It has been a cold, snowy winter for much of the United States - and a great year to be in the salt business.
Sales of rock salt used to combat ice and snow buildup on highways are up 60 percent over last year, and a series of big snowstorms has left officials in some areas of the Midwest and New England scrambling to secure more salt.
FOR THE RECORD - Because of inaccurate information from the Salt Institute, the total amount of salt spread on U.S. highways each year was incorrectly stated on Page 4A of yesterday's editions. The actual quantity of salt spread is 18 million tons.
The Sun regrets the error.
Trucks were expected to be out spreading salt today in the Baltimore metro area, for the second time this week, in preparation for a forecast snow and ice storm.
"It's a terrible business we're in," said Morton Satin, technical director for the Salt Institute, an industry trade group. "Everybody says the weather's terrible and we say, `Isn't that great.'"
By lowering the freezing point of water, salt keeps it from forming ice on roads or, if it is already frozen, turns it back to a liquid.
While pure water freezes at 32 degrees Fahrenheit, a solution of 10 percent salt water freezes at 20 degrees and a 20 percent solution freezes at 2 degrees. This is why ice cream is made using icy brine to chill the mixing container - the temperature has to be lower than 32 degrees for the thick cream mixture to freeze.
But ice cream making - and salt used to flavor food - account for a minority of the salt market. "We use a tremendous amount of salt for roads, so much so that it's by far the largest use," said Satin.
About 18,000 tons of salt are spread in the U.S. each year at a cost of about $500 million to municipalities hoping to keep roads clear. Satin said about 60 percent of salt sold in the United States each year is used on roads.
The practice of salting roads began in the 1940s and grew with the U.S. interstate highway system. Salt sales hit a plateau in the 1970s and '80s, but demand picked up again in the 1990s as U.S. industries adopted "just-in-time" manufacturing practices. They stopped carrying large inventories of parts and raw materials needed to make products and instead relied on trucks to deliver the materials "just in time."
"So the highways needed to be reliable," said Richard L. Hanneman, the Salt Institute's president. "If you miss the guy who's delivering engines or brake pads, you can't keep building a car."
The wide application of salt has raised concerns among environmentalists that salt could harm wildlife and taint rivers and wells.
"It certainly has harmful environmental impacts," said Leila Goldmark, watershed program director for Riverkeeper, a New York-based group. "You're changing the ecology of the landscape; salt can be toxic. That can impact wetland species and roadside plants."
She said great blue herons in New York can be seen standing on roadsides for long periods because they are "salt drunk" and too dazed to fly. On the other side of the country, the Washington State Department of Transportation is investigating whether flocks of finches are gathering next to roads in unusually large numbers because they've eaten rock salt. Riverkeeper is encouraging local governments in New York to fine-tune their use of salt on roadways.
Several new technologies have emerged to do that. Until the mid-1990s, salt was typically applied to roads after snow or ice had already formed, by trucks that dumped out uneven layers of the crystals. "They used to leave bumps at stop signs," said Hanneman.
The current state of the art in salt application is trucks that can spray a salt solution on the road before a storm and adjust the amount deposited according to the speed of the truck. When the truck is moving fast it sprays more salt; when it's moving slow it sprays less. So an even coat of salt is laid down.
It also has the advantage of preempting a storm - and the ice-to-road bond that, once formed, can be difficult to break.
"The physics of ice on pavement is that you can get a very strong bond," said Satin. "If you spray brine prior to a snowstorm, you end up with a film of salt that prevents the bond from forming between the road and the ice."
With traditional techniques, 12 to 15 percent of the dry salt crystals bounce off the road, according to Salt Institute estimates. But Satin said the liquid form allows for a more precise amount of salt to be applied. "It lets you use only enough salting that's necessary for the job," he said.
In mountainous or northern regions of the country, it's too cold for saline solutions to work, so trucks spray liquid magnesium and calcium chloride to de-ice roads.
Each year 7,000 deaths and 800,000 injuries on U.S. roads are weather-related, according to the Salt Institute. A 2000 study by the Iowa Department of Transportation found that accident rates were multiplied by 130 during winter storms.
Another study by civil engineers at Marquette University in Wisconsin found that salting roads after a snowstorm led to an 88 percent drop in traffic accidents that result in injuries. A German study came to a similar conclusion.
And where does all this life-saving salt come from? Salt mines, of course.
The United States produces more salt than any other country in the world. But port cities on the East Coast - such as Baltimore, New York and Boston - get much of their rock salt from mines in Chile and the Caribbean because shipping salt is cheaper than delivering it by truck or rail.
Much of the Midwest is supplied via the Mississippi River from a mine in Louisiana that produces the greatest tonnage of any U.S. mine. New York and Ohio also have large mines.
Unlike table salt, which is produced by pumping water into a mine, then drawing the saline solution back out, rock salt mines are dry operations.
"Try and picture a hallway that's 40 feet across and 40 feet high that goes on for a couple of miles," said Satin, describing the scale of rock salt mines. "And there is another hallway next to that, and another next to that - and so on."