Hi Francis,
Here’s a picture of my filters after just 4 days in a bus in China! We spent 2 days driving in Sichuan to do our mountain climbing, and then 2 days more in the bus on the way back.
Totobobo filter after 4 days use in a bus in China
This is me in the bus.
Kenneth using Totobobo mask in a bus in China
There’s a write-up and a nice video on my website:
Regards,
Kenneth Koh
www.adventurenomad.blogspot.com