I can tell you but Im not sure you will believe me. I had a hard time believing it at first but became convinced. My brother in law worked in the gas service industry for about 40 years. He and I were discussing things one day and I asked about that.
He told me that very often the air is laden with chemicals in the form of vapor. He used perfume as only one such example but he said it could be anything from cleaning fluids, deodorants, mens cologne, etc., etc… He went on to tell me that when those fumes or vapors come into contact with a standing pilot light or any open flame it will develop a very fine ash that falls to the surface below it. If you think about it that makes sense as the ash is alwasy right below the pilot and there is usually only a small amount.
He found out what it was when he would go huge plants, mills and factories that were using a lot of chemicals and the air would be very heavy with the fumes or vapors. He did some research, asked some very smart people and they all told him the same thing. Incidently, in those plants and factories, gas heat exchangers in those big overhead gas blower furnaces would burn out in half the time as normal. He would end up on numerous call backs and would always find the white powdery ash deposits. I have noticed since then that on gas ranges you will sometimes find the same ash around the wires for a Piezo electric igniter on the burners.