One might think that an unfinished wood floor is devoid of synthetic chemicals. It sure looks that way--but toxic preservatives may lie in plain sight.
Moist lumber is susceptible to fungal staining. This staining does not cause physical decay, but it looks bad. Commonly called "blue stain," the offending fungi may be yellow, orange, purple, gray, or red in addition to shades of blue. The stain penetrates into the sapwood and cannot be removed by resurfacing.