Hand sanitizer should be used as a substitute for washing hands with soap and water after:

Hand sanitizer is effective for reducing the number of germs on hands and is particularly useful in certain situations where soap and water may not be available. It is designed to be used as a convenient alternative under specific circumstances rather than a blanket replacement for handwashing.

Using hand sanitizer after a possible contamination event is crucial because it can help eliminate pathogens that may have been introduced during the incident. In such situations, especially if soap and water are not readily accessible, hand sanitizer serves as a practical solution to mitigate the risk of spreading germs. This is why this scenario is highlighted as an appropriate instance for using hand sanitizer.

In contrast, while hand sanitizer can assist after handling food, it's not a substitute for washing hands in this situation, especially since soap and water are preferred for removing food residues and certain pathogens. Similarly, it should not be relied upon solely after personal hygiene tasks or in all situations indiscriminately, where proper washing with soap and water is always recommended for thorough cleanliness.