Alright, one playStore app actually displays mG (milliGauss) instead of uT (microTeslas)
https://play.google.com/store/apps/details?id=com.keuwl.gaussmeter&hl=fr&gl=US
And it displays between 0.1mG and 0.3mG in my room (medium size living room in a big tower)
So 20uT probably translates to 0.2mG (google search says it's 50 times more though, but probably the culprit are the app displaying uT. so just find a mG app to check if you are under 0.4mG.)