The only thing that will get inside you is water. Nothing else can get through your layers of skin. Think of it like pouring muddy water over sand. By the time the water has traveled through the sand to the bottom, it is crystal clean. Only water can travel, via
osmosis, through your skin. The absorption of nutrients thing is mostly a lie. The healthy part is from the abrasiveness of the sand in the mud; it exfoliates your skin and scrapes away dead skin cells. Just don't do it too often or you can scrape away healthy skin cells too.
When it comes to the micro-world, your body is pretty much fort knox, so don't worry about much getting into your system. (Unless you eat the mud

)
Hope this helps.