Comparison of lung burdens of inhaled particles of rats exposed during the day or night

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Abstract
Inhalation studies frequently involve daytime exposures of nocturnal animals to toxicants. Such exposures may result in different respiratory tract depositions than would be obtained if rodents were exposed at night. Our study assessed the effect of night versus day exposures on lung burdens of particles inhaled by Fischer 344 rats. One group of 15 female rats was exposed to 0.3 μm volume median diameter particles of gallium oxide (Ga 2 O 3 ) for 11.2 h during the day and a second group of 15 female rats was exposed to the same aerosol for 11.2 h at night. Gallium in the lungs at the end of exposure was measured by electrothermal atomic absorption spectrometry. Rats exposed during the night had significantly (p < 0.05) higher lung burdens than day‐exposed rats when burdens were expressed as either μg Ga 2 O 3 /lung (X±SD = 896 ± 175 versus 698 ± 150) or μg Ga 2 O 3 /g lung (X±SD= 683 ± 134 versus 465 ± 103). The greater amount of material in lungs of rats exposed at night probably reflected increased ventilation accompanying nocturnal activity.