Defogger problems of the 2007 Lexus RX350

For the last 3 years our vehicle has been emitting white powder from the interior vents. It is very noticeable as the powder settles all over the dash board and becoming progressively worse each year. We live in florida and only notice this problem during the winter when the heater is used. We are worried of this powder being potentially hazardous to our health as we breath it in. I have heard other owners of Lexus RX 350 with similar issues. The dealers fail to acknowledge the defect and potential hazard.