How to know when to throw it out.
Hand sanitizer is great for killing germs and bacteria when washing your hands is not an option. However, as much as you stock up on these essential bottles it's important to remember that they do expire. Expired hand sanitizer does not necessarily mean you have to throw it away, in fact you could still use it.
In general, an effective bottle of sanitizer contains at least 60% of alcohol in order to remove bacteria and germs.
An opened bottle of hand sanitizer will cause the alcohol content to dissolve over time. Sanitizer expires because the alcohol content has evaporated over time. When the bottle drops below 60% alcohol it will not be as effective in killing germs. This means that an opened bottle of hand sanitizer is still safe to use, however it will not be as effective.
On the other hand, if a bottle of hand sanitizer is unopened at the date of the expiration date it will still have retained much of its alcohol strength. This means the expired unopened sanitizer is more effective than the expired bottle that has already been opened.
That being said, your best option is still washing your hands thoroughly with soap and water. Use hand sanitizers as an alternative if hand washing with soap and water is not available.
For more important hygiene facts read this article.