With the increased use of ethanol-blend gasoline as an alternative to gasoline, there is a demand for new data to assess the potential health risk of ethanol exposure. Currently, there is very limited information from the scientific database on the adverse effects of inhaled low doses of ethanol (ETOH). The aim of this study was to determine the concentration of ETOH and acetaldehyde (ACTDH), a toxic metabolite of ETOH, in the alveolar air (AA) of subjects exposed to low ETOH vapor concentrations by inhalation. Five healthy adults (2 women, 3 men), nonsmoking, 25-55 yr of age, were exposed for 6 consecutive hours to 25, 100, or 1000 ppm ETOH and were asked to provide AA samples at various intervals during and after each exposure session, for ETOH and ACTDH measurements. Results showed that the concentrations of ACTDH and ETOH in AA measured after 2 h of exposure at 25 ppm were 0.06 ppm and 7.5 ppm, respectively. Overall, there was a significant correlation between ETOH exposure and ETOH (R(2) =.92, p <.001) and ACTDH (R(2) =.99, p <.001) in AA. The ratios between ACTDH and ETOH in AA after 4 h of exposure to 25.9 ppm, 101.7 ppm, and 990.8 ppm ETOH were 0.005, 0.008, and 0.006, respectively. In conclusion, this study showed that inhalation of ETOH at low concentrations resulted in measurable levels of ACTDH in AA and suggests that ACTDH may be used, with good toxicological relevance, as an indicator of exposure to low levels of ETOH in air.