Thanks for the replies.
When I opened my scooter’s CVT box I found a layer of sticky belt dust inside the cover, but I couldn’t see any other dirt or dust. Dust could be trapped in the rubber dust inside the housing, I guess that could be the reason why I didn’t see any normal dust.
I think air filters might be good, but it will cause overheating problems once it got dirty. According to info on the web a dirty CVT air filter will shorten the lifespan of a CVT belt due to overheating problems.
By the way - most of our local roads are similar to roads in the US, there are no differences in riding conditions. The only differences I can think of - we ride on the “wrong†side of the road, our winters are not that cold, and we hardly ever get snow on our roads.
Anyway, I think the high mileage without problems mentioned here is proof that there should be no reason to worry about the absence of an air filter.
Thanks again.