It is generally safe to leave the microwave door closed when it is not in use. In fact, it is recommended to keep the microwave door closed when not in operation. Closing the door helps to prevent dust, dirt, and other debris from entering the microwave, which can affect its performance and potentially cause damage.
Leaving the microwave door open when not in use might also be a safety concern, especially if you have small children or pets around. Open access to the microwave can increase the risk of accidental burns or injuries.
However, if you have just finished using the microwave and it is still warm inside, it is a good idea to leave the door slightly ajar for a few minutes to allow the heat to dissipate. This can help prevent condensation from forming inside the microwave and potentially causing moisture-related issues.
In summary, it is generally safe and recommended to keep the microwave door closed when it is not in use, except for a short period after cooking to allow for cooling.