Academic journal article Environmental Health Perspectives

Formation of Strong Airway Irritants in Mixtures of Isoprene/ozone and Isoprene/ozone/nitrogen Dioxide. (Articles)

Academic journal article Environmental Health Perspectives

Formation of Strong Airway Irritants in Mixtures of Isoprene/ozone and Isoprene/ozone/nitrogen Dioxide. (Articles)

Article excerpt

We evaluated the airway irritation of isoprene, isoprene/ozone, and isoprene/ozone/nitrogen dioxide mixtures using a mouse bioassay, from which we calculated sensory irritation, bronchial constriction, and pulmonary irritation. We observed significant sensory irritation (approximately 50% reduction of mean respiratory rate) by dynamically exposing the mice, over 30 min, to mixtures of isoprene and [O.sub.3] or isoprene, [O.sub.3], and N[O.sub.2]. The starting concentrations were approximately 4 ppm [O.sub.3] and 500 ppm isoprene (+ approximately 4 ppm N[O.sub.2]). The reaction mixtures after approximately 30 sec contained < 0.2 ppm [O.sub.3]. Addition of the effects of the residual reactants and the identified stable irritant products (formaldehyde, formic acid, acetic acid, methacrolein, and methylvinyl ketone) could explain only partially the observed sensory irritation. This suggests that one or more strong airway irritants were formed. It is thus possible that oxidation reactions of common unsaturated compounds may be relevant for indoor air quality. Key word: airway irritation, indoor air chemistry, isoprene, mouse irritation bioassay, nitrogen dioxide, ozone. Environ Health Perspect 109:937-941 (2001). [Online 24 August 2001]

http://ehpnet1.niehs.nih.gov/docs/2001/109p937-941wilkins/abstract.html

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It has been proposed that reactions between unsaturated volatile organic compounds (VOCs) and oxidants (e.g., terpenes and ozone) may produce chemically reactive products that irritate the eye and airway (1). Some of the known reaction products are aldehydes, carboxylic acids, and hydroperoxides, which may cause irritation at concentrations relevant for indoor air (1,2). Some epidemiologic studies of airway irritation symptoms are consistent with the hypothesis that ozone in combination with unsaturated VOCs (e.g., from human activities or building furnishings) contribute to nasal resistance and eye irritation (3). However, until the recent report of the formation of irritants from the reactions of [O.sub.3] and (+)-[alpha]-pinene (4) and [O.sub.3] and limonene (5), the only experimental evidence that supports this hypothesis was the observations of indoor air oxidation reactions reported by Weschler and Shields (6,7).

Our objective was, using a mouse bioassay, to provide experimental evidence for the formation of irritating substances in mixtures of [O.sub.3] and isoprene, a common plant and microbial metabolite (8,9) and one of the major organic constituents of air exhaled by humans (10,11). This assay analyzes the respiratory pattern of mice exposed to airborne chemicals (e.g., VOCs). When the upper airway is exposed to irritants, the respiratory rate is reduced because stimulation of the nasal trigeminal nerves reflexively induces a break in breathing after inhalation. When pulmonary irritants are present, the vagal nerves are stimulated, which often creates a pause in breathing before inhalation and thus also a reduction of the respiratory rate. These effects are concentration-dependent over a wide range of concentrations and they are distinguished by analysis of the respiratory parameters (12). They are usually expressed as percent of baseline or percent decrease from baseline. Thus the threshold concentration for reduction of the respiratory rate (R[D.sub.0]), which can be estimated from the dose-response relationship, corresponds to the no-effect level (NOEL). The R[D.sub.50] used here is the concentration of a substance required to cause 50% decrease in respiratory rate.

The atmospheric chemistry of isoprene with ozone and nitrate radicals has been investigated extensively (13-18). The ozone reaction is reported to give methacrolein, methyl vinyl ketone, hydroxy hydroperoxides, the two isomeric monoepoxides, 3-methylfuran, propene, and many secondary oxidation products of these, depending on the reaction conditions (13-15). The reported reaction products of isoprene with N[O. …

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