Why would you eat snow in the first place? Because it’s fun and harmless. It’s free ice cream. A new study, however, suggests snow isn’t as pure as think. Being colder, the temperature gradient leads noxious pollutants expelled from the tails of vehicles to become absorbed by the snow, and in your organism – if you decide to eat it. That being said, if you’re having fun in the park, it’s better not to eat it. We don’t know yet how harmful this is, but ingesting benzene, ethylbenzene, toluene, and xylenes can’t really be good.
The study was led by Dr. Parisa Ariya, professor of chemistry and atmospheric sciences at McGill University in Canada. She and colleagues conducted an experiment where snow was placed near exhaust fumes in a chamber. It was soon found that the iced water and fumes went through a chemical reaction in which some of the fumes were absorbed by the snow. During one hour of keeping the engine running, the concentration of harmful chemicals typically found in the exhaust of cars increased dramatically, from “0.218 ± 0.014 to 0.539 ± 0.009 mg L−1, and over 40 additional (semi)volatile organic compounds and a large number of exhaust-derived carbonaceous and likely organic particles were identified,” the researchers wrote in the study published in the journal Environmental Science.
“Snow flakes are ice particles with various types of surfaces, including several active sites, that can absorb various gaseous or particulate pollutants,” she told The Huffington Post. “As a mother who is an atmospheric physical chemist, I definitely do not suggest my young kids to eat snow in urban areas in general.”
“Without considering snow and ice, one will not be able to properly evaluate the effect of exhaust emission, and subsequently health and climate impacts, for the cities which receive snow,” Ariya said. “Further research — lab, field and model — is recommended to address various aspects of such experiments under various environmental conditions, for adequate implementation in future modeling. Further advisory policy will also be required.”
The findings beg the question: what happens to the snow when it melts? Trapped in the ice, these chemicals have no place else to go but back in the atmosphere. This means that once spring comes, cities should see a spike in key chemical signatures concentration. This warrants further studying.
Snow is crystallized water which makes it purer that most forms of precipitations. Campers and mountaineers all over the world use snow as their primary water source. I make tea from snow all the time. But in urban environments, as this study suggests, you ought to stay away from snow.
There are other things about snow you should consider, though maybe not as alarming as air pollutants trapped in it. Pure water freezes at -40 °C. Come again? Yes, water turns into ice at precisely 0°C, but only if some small particle (dust, protein, etc.) that will force water particles into the correct formation to form a crystal. These particles are called nucleators since these nucleate the ice crystal. With pollen or inorganic molecules nucleators water freezes at -11 °C.
Some airborne bacteria have proteins that are great nucleators. In fact, bacteria — most notably Pseudomonas syringae — cause frost on the surface of plants this way, which breaks open the cells for the animals to feed on the sugar and released proteins. Some scientists believe we have bacteria in the first place to thank for snow and rain! Nevertheless, Psedomanas s. not only falls on leafs of course, it falls everywhere. Along with it, many other bacterial species. When researchers analyzed 20 samples of snow taken from various places around the world, including even remote places such as Montana, the Yukon and Antarctica, they found bacteria levels were high in all the samples. Another team of researchers found that one-third of the ice crystals in clouds over Wyoming had formed around biological particles, while mineral dust accounted for 
50% of the residues. Considering these bacteria are airborne you likely come in contact with them all the time, the snow is safe to eat. I mean, I’d eat it. But don’t my word for it. I’ve yet to find a paper that says the bacteria found in snow is 100% safe.