The aim of this study was to measure nicotine concentrations in inspired and expired air so as to learn more about respiratory (nasopharyngeal cavity and lung) nicotine absorption from inspired air and to estimate the nicotine intake during passive smoking. A total of 17 young non-smoking women were exposed to experimental passive smoking. Inspired and expired air was sucked at a constant rate into samplers filled with acid-treated diatomite (Uniport-S) to absorb nicotine in the air. Absorbed nicotine was assayed by gas chromatography. The range of nicotine concentration in the inspired air was 40-200 micrograms/m3. In this setting, 47 samples obtained from the 17 subjects were assayed. Nicotine absorption, which was calculated as [(nicotine concentration in inspired air-nicotine concentration in expired air)/nicotine concentration in inspired air] x 100, remained at 60%-80% (mean +/- SD, 71.3% +/- 10.2%) without being affected by the nicotine concentration in the inspired air. From this result, it was estimated that the average intake of nicotine was 0.026 mg/h in a group of non-smokers exposed in a room containing a nicotine concentration of 100 micrograms/m3, which is equivalent to fairly severe involuntary tobacco smoking. This is the first report on the estimation of respiratory nicotine absorption and nicotine intake during passive smoking based on the direct measurement of nicotine concentrations in both inspired and expired air.