Maryland highway officials said yesterday they are investigating whether moist air could have affected a chemical spray used to treat roads before a snowstorm last week, causing some vehicles on Interstate 270 to spin out on the slippery surface.

The state also is investigating whether the spray is connected to similar slippery conditions on a Capital Beltway exit ramp in December, a spokeswoman said.

Maryland has stopped pre-treating roads with that chemical spray or salt brine until the State Highway Administration figures out what made 1,000 feet of I-270 at Falls Road so slippery about 4 p.m. Thursday.

Valerie Burnette Edgar, a spokeswoman for the agency, said highway officials are not certain that the spray caused the slippery pavement, but they are focusing on it as the only unusual occurrence. That stretch, as well as other areas of I-270 that did not become slippery, had been treated about three hours earlier.

Edgar said salt brine has not posed any problems, but no Maryland highways will be pre-treated until the agency figures out the problem.

The chemical spray -- liquid magnesium chloride -- has been used for years to prevent snow from freezing on roads throughout the Washington region and across the country without major incident, local and national highway officials said.

It often is used in urban areas because it gives road crews a jump-start on the snow before heavy traffic packs it into a frozen sheet, making it difficult to clear. Used with salt after snow begins, the chemical prevents freezing at temperatures well below zero, while salt alone works only into the upper teens, highway officials said.

Edgar said the Maryland highway agency heard yesterday about some slippery encounters from colleagues in other states.

"Some other highway agencies have said this material absorbs moisture from the air, so that's what they're looking at," Edgar said. "The chemical could have become too diluted and wouldn't act the way you'd think it would."

Edgar said a Beltway exit ramp became slippery Dec. 5 on the Maryland side of the Woodrow Wilson Bridge. The ramp had not been sprayed, but vehicles tracked some of the spray onto it from other areas, Edgar said.

The state continued using the spray because there was no proof it caused the problem and because it was the first time highway crews in that area had applied it, Edgar said. However, the Gaithersburg area highway crews that treated I-270 had several years of experience with it, making state officials more suspicious of the spray, Edgar said.

Virginia and the District have used the spray for at least several years. Spokesmen for agencies in both jurisdictions said they had not had problems with slipperiness.

Ken Kobetsky of the American Association of State Highway and Transportation Officials said magnesium chloride has been used throughout the country since the early 1990s. He said he has not heard of any widespread problems beyond some incidents in Wisconsin when the chemical spray was not applied at the proper time.