Function Of Ozone Concentration Research Articles

Room temperature optical detection of ultra-low ozone concentration using photoluminescent ZnO nanohybrids

The highly sensitive room temperature (RT) photoluminescence (PL) emission of zinc oxide (ZnO) in the presence of ozone (O3) is demonstrated for the first time. The optical materials examined consist of commercially available luminescent ZnO nanoparticles (ZnO NPs), as well as ZnO/polymer nanohybrids, with different polymeric matrices (poly(poly(ethylene glycol) methyl ether methacrylate) (PPEGMA) and polydimethylsiloxane (PDMS)), which are excited with a UV pulsed laser source (λex= 248 nm, τex= 15 ns).The PL emission of the ZnO NPs and their nanohybrids (ZnO/PPEGMA, ZnO/PDMS) is investigated upon exposure to ozone gas and their sensing characteristics, such as response (%ΔI), reversibility, response/recovery time are determined, as a function of ozone concentration in synthetic air.Sensing tests revealed that all materials allowed the detection of ozone gas in a wide range of concentrations in synthetic air (1600 down to 50 ppb), with the ZnO/PDMS nanohybrid exhibiting the highest response and recovery time of about 50 s (at 750 ppb ozone concentration) and 100 s, respectively. Finally, its remarkable optical response of about 20% at 50 ppb ozone concentration at RT, underlines the great potential of the ZnO/PDMS nanohybrid to serve as a sensitive probe for ozone detection, thereby introducing new developments in optosensing applications.

Photosensitised heterogeneous oxidation kinetics of biomass burning aerosol surrogates by ozone using an irradiated rectangular channel flow reactor

Abstract. Heterogeneous reaction kinetics involving organic aerosol and atmospheric oxidants such as ozone can be enhanced under visible or UV irradiation in the presence of a photosensitiser, with subsequent implications for the climate, cloud radiative properties, air quality, and source appointment. In this study we report the steady-state reactive uptake coefficient, γ, of O3 by levoglucosan and 5-nitroguaiacol acting as surrogates for biomass burning aerosol particles, with and without the presence of Pahokee peat acting as a photosensitiser. The reactive uptake has been determined in the dark and as a function of visible and UV-A irradiation and ozone concentration. In addition, γ was determined for 1 : 1, 1 : 10, and 1 : 100 by mass mixtures of Pahokee peat and 5-nitroguaiacol, and for a 10 : 1 : 3 mixture of levoglucosan, Pahokee peat, and 5-nitroguaiacol. We developed a novel irradiated rectangular channel flow reactor (I-RCFR) that was operated under low pressures of about 2–4 hPa, and allowed for uniform irradiation of the organic substrates. The I-RCFR was coupled to a chemical ionisation mass spectrometer and has been successfully validated by measuring the kinetics between various organic species and oxidants. γ of O3 and levoglucosan in the dark and under visible and UV-A irradiation was determined to be in the range of (2–11) × 10−6 and did not change in the presence of Pahokee peat. The determined γ of O3 and 5-nitroguaiacol in the dark was 5.7 × 10−6 and was only enhanced under UV-A irradiation, yielding a value of 3.6 × 10−5. γ of the 1 : 1 Pahokee peat/5-nitroguaiacol substrate was enhanced under visible and UV-A irradiation to 2.4 × 10−5 and 2.8 × 10−5, respectively. Decreasing the amount of Pahokee peat in the 5-nitroguaiacol/Pahokee peat substrate resulted in lower values of γ under visible irradiation, however, γ was consistent under UV-A irradiation regardless of the amount of Pahokee peat. The 10 : 1 : 3 mixture by mass of levoglucosan, Pahokee peat, and 5-nitroguaiacol, under both visible and UV-A irradiation yielded γ values of 2.8 × 10−5 and 1.4 × 10−5, respectively. γ was determined as a function of photon flux for O3 with the 1 : 1 Pahokee peat/5-nitroguaiacol substrate, yielding a linear relationship under both visible and UV-A irradiation. γ of O3 with the 1 : 1 Pahokee peat/5-nitroguaiacol substrate was determined as a function of ozone concentration and exhibited an inverse dependence of γ on ozone concentration, commonly interpreted as a Langmuir–Hinshelwood mechanism. The reactive uptake data have been represented by a Langmuir-type isotherm. From the O3 uptake data under visible irradiation, the following fit parameters have been derived: ks = (5.5 ± 2.7) × 10−19 cm2 s−1 molecule−1 and KO3 = (2.3 ± 2.0) × 10−12 cm3 molecule−1; and under UV-A irradiation: ks = (8.1 ± 2.0) × 10−19 cm2 s−1 molecule−1 and KO3 = (1.7 ± 0.7) × 10−12 cm3 molecule−1. The oxidative power, or the product of γ and [O3], was determined for O3 with the 1 : 1 Pahokee peat/5-nitroguaiacol substrate and was in the range of (1.2–26) × 106 molecule cm−3. Atmospheric particle lifetimes were estimated for a 0.4 μm 5-nitroguaiacol particle as a function of visible and UV-A irradiation and ozone concentration.

Investigation of Ozone Concentration Measurement by Visible Photo Absorption Method

The measurement of ozone concentration based on the visible photo absorption method is investigated. An LED having an emission peak at 609 nm and a photo diode sensitive at visible light frequencies are used as the light source and the photo detector, respectively. The transmittance is measured as a function of ozone concentration, which is in reasonable agreement with the Beer–Lambert relationship. The effective absorption cross-section can therefore be estimated from the fitting procedure. The concentrations determined by both the UV and visible photo absorptions agree well with each other in the range from 7.1 to 68.4 g/m3(N). The relative error in these concentrations is discussed on the basis of the Twyman–Lothian curve. The ozone measurement for the air-fed ozone generation is also investigated. The effect of photo absorption due to NOx species is unavoidable. The present results provide useful data if the ozone measurement based on the visible photo absorption method is applied to ozone monitoring instruments or used for laboratory experiments.

Correlation and prediction of azo dye degradation by nonlinear least-square regression in combined ozonation and ultrasonolysis processes

Correlative reaction kinetics based on nonlinear least square regression were obtained to predict the degradation of p-aminophenol (PAP), C.I. Reactive Yellow 84 (RY84), C.I. Reactive Blue 19 (RB19), and C.I. Direct Red 23 (DR23) in combined ultrasonolysis and ozonation. Effects of ultrasonolysis and ozonation as individual processes, as well as the synergetic effect for the degradation of azo dyes, were studied by linear and nonlinear regression methods. An initial dye concentration correlation index (φ) for each compound was developed to study the effects of initial concentrations of organics on their reaction rate constants. Reaction rate constants for the azo dyes’ degradation in ozonation and ultrasonolysis alone and their combination were obtained for different operating conditions. Results showed that ultrasonolysis alone was almost ineffective for decolorization and mineralization of azo dyes, but the combination of ozonation and ultrasonolysis had a significant effect on their degradation. Both ozone dose and ultrasonic density had a direct effect on the reaction rate constants, where the impact of ozone was greater with respect to that of the ultrasound. The synergetic effect of combined processes as a function of ozone concentration and ultrasound density varied from 0 to 5, 0 to 4, and 0 to 35 for RY84, RB19, and DR23, respectively.

Ozone sonde cell current measurements and implications for observations of near-zero ozone concentrations in the tropical upper troposphere

Abstract. Laboratory measurements of the Electrochemical Concentration Cell (ECC) ozone sonde cell current using ozone free air as well as defined amounts of ozone reveal that background current measurements during sonde preparation are neither constant as a function of time, nor constant as a function of ozone concentration. Using a background current, measured at a defined timed after exposure to high ozone may often overestimate the real background, leading to artificially low ozone concentrations in the upper tropical troposphere, and may frequently lead to operator dependent uncertainties. Based on these laboratory measurements an improved cell current to partial pressure conversion is proposed, which removes operator dependent variability in the background reading and possible artifacts in this measurement. Data from the Central Equatorial Pacific Experiment (CEPEX) have been reprocessed using the improved background treatment based on these laboratory measurements. In the reprocessed data set near-zero ozone events no longer occur. At Samoa, Fiji, Tahiti, and San Cristóbal, nearly all near-zero ozone concentrations occur in soundings with larger background currents. To a large extent, these events are no longer observed in the reprocessed data set using the improved background treatment.

Two Reactivity Modes in the Heterogeneous Cyclohexene Ozonolysis under Tropospherically Relevant Ozone-Rich and Ozone-Limited Conditions

Important mechanistic differences regarding C=C double-bond oxidation processes under ozone-limited and ozone-rich reaction conditions for cyclohexene-functionalized fused silica substrates serving as model systems for studying heterogeneous C=C double bond oxidation chemistry in the troposphere are evaluated. By using broadband vibrational sum frequency generation (SFG), we track heterogeneous ozone reactions in real time. Ozone levels span three orders of magnitude and represent environments ranging from pristine remote continental regions to highly polluted urban centers, ranging from 30 ppb to 3 ppm (from 7 x 10(11) molecules cm(-3) to 7 x 10(13) molecules cm(-3)). We determine reaction rates and reactive uptake coefficients (gamma values). At these tropospherically relevant ozone levels, the heterogeneous reaction rates follow a Langmuir-Hinshelwood-type mechanism. The product formation rates, which we determine as a function of ozone concentrations, are found to be half of the olefin reaction rates. This ratio is consistent with the previously proposed reaction pathway involving the breaking of one C=C double bond containing two olefinic CH moieties to form a product containing only one methyl group and one polar carbonyl moiety. Contact angle histograms show that out of a total of 60 measurements, there are about 25 more measurements with contact angles up to ten degrees below the mean recorded prior to reaction when ozone levels resemble remote continental conditions (50 ppb) than when ozone levels resemble urban conditions (1 ppm). The implication of these results are that the methyl formation pathway in heterogeneous ozonolysis may be less favorable than the carboxylic acid- and secondary ozonide-production pathway for ozone-limited conditions (i.e., in the remote continental troposphere or during urban nighttime) as opposed to ozone-rich (i.e., polluted urban atmosphere) conditions.

Ozone-initiated disinfection kinetics of Escherichia coli in water

The effect of ozonation on the rate of disinfection of Escherichia coli was investigated as a function of ozone concentration, ozonation duration and flow rates. Ozone was generated in situ using Corona discharge method using compressed oxygen stream and depending on the oxygen flux the ozone concentrations ranged from 0.91–4.72 mg/L. The rate of disinfection of all the three microbes followed pseudo–first-order kinetics with respect to the microbe count and first order with respect to ozone concentration. The influence of pH and temperature the aqueous systems on the rate of ozone initiated disinfection of the microbe was investigated. The inactivation was faster at lower pH than at basic pH. Molecular ozone is found more effective in disinfection than hydroxyl radicals. Two reported mechanisms for antimicrobial activity of ozone in water systems from the literature are discussed. Based on the experimental findings a probable rate law and mechanism are proposed. Ozonation of natural waters significantly decreased the BOD levels of the control and microbe contaminated waters.

Room Temperature Reaction of Ozone and Dimethyl Methylphosphonate (DMMP) on Alumina-Supported Iron Oxide

The reaction of dimethyl methylphosphonate (DMMP) with ozone on a reactive adsorbent, alumina-supported iron oxide, has been examined at room temperature. The surface oxidation reaction was found to follow the Langmuir−Hinshelwood mechanism, involving a surface adsorbed and activated DMMP molecule or fragment at a Lewis acid site and an active oxygen species on the surface formed from ozone. Carbon dioxide is the primary reaction product observed, and the amount formed was found to plateau as a function of ozone concentration after a concentration of approximately 300−400 ppm. The surface reaction with ozone on the 10 wt % Fe2O3/Al2O3 adsorbent generates approximately 2.7 times as much gas-phase carbon, in the form of CO2 and CO, as does the same surface in the absence of ozone, in the form of methanol. The ozone appears to increase the number of active sites for DMMP decomposition by liberating some of the reaction products from the surface and recycling the surface sites. The supported iron oxide materi...

Kinetic and Product Yield Study of the Heterogeneous Gas−Surface Reaction of Anthracene and Ozone

Little quantitative information exists regarding the products of the heterogeneous reaction of polycyclic aromatic hydrocarbons (PAHs) and ozone. We have, therefore, performed the first quantitative study investigating the kinetics and products of the heterogeneous gas-surface reaction of anthracene and ozone as a function of ozone concentration and relative humidity (RH). The reaction exhibited pseudo-first-order kinetics for anthracene loss under dry conditions (RH < 1%) and the pseudo-first-order rate coefficients displayed a Langmuir-Hinshelwood dependence on the gas-phase ozone concentration, which yielded the following fitting parameters: the equilibrium constant for ozone adsorption, K(O3) = (2.8 +/- 0.9) x 10(-15) cm3 and the maximum pseudo-first-order rate coefficient, k(I)max = (6.4 +/- 1.8) x 10(-3) s(-1). The kinetics were unchanged when experiments were performed at approximately 50% and 60% RH. In the product study, a nonlinear dependence, similar to a Langmuir adsorption plot, of the anthraquinone product yield as a function of ozone concentration was observed and resulted in the following fitting parameters: K(O3) = (3.4 +/- 1.5) x 10(-15) cm3 and the maximum anthraquinone yield, ANQmax % = 30 +/- 18%. Experiments performed under higher relative humidity conditions ( approximately 50% and 60% RH) revealed that the anthraquinone yield was unaffected by the presence of gas-phase water. It is noteworthy that both the anthracene loss kinetics and the anthraquinone yields have a similar dependence on the degree of ozone partitioning to the surface. This can be understood in terms of a mechanism whereby the rate-determining steps for anthracene loss and anthraquinone formation are both driven by the amounts of ozone adsorbed on the surface. Our results suggest that at atmospherically relevant ozone concentrations (100 ppb) the anthraquinone yield from the ozonolysis of anthracene under dry and high relative humidity conditions would be less than 1%.

Diffuse Reflectance FTIR Study of the Interaction of Alumina Surfaces with Ozone and Water Vapor

The interaction of ozone with alumina has been examined at ambient temperature as a function of ozone concentration and relative humidity. The experiments used diffuse reflectance FTIR spectroscopy in a small flow reactor, which provided control of the temperature, pressure, and composition of the gas mixture to which the sample was continuously exposed. Treatment of alumina with ozone produced a new spectroscopic feature at 1380 cm(-1), which we attribute to an aluminum oxide species formed by interaction of O3 with Lewis acid sites on the alumina surface. After exposure of the alumina sample to O3 was stopped, subsequent exposure of the sample to humidified nitrogen resulted in the slow removal of the peak at 1380 cm(-1). Simultaneously, the uptake of water by the alumina increased as indicated by the growth of the adsorbed water features which extend from approximately 3700 to 2500 cm(-1). Treatment of dry alumina with humidified ozone strongly inhibited both the rate of formation of the spectral feature at 1380 cm(-1) and its limiting extent of formation. These observations are analyzed in terms of the adsorption and surface reaction properties of ozone on alumina. The observation that the new oxide feature on alumina, produced by reaction with ozone, can be removed by water is important for assessing the ability of mineral dust aerosols to process atmospheric trace gases over a significant time scale. We believe the work reported here to be the first direct and quantitative kinetic study of the competition between O3 and water for adsorption sites on alumina.

A Method for Identifying Exposure-Response Models for the Acute Respiratory Effects of Inhaled Irritants in Humans: Effects of Concentration, Minute Ventilation, and Duration

In order to conduct quantitative risk assessment with minimal uncertainty for short-term exposure to ozone and other respiratory irritants, one must identify exposure-response (E-R) models that accurately predict the distribution of the magnitudes of response (or the proportion of individuals experiencing a given response) across the range of ambient exposure scenarios and personal characteristics of interest. The purpose of this manuscript is to describe a method for identification of such models and to demonstrate the method and evaluate the resulting model's performance using ozone as a representative respiratory irritant. This approach to model identification utilizes a general knowledge of mechanistic information and E-R characteristics for the specific pollutant and traditional statistical techniques for estimating model coefficients and prediction uncertainty. For ozone we identified a two-compartment model consistent with known mechanisms and E-R characteristics. Using data from 485 volunteers exposed for two hours to one of six ozone concentrations at one of three activity levels, we estimated model coefficients and evaluated the predictive ability of the model for lung function and symptom responses. We found that the model accurately predicted responses as a function of ozone concentration, minute ventilation during exposure, and duration of exposure and concluded that this approach to model identification was successful for ozone, and we hypothesized that a similar approach would be useful for other respiratory irritants.

A Statistical Model for FEV1 Response to Arbitrary Dynamic Ozone Exposure Conditions

ABSTRACT Lung function response to inhaled ozone at ambient air pollution levels is known to be a function of ozone concentration, exposure duration, and minute ventilation. Most data-driven exposure-response models address exposures under static condition (i.e., with a constant ozone concentration and exercise pattern). Such models are simplifications, as both ambient ozone concentrations and normal human activity patterns change with time. The purpose of this study was to develop a dynamic model of response with the advantages of a statistical model (a relatively simple structure with few parameters). A previously proposed mechanistic model for changes in specific airways resistance was adapted to describe the percent change in forced expiratory volume in one second (FEV1). This model was then reduced using the fit to three existing exposure-response data sets as criterion. The resulting model consists of a single linear differential equation together with an algebraic logistic equation. Under restricted static conditions the model reduces to a logistic model presented earlier by the authors.

Simulation of planetary UV radiation climate on the example of the early Earth

Solar UV radiation is a dynamic driving force of organic chemical evolution. However, it may have severe constraints in biological evolution, for instance in the early history of life on Earth, before atmospheric evolution of a protective ozone layer. Today, the full spectrum of solar UV radiation is experienced only in space. In the experiment RD-UVRAD on D-2 the biological response to solar UV as a function of ozone concentrations down to very low values was investigated. Biofilms, biological UV dosimeters, were exposed to the extraterrestrial solar radiation on one of the two exposure trays mounted on the User Support Structure (USS) with free access to space. Different cut-off filters, which exclude UV of short wavelengths, and neutral density filters were used. After the flight the biofilms were developed and analyzed on ground. As a result a strong correlation between the increase in biologically effective UV radiation with decreasing (simulated) ozone concentrations was observed. Without any ozone the solar UV radiation is about three orders of magnitude more effective than today on Earth. This space experiment resulted in new insights in the environmental conditions of life on the early Earth and in the deleterious biological consequences of future stratospheric ozone depletion.

Individual variability in human lung function responses to ozone exposure

Ozone is a common photochemical air pollutant which is present in the ambient air of many urban areas at concentrations sufficient to produce acute respiratory effects in humans. Because individuals vary considerably in the magnitude of their responses to ozone exposure, it is difficult to estimate the number of individuals in a given population who are experiencing adverse effects. Consequently risk and benefits analysis for various regulatory scenarios cannot be carried out with precision. As an aid to risk assessment this paper presents a method of predicting the proportion of individuals in the population who experience a particular health effect. Risk equations predicting the proportion of individuals experiencing lung function decrements as a function of ozone concentration, duration of exposure, and age are presented.

Proportion of moderately exercising individuals responding to low-level, multi-hour ozone exposure.

The purpose of this study was to describe the proportion of moderately exercising individuals experiencing significant respiratory responses to low-level, multi-hour ozone exposure as a function of ozone concentration and exposure duration. Sixty-eight healthy, nonsmoking adults, ages 18 to 34 yr, underwent two or more 6.6-h exposures to 0.0, 0.08, 0.10, or 0.12 ppm ozone. Five hours of exercise was performed during exposure, and lung function was measured before exposure and following each hour of exposure. For each combination of concentration and duration, each individual was determined to either have or not have experienced a 10% or greater decrement in FEV1. A logistic function was used to model the proportion of individuals experiencing such a decrement as a function of concentration and exposure duration. Bootstrap 90% confidence intervals (CIs) were calculated around the predictions. The model was found to give predictions that were in good agreement with observed data. The lowest level of exposure (C x T) for which the 90% CI excluded zero was approximately 0.2 ppm-h. For exposure to 0.12 ppm ozone for 6.6 h, 47% (90% CI = 30 to 65%) of exposed individuals were predicted to experience a 10% decrement in FEV1. A greater proportion of younger adults than of older adults were found to experience a given effect for a given exposure.

Application of the EPR spin-trapping technique to the detection of radicals produced in vivo during inhalation exposure of rats to ozone

Ozone is known to induce lipid peroxidation of lung tissue, although no direct evidence of free radical formation has been reported. We have used the electron paramagnetic resonance (EPR) spin-trapping technique to search for free radicals produced in vivo by ozone exposure. The spin trap α-(4-pyridyl-1-oxide)- N-tert-butylnitrone (4-POBN) was administered ip to male Sprague-Dawley rats. The rats were then exposed for 2 hr to either 0, 0.5, 1.0, 1.5, or 2.0 ppm ozone with 8% CO 2 to increase their respiratory rate. A six-line 4-POBN/radical spin adduct signal ( a N = 15.02 G and a β H = 3.27 G) was detected by EPR spectroscopy in lipid extracts from lungs of rats treated with 4-POBN and then exposed to ozone. Only a weak signal was observed in the corresponding solution from rats exposed to 0 ppm ozone (air with CO 2 only). The concentration of the radical adduct increased as a function of ozone concentration. After administration of 4-POBN, rats were exposed for either 0.5, 1.0, 2.0, or 4.0 hr to either 0 or 2.0 ppm ozone (with CO 2). The radical adduct concentration of the ozone-exposed groups at exposure times of 2.0 and 4.0 hr was significantly different from that of the corresponding air control groups. A correlation was observed between the radical adduct concentration and the lung weight/body weight ratio. These results demonstrate that ozone induces the production of free radicals in rat lungs during inhalation exposure and that radical production may be involved in the induction of pulmonary toxicity by ozone. This is the first direct evidence for ozone-induced free radical production in vivo.

Application of the EPR spin-trapping technique to the detection of radicals produced in vivo during inhalation exposure of rats to ozone

Abstract Ozone is known to induce lipid peroxidation of lung tissue, although no direct evidence of free radical formation has been reported. We have used the electron paramagnetic resonance (EPR) spin-trapping technique to search for free radicals produced in vivo by ozone exposure. The spin trap α-(4-pyridyl-1-oxide)- N-tert -butylnitrone (4-POBN) was administered ip to male Sprague-Dawley rats. The rats were then exposed for 2 hr to either 0, 0.5, 1.0, 1.5, or 2.0 ppm ozone with 8% CO 2 to increase their respiratory rate. A six-line 4-POBN/radical spin adduct signal ( a N = 15.02 G and a β H = 3.27 G) was detected by EPR spectroscopy in lipid extracts from lungs of rats treated with 4-POBN and then exposed to ozone. Only a weak signal was observed in the corresponding solution from rats exposed to 0 ppm ozone (air with CO 2 only). The concentration of the radical adduct increased as a function of ozone concentration. After administration of 4-POBN, rats were exposed for either 0.5, 1.0, 2.0, or 4.0 hr to either 0 or 2.0 ppm ozone (with CO 2 ). The radical adduct concentration of the ozone-exposed groups at exposure times of 2.0 and 4.0 hr was significantly different from that of the corresponding air control groups. A correlation was observed between the radical adduct concentration and the lung weight/body weight ratio. These results demonstrate that ozone induces the production of free radicals in rat lungs during inhalation exposure and that radical production may be involved in the induction of pulmonary toxicity by ozone. This is the first direct evidence for ozone-induced free radical production in vivo .

Stress ethylene evolution: A measure of ozone effects on plants

To determine if ethylene evolution by plants was correlated with ozone stress, a range of plant species and cultivars was exposed to varying ozone concentrations. Following exposure, the plants were encapsulated in plastic bags and incubated for up to 22 h. The stress-induced ethylene that accumulated in the bag was monitored and correlated with the effects of ozone on plants. The rate of stress-induced ethylene production (slope parameter B) as a function of ozone concentration was used as a measure of plant sensitivity. Applying this ranking scale, ponderosa pine, eucalyptus and soybean were the most sensitive species; holly, squash and marigold were least sensitive. There was a good correlation between ozone-induced stress ethylene production and foliar injury. However, the coefficient of variability associated with the ethylene determination was substantially less than with the visual injury estimate. The production of stress-induced ethylene generally lasted for less than 48 h following exposure. The measurement of stress ethylene appeared to be a fast, reliable, sensitive and reproducible technique to measure ozone stress on plants.

Glutathione peroxidase system and lysozyme in rats exposed to ozone or nitrogen dioxide

Exposure of rats to low concentrations of ozone (as low as 0.2 ppm) continuously for 8 days, or intermittently (8 hours/day) for 7 days, significantly increased the activities of the glutathione (GSH) peroxidase system in lung tissue. Linear regression analysis showed the increased enzymic activities to be a function of ozone concentration. Lysozyme activity was significantly increased in lung soluble and plasma, during continuous but not intermittent ozone exposure. The results indicated that lipid peroxidation damage occurred in lungs of rats exposed to relatively high levels of ozone. Rats were exposed to relatively higher levels of nitrogen dioxide for 4 days. The activities of GSH reductase and glucose-6-phosphate dehydrogenase were significantly increased during exposure to 2.3 ppm and 6.2 ppm nitrogen dioxide, respectively. There was no significant increase in the activity of GSH peroxidase in lung soluble, or of lysozyme in the plasma, during exposure to nitrogen dioxide. The results suggest that the mechanism of action of nitrogen dioxide is different from that of ozone.