Sorry, again not true. Seals are indeed designed to retain vapor. The crankcase of modern cars is designed to contain the gasses that get past the rings and route them back into the air intake to be burned. The same is true for fuel tanks which have similar seals that retain gasoline vapor as well as liquid. There are even sensors that insure this seal is maintained and will throw a check engine light to let you know when this no longer occurs.
What you are seeing in your engine bay is minor leakage from seals and gaskets as well as crud off the road from other cars and trucks that then gets blown around the area in normal driving. Not soot unless it contacts something hot - and then you will smell it. It is not odorless. Transmissions have vent tot the atmosphere that do not capture any vapor but release it. These are for expansion and contraction of the transmission. If there is significant vapor coming out, your tranny is in serious trouble and, again, you WILL smell it. These minor leaks can be easily fixed by replacing the offending gaskets and periodically cleaning the engine compartment.
You can see I’m not the only one who has replied this way.
Since you changed cars and fixed the problem went away, what is your point to all this? To convince us of your findings? To convert us to your cause? Your conclusion is based on lots of inaccuracies. I am glad you symptoms are gone. They were solved in a way I think most people would approach the problem - If it makes me sick, I shall stop doing it!