By the time April rolls around, thawing snowbanks are a welcome sign for many winter-weary Canadians.

But for one researcher, that dirty grey pile of slush on your sidewalk is a cocktail of toxic molecules, pollution from air that's embedded itself in the snow.

"Snow is very, very effective at accumulating compounds, including toxic compounds," Parisa Ariya, an atmospheric science professor at McGill University in Montreal, told CBC Nova Scotia's Information Morning.

However, that does not mean every snow pile is dangerous. Some compounds actually become less toxic when mixed with snow. Also, some of the dirt is nothing but harmless dust.

"It's not wise to eat it, for example," Ariya said. "But it's not dangerous to walk around in it. There is nothing to be scared of. We have to just understand it."

And that's what she's trying to do. She said she hopes her research will reveal new details of how molecules react to melting snow, whether they remain in the water, seep into soil or turn into different compounds.

"This is basically the beginning of a big story we haven't explored yet," said Ariya. "I don't want to be afraid of the unknown."

The snow is typically dirtier in cities because of car exhaust. But pollution can travel from its origin. In the 1970s, for example, Halifax suffered from acid rain caused by emissions from the coal and steel industry in Pennsylvania.