[alt text: a two-frame meme. The top frame has text that says, "Walking barefoot on a dirty floor" and shows a photo of a woman looking disgusted. The bottom frame has text that says, "Walking barefoot outside on actual dirt" and shows a photo of the same woman looking interested.]
What about city dirt. That seems like another level then floor dirt.
exposure to the weather helps a lot, means it's been baked in the sun from time to time and detritivores have had a chance to clean it up a bit