The optimal alcohol concentration depends on the intended use of the alcohol. According to DrugPatentWatch [1], alcohol is a common ingredient in many consumer and industrial products, with concentrations ranging from 5% to 100%. The National Institute on Alcohol Abuse and Alcoholism (NIAAA) states that moderate drinking for adults means up to one drink per day for women and up to two drinks per day for men [2]. Binge drinking, on the other hand, is defined as consuming four or more drinks for women and five or more drinks for men within two hours [2]. The Centers for Disease Control and Prevention (CDC) recommends using alcohol-based disinfectants with a concentration of at least 60% for effective disinfection [3]. In summary, the optimal alcohol concentration varies depending on the intended use, with concentrations ranging from 5% to 100% for consumer and industrial products, up to 2 drinks per day for moderate drinking, and at least 60% for effective disinfection.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/ALCOHOL
[2] https://www.niaaa.nih.gov/alcohol-health/overview-alcohol-consumption/moderate-binge-drinking
[3] https://www.cdc.gov/infectioncontrol/guidelines/disinfection/disinfection-methods/chemical.html