In school I think I was told snowflakes have to form around some small particle, such as a speck of dirt. I don’t know if that’s really the case (the Wikipedia page on snow didn’t mention it, and I’m too lazy to research it further). Whether dirt is inherent in the formation of snowflakes or it just picks up gunk from the atmosphere on the way down (probably both), nothing illustrates how much dirt is in snow like watching the big piles in the parking lot melt.
Seriously, it’s disgusting. They’re basically big piles of black grime, with little bits of white showing through now and then (I’ll spare you by not posting a picture). As a kid I ate my fair share of snow, and now it’s gross to think that I was basically drinking dirty water.