The meter is this one:
https://www.amazon.com/Air-Pollution-Formaldehyde-Detector-T...
After these words I'm seriously starting to doubt the correctness of the device. I can't believe I wasn't able to find any indoor location with a low enough concentration for below 0.1 ppm, or only outdoors. I mean, these are rooms where I, my colleagues, friends and family are non-stop, we should all be good as one by now with those concentrations.
Looking at the link, this device measures formaldehyde in mg/m3. You'll need to convert this to ppm, just FYI.
1 ppm in air = 1.24 mg/m3 @ 25° C
If you're measuring 0.1 on the meter, that's about 0.08ppm (still pretty high).
It can show the measurement in ppm also.
However, I've read the reviews and dug a bit deeper, talked to some people. Basically, that value might be elevated by some unknown amount, but the device should not be trusted for the absolute levels.
Just for reference, NIOSH recommends 0.016 ppm as the max exposure limit for a workday. CARB recommends a max safe level of 0.05 ppm. WHO recommends max level of 0.08 ppm for 30 minutes, etc.
0.3 ppm is extremely high for chronic exposure.