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Some windshields can melt an icy coating, cut the sun's glare or resist shattering when rocks hit them at 50 mph.

But there's one thing common to all windshields, and it can interfere with driving: dirt.But cleaning windshields may cause long-term problems if it isn't done properly, says a Fairleigh Dickinson University professor.

Oswald Haase, who teaches physics at the Rutherford, N.J., school, has studied windshield wear and how light is diffused by the grooves and pits left behind by dirt or flying pebbles.

Windshield washers and wiper blades may take away some dirt, but the abrasion may leave behind scratches that can cause problems down the road.

"They help you see in the short run, but louse it up in the long run," Haase says.

The research, done during the last four years in Sweden and Cologne, West Germany, showed scratches and pits caused significant vision problems in less than 2 percent of the windshields tested, while dirt accounted for serious problems in about 25 percent.

He says it isn't clear yet if recent automobile design trends making windshields less vertical will help move dirt and small objects over the car or if longer wiper blades needed by larger windshields will hurt.

"The fact that the windshield is fitted is good. You have to get a better windshield washer," he says.