Hi mattress strain,
As you probably know formaldehyde exists in nature and there are natural sources for formaldehyde that is at background levels in ambient air outside of any VOC’s that come from products or building materials (including plywood and engineered wood products … see here) that you may have in your home. There is some interesting information about formaldehyde exposure and background levels in homes and various cities here and here that may be of interest to you.
Some of the comments in posts #32 - 36 here and in posts #5 - #21 here may also be of interest.
Phoenix