With temperatures consistently below freezing, we're asking an expert: is it really too cold to salt the streets?

As people struggle to get around the city, sidewalks at Dalhousie University are clear. That's because the school is salting and sanding today.

Mary Anne White, a professor at Dalhousie University's faculty of chemistry, said salt is actually effective until it's –21 C outside. She said it's a misconception that salt may not work when it's colder than –9 C.

What is the deal with salt? Can it be too cold to work?

"If it's below about –21 C, then it is too cold for salt to work because salt reduces the freezing point of the ice — and that's the whole point," said White.

"If you can get, with the salt, the whole mess on the sidewalk to be at a freezing point of, let's say, –15 C and it's –10 C out, then that means it's going to be a liquid on the sidewalk and you're going to be able to walk through that slush, as unpleasant as that it. It's going to be much safer than the ice."

When does Halifax use salt?

Jennifer Stairs, speaking for the Halifax Regional Municipality, said she doesn't dispute White's salt assessment. 

"What I've been told by our winter works staff is that salt starts to lose its effectiveness at temperatures below –12 C. That's not to say it doesn't work, just not as well as in warmer temperatures," she said. 

"Over the past few days we've been using sand or crusher dust mixed with a bit of salt to give a degree of traction on the slippery surfaces. The sand and gravel does nothing to melt the ice."

There are, in fact, different kinds of salt

"Salts mean ionic compounds and there are different types of salt. The one we usually use is sodium chloride, but you can also use calcium chloride and it's good until a lower temperature, until –29 C," said White.

Does that –21 C include windchill?

road salt

As people struggle to get around the city, sidewalks at Dalhousie University are clear. That's because the school is salting and sanding today. (Carolyn Ray/CBC)

"No, that's temperature, –21 C is temperature. So it might be –10 C outside and it feels like –21 C but that means that your flesh will freeze as quickly with that wind as if the temperature had been –21 C," White said.

"But it's the actual temperature that counts. If it's –10 C outside, salt is going to work."

Does it work as quickly?

"It wouldn't work as quickly [at lower temperatures] as at –5 C but it's going to work. It's going to give us some grit — that's a good thing — but it also gives us freezing point depression, as we call it," White said.

Should the city be salting today?

"I would say that they should get the salt out. The salt gives a grip and it also helps get through this because we have a huge pile of ice," said White.

"Anything you can do to create a little, open spot in that ice, then the sidewalk shows and the sidewalk absorbs more of the light from the sun than the ice does. The ice reflects the sun, to a large extent. The sidewalk absorbs more of it and then it's an accelerated process — you get one open spot and that leads to more open spots and then you can chisel away at it."

Salting technique?

"I'd say get a few [bare, salted] spots going and then that will help to get some leverage underneath and get some ice melting underneath so you can get more spots through," White said.

"When that snow packs down from so many people walking on it, packs down into that thick ice, that's the most difficult thing to remove of all and that's one of the things the sidewalk plows are really adding to this winter."

Are there alternatives to salt?

"Salt doesn't work in all circumstances, either. If you're on a well, you don't want salt because the salt is going to leave a residue," White said.

"The other possibilities are anything that gives grit. Sand is good for giving grit, even kitty litter is OK, because it's just a clay and it's giving you some grit so at least you're safer on the sidewalks walking than with nothing on the sidewalk at all."