Is there evidence that eating brake dust in tiny quantities is bad for you?
It's actually a good question, and I'm disappointed to read middlebrow dismissal of it.
As long as we want to go fast, there will be some brake dust in the air. Brake pads can be made out of a variety of materials, and we want to use the ones which are least harmful to our lungs and other organs.
That takes research, not emotional appeals to disgust (from the Latin, meaning "bad to eat").
For example, I wouldn't sprinkle dirt all over my food. But I'm perfectly happy for my food to grow in dirt. Does that mean contact with dirt = bad? No.
As for the dirt: plants act as natural filters, they select that which is good for them using their root systems and use that to build the rest of the plant as they go along. Some plants can be used to extract certain materials from the soil, others do the opposite and will react by rejecting the contaminants. What exactly happens depends on the plant, soil conditions, humidity, amount of sunlight and lots of other factors besides. Not all dirt is good, not all plants grown in all dirt are good, not all dirt is the same.