white "powder"

If you’re located in a climate where indoor air gets dry in the winter, and there is a humidifier upstream of the filter, the white particles may be mineral deposits left behind by evaporating water. A properly running humidifier should be putting only water vapour into the air, leaving the mineral deposits behind in the unit. A malfunctioning unit may be putting liquid water into the air stream, along with the minerals dissolved in the liquid.

Chris

If you have a humidifier upstream it could be mineralization precipitating out of the water. Otherwise, it could be dust, dry skin, etc. in the dry household air during the winter.

Only the ultra-sonic style mist humidifiers can do that - evaporation humidifiers don't put out minerals.

That problem is a common one when using an atomizing (mist spraying) type of humidifier rather than an evaporating type. How serious it is varies with how much mineral content is in your water supply.

I suggest you get an evaporative type of humidifier since the furnace filter isn't getting all those solids from the air and breathing that stuff may not be healthy.

I didn’t know that furnace mounted atomizing type humidifiers were available. I recall we bought one of the standalone types for a single room but we only used it for a short while before we recognized why every horizontal surface in the room was turning white.

Chris