White, cold, freshly fallen snows begs to be lumped into snowballs — or sometimes, to be eaten. In some parts of the country, a treat known as snow cream (aka snow ice cream), made with fresh snow and a dairy-based liquid such as sweetened condensed milk, is popular. But is snow safe to eat?

Snow, no matter how white and fresh it looks, is hardly pure as, well, driven snow. In fact, the white stuff can become contaminated not only once it's on the ground but even on its journey from sky to earth.

“Air pollution can affect the cleanliness of snow in lots of ways, especially if there’s a large amount of dust present,” says Mark Williams, PhD, a professor of geography at the University of Colorado, Boulder. On windy days flakes can be especially filthy, since the moving air incorporates dirt and other particles into the swirl.

Once snow hits the ground, it can be contaminated with pesticides and other chemicals in the soil or plants it lands on.

Even so, ingesting small amounts of snow won’t hurt you. In fact, if you’re camping in the wilderness, snow can be a valuable resource. “I’ve been on hut trips in Colorado where all of the water used for drinking and cooking has been collected from the snow right outside and melted down on wood stoves,” explains Williams.

Related: Tips for Warm and Safe Winter Camping

If you go foraging for snow to make snow ice cream, keep these factors in mind before you scoop some up.

Location. Snow that falls over large metropolitan areas is dirtier than, say, the stuff that lands in the mountains of Vermont. Cars are to blame. “Urban areas have more local sources of pollution, particularly black carbon from vehicle emissions,” explains Williams.

Color. If snow is gray, dirty-looking or, of course, yellow, stomp those boots on by. Stay away from pink snow, too. According to a report from the University of California at Santa Barbara (UCSB), a pinkish shade on snow is likely caused by red algae, which could contain potentially toxic bacteria.

Timing. Where snow lands doesn’t always determine whether it’s edible. In fact, snow is just as "clean" on sidewalks as it is on your front lawn, according to Williams. But once it begins to melt and flow, contaminants are dragged into it.

Proximity to plowing. “Plows disturb the ground and mix all kinds of dirt into snow,” points out Williams. The same is true of shoveled snow piled up along sidewalks and driveways (oil and exhaust from nearby cars can leach harmful chemicals).

Related: How to Shovel Snow Without Ending Up in the ER

Once you find snow that looks good enough (and safe enough) to eat, scrape it from the surface into a clean bowl using a large clean kitchen spoon or measuring cup. Don't dig too deep. To make snow ice cream, mix in one cup of milk, a quarter cup of sugar and a teaspoon of vanilla and enjoy — right away, before it melts. (If it does, stick it in the freezer for a bit.)

Just don't gorge yourself. No matter how pure the snow you gather is, it still isn’t the same as the H2O that flows from the tap in your home. Snow is more like distilled water, which means it’s missing important electrolytes including salt and other minerals. According to the World Health Organization, drinking too much distilled water can cause an electrolyte imbalance, which may leave you with a headache, weakness, muscle cramps and fatigue.

Related: How Safe is Your Drinking Water?

“Once in a while, it’s absolutely okay to enjoy some snow,” says Williams. In fact, snowfall anywhere in the United States is of better quality than some municipal water systems, he adds.

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Jennifer Kelly Geddes is a New York City-based writer and editor who specializes in parenting, health and child development. She’s also the mom of two teen girls.