Just did some non extraction sanding of Oak and that meter went mental

especially PM2.5 and PM10 ... The Formaldehyde increased only when the heat built up and burned!! Gasses?
Sorry, late to this thread. The PM readings are for dust (Particulate Matter) not formaldehyde. PM2.5 and PM10 (2.5 and 10 microns) are relatively large - the dust particles you need to worry about especially are those at <0.3 micron. Those handheld meters aren’t the most accurate either, but as long as they’re consistent you can use them for relative readings, rather than absolute ie ‘the reading is higher than it was yesterday, I need to put the air scrubber on / open a window.‘
As others have said in the thread, formaldehyde is a naturally occurring chemical present in all timbers. It’s also commonly used in many industrial processes, particularly as a binder in the production of manufactured boards eg plywood and MDF, chipboard etc...
If you want to scare yourself, put that meter inside your living room and get a reading, or by your car or van exhaust with the engine running.
My wife worked in the clothing industry on the technical side all her career, and formaldehyde is used extensively as a finishing agent; you’ll come into contact with more formaldehyde on a market-stall t-shirt than you ever will from working with wood.
HTH P