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Yes, you are correct. Alcohol evaporates. Alcohol on your hands will evaporate in approximately 30 seconds. Alcohol in a hand sanitizer will also evaporate in an open container. It is very important to keep an alcohol containing hand sanitizer in a closed container. If the container is closed, the alcohol in the hand sanitizer will very quickly reach an equilibrium with the alcohol in the air in the bottle an no more alcohol will evaporate into the air in the bottle. It is possible that a very large bottle opened to many times might result in the alcohol concentration dropping below the effective level (62%). However, it is not probable. A hand sanitizer containing 70% alcohol is better than a sanitizer containing 62% alcohol. With 70% alcohol you can loose 10% of the alcohol to evaporation and it will still be effective. With 62% alcohol you cannot loose any alcohol to evaporation and still be effective. It also helps if your hand sanitizer contains a second germ killer like Chlorhexidine gluconate or Benzalkonium chloride. If the alcohol evaporates below the critical level (62%) the second germ killer will still kill germs.