Grey water?

Just curious, what is the cause of interior glass water coming off grey? I’ve always explained a gas fireplace can do that, but feel the HVAC vents need cleaning.

Any other theories?

Saw some grey water after I cleaned the interior windows & skylights. They weren’t done in several years, and they were dirty. So my guess is dirt/daily life stuff.

length of time since last cleaning and/or minimal use of candles/other burning things

black = lots of candle/wood fireplace/burning things use

white film - natural gas use (fireplace, heater)

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