Vents and exhausts need to be regularly cleaned in order to make sure the air flow of the heating unit is maximised and that the flow doesn’t throw nasty dust into the air. Often decreased heater efficiency or unpleasant burning smells can be traced back to dusty vents.
Remove the exhaust lids in the bathroom’s (very carefully) clean the dust within them and rinse under hot water. This will help you to remove the sticky dirt also. Let the exhaust lid dry and then place it back. Same goes with the vents.
This above exercise will allow you to have a free flow of fresh air and not dust within them.