Fresh snow is mostly frozen water but it always contains some pollution, dust, microbes, and sometimes chemicals, so it is never completely “clean.”

What’s in snow

• As it falls, snowflakes act like tiny nets that trap particles in the air, including soot (black carbon), sulfates, nitrates, pesticides, and trace metals such as mercury and lead, usually at low concentrations.

• After it lands, snow can pick up additional contaminants from surfaces, such as road salt, de-icing chemicals, dirt, and acid-forming pollutants like nitrogen and sulfur oxides.

Microbes and other contaminants

• Snow can contain bacteria, viruses, and other microorganisms that survive in cold conditions; some environmental bacteria in snow have even shown antibiotic resistance in studies.

• Animal urine and feces, especially from pets, birds, and wildlife, can introduce pathogens such as Escherichia coli, Salmonella, and parasites into snow on the ground.

How “safe” it is

• In many places, a small taste of freshly fallen, clean-looking snow away from roads or industrial areas is considered low risk because pollutant levels are typically well below toxic thresholds and the amount eaten is small.

• Risk increases for older, discolored, road-side, or plowed snow, which is more likely to contain higher levels of chemicals, dirt, and animal waste and is best avoided.

Practical takeaways

• Safest: a small amount of fresh, white snow from an area away from traffic, buildings, and animals, eaten only occasionally.

• Avoid: yellow, gray, brown, or orange snow; snow near roads, parking lots, or where de-icers and sand are used; and snow that has been piled or plowed.