summer in a polluted urban area. The results show that classroom occupants have a significant impact on indoor ozone, which increases from ~9 to ~20 ppb when the pupils leave for lunch and decreases to ~14 ppb when they return. The concentrations of 4-OPA, formic acid, and acetic acid formed as oxidation products following skin emissions attained maximum concentrations of 0.8, 0.5, and 0.1 ppb, respectively, when pupils were present, increasing from near-zero concentrations in their absence. For acetone, methanol, and ethanol from breath emissions, maximum concentrations were ~22.3, 6.6, and 21.5 ppb, respectively, compared to 7.4, 2.1, and 16.9 ppb in their absence. A rate of production analysis showed that occupancy reduced oxidant concentrations, while enhancing formation of nitrated organic compounds, owing to the chemistry that follows from increased aldehyde production. Occupancy also changes the peroxy radical composition, with those formed through isoprene oxidation becoming relatively more important, which also has consequences for subsequent oxidant concentrations.

Heterogeneous Chlorination of Squalene and Oleic Acid

Washing with chlorine bleach leads to high mixing ratios of gas-phase HOCl. Using two methods that are sensitive to surface film composition-attenuated total reflection fourier transform infrared (ATR-FTIR) spectroscopy and direct analysis in real time mass spectrometry (DART-MS)-we present the first study of the chlorination chemistry that occurs when gaseous HOCl reacts with thin films of squalene and oleic acid. At mixing ratios of 600 ppbv, HOCl forms chlorohydrins by adding across carbon-carbon double bonds without breaking the carbon backbone. The initial uptake of one HOCl molecule occurs on the time scale of a few minutes at these mixing ratios. For oleic acid, ester formation proceeds immediately thereafter, leading to dimeric and trimeric chlorinated products. For squalene, subsequent HOCl uptake occurs until all six of its carbon-carbon double bonds become chlorinated within 1-2 h. These results indicate that chlorination of skin oil, which contains substantial carbon unsaturation, is likely to occur rapidly under common cleaning conditions, potentially leading to the irritation associated with chlorinated bleach. This chemistry will likely also proceed with cooking oils, in the human respiratory system which has unsaturated surfactants as important components of lung fluid, and with organic components of the sea surface microlayer.

Headspace Solid-Phase Microextraction Gas Chromatography-Mass Spectrometry Analysis of Scent Profiles from Human Skin

Volatile organic compounds (VOCs) emanating from human skin contribute to an individual’s body odour. Understanding the modulation of human odour by a fragrance is of significant importance to the cosmetic sector in the design, development and evaluation of new products. The present research describes an in vivo approach for passive headspace sampling of skin volatile emissions in human participants. A wearable headspace solid-phase microextraction (HS-SPME) method has been employed to investigate baseline endogenous skin volatiles and the subsequent modulation of skin volatile profiles after application of a fragrance to skin. Coupled with gas chromatography-mass spectrometry (GC-MS) this method enables characterisation of scent profiles and fragrance longevity in vivo. A total of 51 compounds were identified in participants’ skin, including 19 endogenous and 32 fragrance-derived compounds. The temporal variation in volatile profiles at different times after fragrance application was investigated. Fragrance diffusion from skin varied between participants resulting in diversified scent profiles over time. This non-invasive approach could be employed during cosmetic product development for in vivo evaluation of fragrance profiles and for assessment of the retention of fragrance components in skin to reduce reliance on expert panels during product development.

Connecting the Elementary Reaction Pathways of Criegee Intermediates to the Chemical Erosion of Squalene Interfaces during Ozonolysis

Criegee intermediates (CI), formed in alkene ozonolysis, are central for controlling the multiphase chemistry of organic molecules in both indoor and outdoor environments. Here, we examine the heterogeneous ozonolysis of squalene, a key species in indoor air chemistry. Aerosol mass spectrometry is used to investigate how the ozone (O₃) concentration, relative humidity (RH), and particle size control reaction rates and mechanisms. Although the reaction rate is found to be independent of RH, the reaction products and particle size depend upon H₂O. Under dry conditions (RH = 3%) the reaction produces high-molecular-weight secondary ozonides (SOZ), which are known skin irritants, and a modest change in particle size. Increasing the RH reduces the aerosol size by 30%, while producing mainly volatile aldehyde products, increases potential respiratory exposure. Chemical kinetics simulations link the elementary reactions steps of CI to the observed kinetics, product distributions, and changes in particle size. The simulations reveal that ozonolysis occurs near the surface and is O₃-transport limited. The observed secondary ozonides are consistent with the formation of mainly secondary CI, in contrast to gas-phase ozonolysis mechanisms.

The micro-environmental impact of volatile organic compound emissions from large-scale assemblies of people in a confined space

Large-scale assemblies of people in a confined space can exert significant impacts on the local air chemistry due to human emissions of volatile organics. Variations of air-quality in such small scale can be studied by quantifying fingerprint volatile organic compounds (VOCs) such as acetone, toluene, and isoprene produced during concerts, movie screenings, and sport events (like the Olympics and the World Cup). This review summarizes the extent of VOC accumulation resulting from a large population in a confined area or in a small open area during sporting and other recreational activities. Apart from VOCs emitted directly from human bodies (e.g., perspiration and exhaled breath), those released indirectly from other related sources (e.g., smoking, waste disposal, discharge of food-waste, and use of personal-care products) are also discussed. Although direct and indirect emissions of VOCs from human may constitute <1% of the global atmospheric VOCs budget, unique spatiotemporal variations in VOCs species within a confined space can have unforeseen impacts on the local atmosphere to lead to acute human exposure to harmful pollutants.

Kinetics and Products from Heterogeneous Oxidation of Squalene with Ozone

Motivated by the importance of the heterogeneous chemistry of squalene contained within skin oil to indoor air chemistry, the surface reaction of squalene with gas-phase ozone has been investigated. Using direct analysis in real time mass spectrometry (DART-MS) to monitor squalene, the reactive uptake coefficients were determined to be (4.3 ± 2.2) × 10^-4 and (4.0 ± 2.2) × 10^-4 for ozone mixing ratios (MR_O3) of 50 and 25 ppb, respectively, on squalene films deposited on glass surfaces. At an MR_O3 of 25 ppb, the lifetime for oxidation was the same as that in an indoor office with an MR_O3 between 22 and 32 ppb, suggesting that O₃ was the dominant oxidant in this indoor setting. While the heterogeneous kinetics of squalene and O₃ were independent of relative humidity (RH), the RH significantly affected the reaction products. Under dry conditions (<5% RH), in addition to several products between m/z 300 and 350, the major condensed-phase end products were levulinic acid (LLA) and succinic acid (SCA). Under humid conditions (50% RH), the major end products were 4-oxopentanal, 4-oxobutanoic acid, and LLA. The molar yields of LLA and SCA were quantified as 230 ± 43% and 110 ± 31%, respectively, under dry conditions and 91 ± 15% and <5%, respectively, at 50% RH. Moreover, high-molecular weight (molecular weight of >450 Da) products were observed under dry conditions with indications that LLA was involved in their formation. The mechanism of squalene oxidation is discussed in light of these observations, with indications of an important role played by Criegee intermediates.

Roles of the human occupant in indoor chemistry

Over the last decade, influences of the human occupant on indoor chemistry have been investigated in environments ranging from simulated aircraft cabins to actual classrooms. We have learned that ozone reacts rapidly with constituents of skin surface lipids on exposed skin, hair, and clothing, substantially reducing indoor ozone concentrations but increasing airborne levels of mono- and bifunctional compounds that contain carbonyl, carboxyl, or α-hydroxy ketone groups. Moreover, occupants transfer skin oils to and shed skin flakes (desquamation) onto indoor surfaces. Evidence for the presence of skin flakes/oils has been found in airborne particles, settled dust, and wipes of indoor surfaces. These occupant residues are also anticipated to scavenge ozone and produce byproducts. Under typical conditions, occupancy is anticipated to decrease the net level of oxidants in indoor air. When occupants scavenge ozone, the level of SOA derived from ozone/terpene chemistry decreases; the fraction of SVOCs in the gas-phase increases, and the fraction associated with airborne particles decreases. Occupants also remove organic compounds, including certain chemically active species, via bodily intake. Studies reviewed in this paper demonstrate the pronounced influences of humans on chemistry within the spaces they inhabit and the consequences of these influences on their subsequent chemical exposures.

Atmospheric Oxidation of Squalene: Molecular Study Using COBRA Modeling and High-Resolution Mass Spectrometry

Squalene is a major component of skin and plant surface lipids and is known to be present at high concentrations in indoor dust. Its high reactivity toward ozone makes it an important ozone sink and a natural protectant against atmospheric oxidizing agents. While the volatile products of squalene ozonolysis are known, the condensed-phase products have not been characterized. We present an analysis of condensed-phase products resulting from an extensive oxidation of squalene by ozone probed by electrospray ionization (ESI) high-resolution mass spectrometry (HR-MS). A complex distribution of nearly 1300 peaks assignable to molecular formulas is observed in direct infusion positive ion mode ESI mass spectra. The distribution of peaks in the mass spectra suggests that there are extensive cross-coupling reactions between hydroxy-carbonyl products of squalene ozonolysis. To get additional insights into the mechanism, we apply a Computational Brewing Application (COBRA) to simulate the oxidation of squalene in the presence of ozone, and compare predicted results with those observed by the HR-MS experiments. The system predicts over one billion molecular structures between 0 and 1450 Da, which correspond to about 27 000 distinct elemental formulas. Over 83% of the squalene oxidation products inferred from the mass spectrometry data are matched by the simulation. The simulation indicates a prevalence of peroxy groups, with hydroxyl and ether groups being the second-most important O-containing functional groups formed during squalene oxidation. These highly oxidized products of squalene ozonolysis may accumulate on indoor dust and surfaces and contribute to their redox capacity.

Volatile organic compound conversion by ozone, hydroxyl radicals, and nitrate radicals in residential indoor air: Magnitudes and impacts of oxidant sources

Indoor chemistry may be initiated by reactions of ozone (O₃), the hydroxyl radical (OH), or the nitrate radical (NO₃) with volatile organic compounds (VOC). The principal indoor source of O₃ is air exchange, while OH and NO₃ formation are considered as primarily from O₃ reactions with alkenes and nitrogen dioxide (NO₂), respectively. Herein, we used time-averaged models for residences to predict O₃, OH, and NO₃ concentrations and their impacts on conversion of typical residential VOC profiles, within a Monte Carlo framework that varied inputs probabilistically. We accounted for established oxidant sources, as well as explored the importance of two newly realized indoor sources: (i) the photolysis of nitrous acid (HONO) indoors to generate OH and (ii) the reaction of stabilized Criegee intermediates (SCI) with NO₂ to generate NO₃. We found total VOC conversion to be dominated by reactions both with O₃, which almost solely reacted with d-limonene, and also with OH, which reacted with d-limonene, other terpenes, alcohols, aldehydes, and aromatics. VOC oxidation rates increased with air exchange, outdoor O₃, NO₂ and d-limonene sources, and indoor photolysis rates; and they decreased with O₃ deposition and nitric oxide (NO) sources. Photolysis was a strong OH formation mechanism for high NO, NO₂, and HONO settings, but SCI/NO₂ reactions weakly generated NO₃ except for only a few cases.

Identification of volatile organic compounds in human cerumen

We report here the initial examination of volatile organic compounds (VOCs) emanating from human earwax (cerumen). Recent studies link a single nucleotide polymorphism (SNP) in the adenosine triphosphate (ATP) binding cassette, sub-family C, member 11 gene (ABCC11) to the production of different types of axillary odorants and cerumen. ABCC11 encodes an ATP-driven efflux pump protein that plays an important function in ceruminous apocrine glands of the auditory canal and the secretion of axillary odor precursors. The type of cerumen and underarm odor produced by East Asians differ markedly from that produced by non-Asians. In this initial report we find that both groups emit many of the same VOCs but differ significantly in the amounts produced. The principal odorants are volatile organic C2-to-C6 acids. The physical appearance of cerumen from the two groups also matches previously reported ethnic differences, viz., cerumen from East Asians appears dry and white while that from non-Asians is typically wet and yellowish-brown.

Irritancy and allergic responses induced by exposure to the indoor air chemical 4-oxopentanal

Over the last two decades, there has been an increasing awareness regarding the potential impact of indoor air pollution on human health. People working in an indoor environment often experience symptoms such as eye, nose, and throat irritation. Investigations into these complaints have ascribed the effects, in part, to compounds emitted from building materials, cleaning/consumer products, and indoor chemistry. One suspect indoor air contaminant that has been identified is the dicarbonyl 4-oxopentanal (4-OPA). 4-OPA is generated through the ozonolysis of squalene and several high-volume production compounds that are commonly found indoors. Following preliminary workplace sampling that identified the presence of 4-OPA, these studies examined the inflammatory and allergic responses to 4-OPA following both dermal and pulmonary exposure using a murine model. 4-OPA was tested in a combined local lymph node assay and identified to be an irritant and sensitizer. A Th1-mediated hypersensitivity response was supported by a positive response in the mouse ear swelling test. Pulmonary exposure to 4-OPA caused a significant elevation in nonspecific airway hyperreactivity, increased numbers of lung-associated lymphocytes and neutrophils, and increased interferon-γ production by lung-associated lymph nodes. These results suggest that both dermal and pulmonary exposure to 4-OPA may elicit irritant and allergic responses and may help to explain some of the adverse health effects associated with poor indoor air quality.

Measuring the penetration of ambient ozone into residential buildings

Much of human exposure to ambient ozone and ozone reaction byproducts occurs inside buildings. However, there are currently no experimental data on the ability of ozone to penetrate through building envelopes and into residences. This paper presents a method to determine the penetration factor for ozone in buildings, and applies it in an unoccupied test house and seven single-family residences. The mean (±SD) ozone penetration factor was measured as 0.79 ± 0.13 in the eight homes using this method, ranging from 0.62 ± 0.09 to 1.02 ± 0.15. An analysis of tests across the homes revealed that ozone penetration was significantly higher in homes with more painted wood envelope materials, homes with larger air leakage exponents from fan pressurization tests, and older homes. The test method utilizes a large calibrated fan to elevate air exchange rates and steady-state indoor ozone concentrations to levels that can be accurately measured, so there is a potential for overpredicting ozone penetration factors. However, evidence suggests that this bias is likely small in most of the homes, and, even if a bias exists, the measured ozone penetration factors were lower than the usual assumption of unity in seven of the eight tested homes.

Ozone-initiated particle formation, particle aging, and precursors in a laser printer

An increasing number of researchers have hypothesized that ozone may be involved in the particle formation processes that occur during printing, however no studies have investigated this further. In the current study, this hypothesis was tested in a chamber study by adding supplemental ozone to the chamber after a print job without measurable ozone emissions. Subsequent particle number concentration and size distribution measurements showed that new particles were formed minutes after the addition of ozone. The results demonstrated that ozone did react with printer-generated volatile organic compounds (VOCs) to form secondary organic aerosols (SOAs). The hypothesis was further confirmed by the observation of correlations among VOCs, ozone, and particles concentrations during a print job with measurable ozone emissions. The potential particle precursors were identified by a number of furnace tests, which suggested that squalene and styrene were the most likely SOA precursors with respect to ozone. Overall, this study significantly improved scientific understanding of the formation mechanisms of printer-generated particles, and highlighted the possible SOA formation potential of unsaturated nonterpene organic compounds by ozone-initiated reactions in the indoor environment.

Chemistry in indoor environments: 20 years of research

In the two decades since the first issue of Indoor Air, there have been over 250 peer-reviewed publications addressing chemical reactions among indoor pollutants. The present review has assembled and categorized these publications. It begins with a brief account of the state of our knowledge in 1991 regarding 'indoor chemistry', much of which came from corrosion and art conservation studies. It then outlines what we have learned in the period between 1991 and 2010 in the context of the major reference categories: gas-phase chemistry, surface chemistry, health effects and reviews/workshops. The indoor reactions that have received the greatest attention are those involving ozone-with terpenoids in the gas-phase as well as with the surfaces of common materials, furnishings, and the occupants themselves. It has become clear that surface reactions often have a larger impact on indoor settings than do gas-phase processes. This review concludes with a subjective list of major research needs going forward, including more information on the decomposition of common indoor pollutants, better understanding of how sorbed water influences surface reactions, and further identification of short-lived products of indoor chemistry. Arguably, the greatest need is for increased knowledge regarding the impact that indoor chemistry has on the health and comfort of building occupants.

PRACTICAL IMPLICATIONS

Indoor chemistry changes the type and concentration of chemicals present in indoor environments. In the past, products of indoor chemistry were often overlooked, reflecting a focus on stable, relatively non-polar organic compounds coupled with the use of sampling and analytical methods that were unable to 'see' many of the products of such chemistry. Today, researchers who study indoor environments are more aware of the potential for chemistry to occur. Awareness is valuable, because it leads to the use of sampling methods and analytical tools that can detect changes in indoor environments resulting from chemical processes. This, in turn, leads to a more complete understanding of occupants' chemical exposures, potential links between these exposures and adverse health effects and, finally, steps that might be taken to mitigate these adverse effects.

Surface chemistry of a pine-oil cleaner and other terpene mixtures with ozone on vinyl flooring tiles

Indoor environments are dynamic reactors where consumer products (such as cleaning agents, deodorants, and air fresheners) emit volatile organic compounds (VOCs) that can subsequently interact with indoor oxidants such as ozone (O(3)), hydroxyl radicals, and nitrate radicals. Typically, consumer products consist of mixtures of VOCs and semi-VOCs which can react in the gas-phase or on surfaces with these oxidants to generate a variety of oxygenated products. In this study, the reaction of a pine-oil cleaner (POC) with O(3) (100ppb) on a urethane-coated vinyl flooring tile was investigated at 5% and 50% relative humidity. These results were compared to previous α-terpineol+O(3) reactions on glass and vinyl surfaces. Additionally, other terpene and terpene alcohol mixtures were formulated to understand the emission profiles as seen in the POC data. Results showed that the α-terpineol+O(3) reaction products were the prominent species that were also observed in the POC/O(3) surface experiments. Furthermore, α-terpineol+O(3) reactions generate the largest fraction of oxygenated products even in equal mixtures of other terpene alcohols. This finding suggests that the judicial choice of terpene alcohols for inclusion in product formulations may be useful in reducing oxidation product emissions.

Ozone-surface reactions in five homes: surface reaction probabilities, aldehyde yields, and trends

Field experiments were conducted in five homes during three seasons (summer 2005, summer 2006 and winter 2007) to quantify ozone-initiated secondary aldehyde yields, surface reaction probabilities, and trends any temporal over a 1.5-year interval. Surfaces examined include living room carpets, bedroom carpets, kitchen floors, kitchen counters, and living room walls. Reaction probabilities for all surfaces for all seasons ranged from 9.4 x 10(-8) to 1.0 x 10(-4). There were no significant temporal trends in reaction probabilities for any surfaces from summer 2005 to summer 2006, nor over the entire 1.5-year period, indicating that it may take significantly longer than this period for surfaces to exhibit any 'ozone aging' or lowering of ozone-surface reactivity. However, all surfaces in three houses exhibited a significant decrease in reaction probabilities from summer 2006 to winter 2007. The total yield of aldehydes for the summer of 2005 were nearly identical to that for summer of 2006, but were significantly higher than for winter 2007. We also observed that older carpets were consistently less reactive than in newer carpets, but that countertops remained consistently reactive, probably because of occupant activities such as cooking and cleaning.

PRACTICAL IMPLICATIONS

Ozone reactions taking place at indoor surfaces significantly influence personal exposure to ozone and volatile reaction products. These field studies show that indoor surfaces only slowly lose their ability to react with ozone over several year time frames, and that this is probably because of a combination of large reservoirs of reactive coatings and periodic additions of reactive coatings in the form of cooking, cleaning, and skin-oil residues. When considering exposure to ozone and its reaction products and in the absence of dramatic changes in occupancy, activities or furnishings, indoor surface reactivity is expected to change very slowly.

Reactions of ozone with human skin lipids: sources of carbonyls, dicarbonyls, and hydroxycarbonyls in indoor air

This study has used proton transfer reaction-mass spectrometry (PTR-MS) for direct air analyses of volatile products resulting from the reactions of ozone with human skin lipids. An initial series of small-scale in vitro and in vivo experiments were followed by experiments conducted with human subjects in a simulated office. The latter were conducted using realistic ozone mixing ratios (approximately 15 ppb with occupants present). Detected products included mono- and bifunctional compounds that contain carbonyl, carboxyl, or alpha-hydroxy ketone groups. Among these, three previously unreported dicarbonyls have been identified, and two previously unreported alpha-hydroxy ketones have been tentatively identified. The compounds detected in this study (excepting acetone) have been overlooked in surveys of indoor pollutants, reflecting the limitations of the analytical methods routinely used to monitor indoor air. The results are fully consistent with the Criegee mechanism for ozone reacting with squalene, the single most abundant unsaturated constituent of skin lipids, and several unsaturated fatty acid moieties in their free or esterified forms. Quantitative product analysis confirms that squalene is the major scavenger of ozone at the interface between room air and the human envelope. Reactions between ozone and human skin lipids reduce the mixing ratio of ozone in indoor air, but concomitantly increase the mixing ratios of volatile products and, presumably, skin surface concentrations of less volatile products. Some of the volatile products, especially the dicarbonyls, may be respiratory irritants. Some of the less volatile products may be skin irritants.

Investigating ozone-induced decomposition of surface-bound permethrin for conditions in aircraft cabins

The reaction of ozone with permethrin can potentially form phosgene. Published evidence on ozone levels and permethrin surface concentrations in aircraft cabins indicated that significant phosgene formation might occur in this setting. A derivatization technique was developed to detect phosgene with a lower limit of detection of 2 ppb. Chamber experiments were conducted with permethrin-coated materials (glass, carpet, seat fabric, and plastic) exposed to ozone under cabin-relevant conditions (150 ppb O(3), 4.5/h air exchange rate, <1% relative humidity, 1700 ng/cm(2) of permethrin). Phosgene was not detected in these experiments. Reaction of ozone with permethrin appears to be hindered by the electron-withdrawing chlorine atoms adjacent to the double bond in permethrin. Experimental results indicate that the upper limit on the reaction probability of ozone with surface-bound permethrin is approximately 10(-7). Extrapolation by means of material-balance modeling indicates that the upper limit on the phosgene level in aircraft cabins resulting from this chemistry is approximately 1 microg/m(3) or approximately 0.3 ppb. It was thus determined that phosgene formation, if it occurs in aircraft cabins, is not likely to exceed relevant, health-based phosgene exposure guidelines.

PRACTICAL IMPLICATIONS

Phosgene formation from ozone-initiated oxidation of permethrin in the aircraft cabin environment, if it occurs, is estimated to generate levels below the California Office of Environmental Health Hazard Assessment acute reference exposure level of 4 microg/m(3) or approximately 1 ppb.

Heterogeneous oxidation of squalene film by ozone under various indoor conditions

Abstract The effects of indoor conditions (ozone concentration, temperature, relative humidity (RH), and the presence of NO(x)) on heterogeneous squalene oxidation were studied with Attenuated Total Reflectance-Fourier Transform Infrared spectroscopy. The heterogeneous kinetics of squalene-ozone reaction revealed a pseudo-first-order reaction rate constant of 1.22 x 10(-5)/s at [O(3)] = 40 ppb. Oxidation kinetics were insensitive to temperature over the range of 24-58 +/- 2 degrees C as well as to RH and presence of NO(x). Products, however, were affected by the environmental parameters. As temperature was increased, fewer surface products and more low molecular weight gaseous products were observed. Lower air exchange rates also enhanced gas phase reactions, allowing for formation of secondary gas phase products. As RH increased, there was a shift in product distribution from ketones to aldehydes, and the presence of NO(x) during squalene ozonolysis resulted in the formation of nitrated oxidation products. Identified surface products included 6-methyl-5-hepten-2-one, geranyl acetone, and long chain ketones and aldehydes, while gas phase products included formaldehyde, acetone, 4-oxopentanal (4-OPA), glyoxal, and pyruvic acid. Practical Implications Heterogeneous oxidation of squalene resulted in surface products including long chain aldehydes and ketones, and gas phase products including formaldehyde, a known human carcinogen (IARC 2006), and bicarbonyl compounds like: 4-oxopentanal (4-OPA), glyoxal, and pyruvic acid that are characterized as asthma triggers and sensitizers (Anderson et al., 2007; Jarvis et al., 2005). In addition, ozonolysis experiments in the presence of NO(x) showed the formation of nitrated surface oxidation products. Such nitrated products may have higher mutagenicity, carcinogenicity, or allergenic potential than their nitrate free counterparts (Franze et al., 2005; Pitts, 1983). Kinetic studies determined that at moderate ozone levels of 40 ppb (Uhde and Salthammer, 2007), and an estimated skin surface density of 4 x 10(15) molecules/cm(2), surface reaction would lead to a minimum product formation flux of 4 x 10(10) molecules cm(2)/s. As squalene is naturally occurring and continually produced by the human body, its concentration in the indoor environment cannot be controlled. However, this study highlights the importance of regulating air exchange rate, temperature, and ozone level in the indoor environment on the formation of potentially harmful or irritating squalene oxidation products.