Why Does a Film of Dirt Build Up On the Inside of My Car’s Windshield?

One good film deserves another, so I’ll borrow my answer from a famous line in the 1967 movie The Graduate: “One word: ‘plastics.’” It’s mainly the plastics in your car that produce the coating on your windshield.

Remember how your car smelled when it was brand-new? It smelled brand-new. A new car’s smell is a potpourri of the many volatile chemicals used in its manufacture, from paint and cement solvents to chemicals used in treating rubber, plastics and fabrics and, if you’re affluent, that “rich Corinthian leather” on the seats.

In fact, every substance in the world is continually evaporating some of its molecules into the air to a greater or lesser degree. (Techspeak: Every substance has a certain vapor pressure.) We smell a substance when some of its evaporated molecules reach the olfactory nerve cells in our noses. And those that don’t wind up in our noses might land anywhere else in the car.

Most of these volatile substances evaporate completely and dissipate long before you’ve paid off the loan, and when that happens your car no longer smells new. But other substances, smelling less noticeably of new debt, are released more slowly over a long period of time.

Plastics, in particular, are the biggest long-term emitters of chemicals: mainly plasticizers, which are waxy chemicals that give them flexibility. When your car is out in the sun, the intense radiation beating down through the windshield, modern automobiles have almost horizontal windshields for streamlining, hits the plastic dashboard cover and drives out plasticizer vapors, which then condense on the slightly cooler glass.

The resulting, sticky film of waxy plasticizer then collects dust particles that blow in through the air ducts around the windshield, whereupon you may either clean it or buy a new car.