Jon Lung rigged up some small fans inside his homemade face shield to defog the mask while riding in winter. On Instructables he writes:
Looking for a way to protect my face from harsh winter conditions, without enduring damp scarves pressed against my mouth or the excessive, mummy like, wrapping of my head, I created headgear with a magnetic visor that is defogged by tiny 5 volt fans.
This instructable will guide you through the process of making a Mk. 1 Winter Face Shield. As a fair warning, it is an involved project that involves programming, sewing, and some various fabrication and building. This process is much easier if you have access to a shop space with tools like table saws and belt sanders. If all you have are some hand tools and a back porch to work in, various details can be scaled to fit your capabilities. We can still do this.
Stop breadboarding and soldering – start making immediately! Adafruit’s Circuit Playground is jam-packed with LEDs, sensors, buttons, alligator clip pads and more. Build projects with Circuit Playground in a few minutes with the drag-and-drop MakeCode programming site, learn computer science using the CS Discoveries class on code.org, jump into CircuitPython to learn Python and hardware together, TinyGO, or even use the Arduino IDE. Circuit Playground Express is the newest and best Circuit Playground board, with support for CircuitPython, MakeCode, and Arduino. It has a powerful processor, 10 NeoPixels, mini speaker, InfraRed receive and transmit, two buttons, a switch, 14 alligator clip pads, and lots of sensors: capacitive touch, IR proximity, temperature, light, motion and sound. A whole wide world of electronics and coding is waiting for you, and it fits in the palm of your hand.