Sustainable Transportation Lab

February 16, 2016

Go ahead and eat that snow

Don MacKenzie

Don MacKenzie

You might recall a little brouhaha a few weeks ago regarding a study out of McGill University in Montreal about car exhaust and the composition of snow. Basically, the researchers were interested in what happens to the air pollutants in car exhaust when they are emitted at low temperatures in the presence of snow, which has implications for modeling and managing air quality in cold environments. The team collected fresh snow from a park in Montreal, put it in an environmental chamber, and pumped in exhaust from an internal combustion engine. Among other things, they reported the composition of the snow before and after contact with the exhaust.

Gizmodo picked up the story, and in what I will generously assume was a colossal failure of reading comprehension, incorrectly claimed that the study “found freshly fallen snowflakes contained… a hefty dose of car exhaust.”

Here is what the study actually found:

Source: Nazarenko, Y., Kurien, U., Nepotchatykh, O., Rangel-Alvarado, R. B., & Ariya, P. A. (2016). Role of snow and cold environment in the fate and effects of nanoparticles and select organic pollutants from gasoline engine exhaust. Environmental Science: Processes & Impacts.

Source: Nazarenko, Y., Kurien, U., Nepotchatykh, O., Rangel-Alvarado, R. B., & Ariya, P. A. (2016). Role of snow and cold environment in the fate and effects of nanoparticles and select organic pollutants from gasoline engine exhaust. Environmental Science: Processes & Impacts.

I took a look at US EPA’s drinking water standards, and compared them with the results above, translating the McGill team’s measurements in the same units used by EPA:

McGill team's measurements versus EPA drinking water standards for regulated pollutants.

Taking a look at the above, you can see that the actual measured level of toluene in freshly fallen snow was 1/57,000 of the EPA drinking water standard. The other organics were even lower relative to the EPA standards. Nitrate was a 1/400 of the EPA standard, although this is based on rounding up to 0.1 the reported level of “<0.1″… a very conservative assumption. Even after exposure to the engine exhaust, the snow still met EPA guidelines for most of the pollutants. Although it exceeded the standard for benzene, the exhaust-tainted snow was still not at an EPA Health Advisories level, meaning it is at a level that EPA would not expect to cause advserse noncarcinogenic effects for up to 10 days of exposure.

Bottom line:
Yellow snow = bad
Pink snow = bad
Freshly fallen snow = A-ok… at least for the sample used by the McGill team
Snow that’s been in a chamber with engine exhaust deliberately pumped over it = not great, but a reasonable quantity isn’t going to hurt you