Currently, Japan’s Road Traffic Act uses the breath alcohol concentration (BrAC), which is the concentration of alcohol in 1 L of breath, to enforce the law. However, 400 drunk driving accidents occurred in 2020 when the BrAC was below 0.15 mg/L, which is legal limit in Japan, and cognitive function may be impaired at this concentration. Therefore, we investigated the effects of alcohol consumption at levels below a BrAC of 0.15 mg/L on cognitive function and its change over time using P300, a type of event-related potential, and the concomitant negative variation (CNV). Beer with a 5% alcohol content was used as the drinking load, and the amount of alcohol consumed was determined using a derivation formula that varied according to the subject’s body weight. Electroencephalogram (EEG) measurements were obtained five times before drinking and 10, 30, 50, and 70 min after, and participants were given a go/no-go task to induce event-related potentials during the measurements. The BrAC was measured immediately before the EEG measurement was performed. The results revealed that the alcohol concentration in the exhaled breath increased significantly (p<0.05) at 10, 30, and 50 min after drinking compared with immediately before. In addition, the P300 latency was significantly prolonged (p<0.05), and the early and late CNV amplitudes were significantly increased (p<0.05) in the event-related potentials during these time periods. Moreover, no significant difference was observed in expiratory alcohol concentration at 70 min post-drinking, but significant changes in event-related potentials still occurred. Therefore, our results suggest that cognitive function is impaired in patients with low levels of alcohol consumption, when the BrAC is below the legal limit, and even when the BrAC recovers after the end of alcohol consumption.

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