Changes in early alveolar particle clearance due to single and repeated nitrogen dioxide exposures in the rabbit

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Abstract
To better understand the potential health risks associated with short‐term NO 2 exposures, a study was conducted to examine the effects of single and repeated NO 2 exposures on the clearance of inert tracer particles from the alveolar region of rabbit lungs. Single 2‐h exposures to 0.3, 1.0, 3.0, and 10.0 ppm produced a concentration‐related acceleration in alveolar particle clearance, which resulted in greater particle removal when compared to control. The greatest response was produced at the lower NO 2 levels, where as much as 40% more particles were cleared when compared to control. Fewer particles were cleared following a 10.0‐ppm NO 2 exposure when compared to the lower NO 2 levels, and there were indications from the clearance pattern that the higher level was beginning to slow clearance, although an actual retardation was not found. Repeated 14‐d exposures (2 hid) to 1.0 or 10.0 ppm NO 2 produced a response similar to a single exposure at the same concentration, suggesting a certain degree of adaptation was produced after the initial exposures. Possible mechanisms for these differences in clearance patterns are discussed. The results of this study demonstrated altered alveolar clearance following short‐term NO 2 exposure at environmentally relevant concentrations; changes in this important host defense mechanism may be indicative of some underlying pathologic condition.