Disposable lenses also come in tear-offs, which are stacked and placed on the motorcycles goggles.
When the tear-offs get dirty, you pull off one layer (like sticky notes) and keep riding. They are installed on the top of the goggles to protect the lens from the roost that flies up from the other riders and racers on the track. They come as single layers, or stacks of 10-15. Usually enough for a mtocross goggle, unless it is a very muddy track.
When the tear-off lens gets muddy, just let go of the bars with your left hand and try and pull just one of the disposable lenses on the stack of lenses, and dispose of it. Most racers will just let the lens go while racing. Not great for the environment, but it is what commonly happens in racing environments. This leaves clean up for the track crew. If you are riding on the trails please don’t throw it on the trail. Stash them in your gear, pack, bike, or anything. Don’t litter on the trails! If you have nowhere else to dispose of them, you may want to look at getting biodegradable lenses that are better for the environment.