You can use a humidifier throughout the day. But, it's not recommended to do so every single day. The humidifier can add excessive moisture to the room and lead to mold or mildew growth if it is kept on 24 hours a day. You should always monitor the humidity level in your home. Turn off your humidifier if it becomes too hot. Humidifiers that run too often can cause a musty, peeling wallpaper or blistered paint. Keep it running for a maximum of 12 hours each morning.