"Insurance and commercial property management companies have noted an increase in claims due to black spot stains on ceilings, walls, furniture, content and HVAC filters from previously unidentified sources. The problem is referred to as “black soot deposition” (BSD) and, as research indicates, it is frequently caused by a common household decorative item.
There is no evidence of a sudden or accidental source for BSD, such as malfunctioning furnaces, gas water heaters, cigarette smoke or cooking by-products. Until recently, there was little compelling evidence to assign proper blame for the source of BSD. Tests have confirmed, however, it is very possible decorative scented candles are a primary culprit."