Response: If the snow is yellow, black, brown, pink or otherwise dirty, it should not be eaten, as the colors each point out the presence of a different kind of pollution — yellow pollution from animals, black and brown pollution from dirt, cars, and people, and pink from bacterial contamination. The snow can collect these pollutants once it falls to the ground by being stepped on, splashed on, peed on, or otherwise having pollution fall on it. More surprisingly to some, perhaps, snow can also become contaminated by pollution as it falls to the ground.
It turns out that snow is a fairly efficient pollution collector when it is in the air. Snow is formed by water vapor that moves in clouds in cold air. As the water vapor moves in the cold air, it can stick to a tiny piece of dust and then have other water molecules attach to it, forming a crystal. Once formed, the crystal can continue to grow and can stay in the air for hours before it falls to the ground. It is during this time that the snow crystal can collect or "scavenge" pollutants that are present in the air.
As a result of this scavenging, snow can contain all kinds of airborne pollution, even if it is white and freshly fallen. They types of pollution that the snow can contain vary by location, but could include metals, acidic pollutants, and persistent organic pollutants (POPs). The amount of pollution in white, fresh snow is generally related to the amount of local pollution that is emitted into the air, of which traffic is a pretty good indicator. As a result, pollution is snow is low in rural areas and is higher in cities and other areas with a lot of traffic.
Even so, should you be worried? For white, freshly fallen snow, probably not, given that scientific studies show low amounts of pollution in fresh snow and given that kids generally don't eat much snow. But for colored snow and for snow that has been sitting on the ground for a while, it is definitely best for your kids to look and not eat. A good way to show them why may be to melt some snow in a container and see what you find. Almost always, you will find dirt, sticks and other debris that were previously hidden.
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Helen Suh MacIntosh is a professor in environmental health at Harvard University and studies how pollution behaves in the environment and how it affects people's health. Please keep in mind that her answers are just her interpretation of available information and should not be taken as the only viewpoint or solution to a problem. Use this column at your own risk. Having said this, please feel free to post any of your environmental health questions to Helen@TreeHugger.com. (Please use a descriptive email subject line and mention if you want to remain anonymous or not).