As Americans spend more time behind the wheel, they may be unknowingly acting as a chauffeur to a menagerie of invisible passengers. Millions of germs and fungi, including bacteria and mold, are probably hitching a ride in your hatchback.
There's no question one of the main reasons a car gathers germs is because few people clean the interiors well or disinfect them.
In some ways, cars are temporary living spaces that we often overlook as a place where microorganisms can grow and thrive.
In a recent British study done for insurance.co.uk, microbiologists randomly tested both the interiors and trunks of 25 cars. They found that the typical British automobile had, on average, 285 types of bacteria present in every square inch of the vehicle. They identified at least 10 major types of bacteria.
Anthony Hilton, a microbiologist from Ashton University who led the study, believed steering wheels would house the most germs. However, stick shifts -- with 356 germs per square inch -- did worse, likely because they have a smaller surface area that concentrates the bugs.
And the nastiest harbor for bacterial refugees -- the carpet of the trunk, where scientists found 300 to 400 bacteria per square inch.
Most of the bacteria in cars came from dead skin cells and soil tracked in on shoes, hands or animal paws. People generally don't get infected by these pathogens. (Although one car that was swabbed for the study contained microbes linked to fecal contamination.)
"Our study found organisms in a car that are not uncommon to be found around a toilet," said Hilton. "If someone gave you some food to eat while you were sitting on the toilet, you would be repulsed."
Yet, plenty of people think nothing of nibbling in their vehicles.
Microbes also enter through the air and heating vents, although that's beginning to change. Newer models in the United Kingdom have more sophisticated ventilation systems, along with pollen or bacteriological filters, to prevent airborne particles from getting in, Hilton explained.
Putting U.S. Cars to the Test
Compared to British cars, American cars aren't much cleaner. In 2006, microbiologist Charles Gerba of the University of Arizona and a colleague did a larger study.
They swabbed the interiors of 100 automobiles in five locations: Arizona, California, Florida, Illinois and Washington, D.C. They took samples from 11 different areas in each car. And the scientists noted whether the driver was male or female, single or married.
When they looked at the results, the cleanest car belonged to a bachelor from Arizona. The dirtiest was driven by a married woman from Florida.
Not surprisingly, vans and SUVs contained more bacteria than smaller models, and married people had germier vehicles than singles.
The four top spots for germs were dashboards -- which ranked first -- followed by change holders, cup holders and children's car seats. These beat out other frequently touched areas such as door handles, window openers and seat belts.
Gerba found evidence that bacteria can grow in food spills and suspects the dashboard was the most germ-laden location, because it's one of the warmest places in the car and has ventilation systems on either side that can aerate spores.
Whether a car is teeming with germs also depends on where you live. Climates that are warmer and more humid like Florida produce more bacteria, because the car's interior acts like a sauna, while Chicago's cold and damp conditions mimic refrigeration inside an automobile and encourage the growth of mold.
Gerba has also done similar studies on planes and buses, and found that cars were the moldiest of all forms of transportation, probably because their interiors get cleaned less frequently. If you're sensitive to mold, your car might trigger an allergic reaction, especially in colder climates.
Driving Away Germs
While the exterior looks of your car might be of primary importance, experts advise you not overlook the inside and clean it as regularly as you would the outside.
Today's car has become more of an extension of the domestic environment -- where people eat and drink and talk on the phone. It's like a room of your house on wheels, said Hilton. Just imagine what a microbiologist might find in your dining room should you choose to foresake cleaning it for a time.
You'll want to periodically clean and wipe the surfaces and mats, especially after food or beverages spill, as well as vacuum the rugs and seats where crumbs and debris can settle. You also need to be more vigilant if your pet regularly joins you for a ride.
"I don't want people to freak out and throw bleach in their cars," said Hilton.
But Gerba takes a stronger stance. He recommends using disinfecting wipes on the dashboard, door handles and kids' car seats, especially if you're transporting lots of children or have a carpool.
It's highly unlikely that the microbes in the average car will make you sick, but under atypical circumstances, they can, pointed out Philip Tierno, a microbiology professor at New York University and author of "The Secret Life of Germs."
The real risk is driving with more passengers in the car, particularly younger ones, which increases the odds of more people sneezing and coughing, leaving a trail of germs behind that you might touch to your nose, mouth or eyes, leading to a cold or flu.
"Children are little bags of germs and if you cart them around, they have more potential pathogens," said Tierno.
Gerba put it another way, "If you're a soccer mom, you're essentially driving a germ-mobile."
Cold & Flu season is here! Visit the ABCNews.com OnCall+ Cold & Flu Center to get all your questions answered about these nasty viruses.