There are certain circumstances where it is not recommended to use fans:

1. During an illness: If you have a fever, cold, or flu, it's not recommended to use a fan. This is because the fan can circulate airborne germs and potentially spread your illness to others. Additionally, a fan can make you feel more uncomfortable by blowing dry air on your skin.

2. In high humidity: In environments with high humidity, a fan may not be effective in cooling you down, as the moist air can prevent the fan from evaporating sweat and reducing your body temperature. In these conditions, it's better to use air conditioning or a dehumidifier.

3. For individuals with respiratory issues: For individuals with respiratory issues like asthma or allergies, a fan can circulate airborne irritants and make symptoms worse. If you have respiratory issues, it's best to avoid using a fan or use it with caution.

4. During extreme temperatures: In extremely hot or cold temperatures, a fan can make you feel more uncomfortable. In extremely hot temperatures, a fan can blow hot air on you, making you feel even warmer. In extremely cold temperatures, a fan can dry out your skin and make you feel colder.

It's important to assess your environment and personal needs before using a fan, and to use it in moderation to ensure you stay comfortable and healthy.