IKEA Vindriktning + ESP32 : Baseline?

hi
i have a vindriktning with an esp32-pico (m5stamp) and it worked great over the last half a year.
the fan power is always-on with 3.3v
uptime today was 38 days.
it is in a room where people smoke.
after a good ventilation, sensor shows green again.
but over the last 6 weeks the clean/base-value rose from 12 to 53, yellow
then i rebooted today.
the value then fell down to 12 again, green.

and i have a second one, identically, but in a non-smoking room.
and it behaved the same, rebooted it, now it’s green again

anybody else have similar experiences (a rising baseline) ?

should have researched first

i made now an auto-reboot based on the uptime:
reboot next midnight after being up longer than 14 days