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Gross car anatomy

October 11, 2010
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If a detailer knew the nature of the bacterium in the motor vehicles they work on they might have second thoughts about being in the business. Or, if they were knowledgeable about the affects and dangers of these germs they could be making a lot more money by more clearly explaining why customer should have their vehicles detailed regularly.

Testing for bacteria and mold
Recently, Dr. Charles P. Gerba and Sheri L. Maxwell set out to test the presence of bacteria and mold in vehicles. It was quite possibly the most scientific test ever done on an automobile interior. There were 100 vehicles involved in the test in the states of Illinois, Arizona, Florida, California and Washington, D.C., to provide a good cross-section of climatic conditions across the country. The study considered several variables, including:

  • Geographic area of the country;
  • Different areas of the vehicle interior;
  • Type of vehicle;
  • Whether there were children in the vehicle;
  • Married or single drivers; and
  • Male or female.

The scientists collected four-inch samples from 11 areas of the vehicle and packaged the sterile swabs in ice to be sent via overnight courier to a University of Arizona laboratory.

The interior sites tested were:

  • Steering wheel;
  • Radio knob;
  • Dashboard;
  • Door handle;
  • Seat;
  • Children’s car seat;
  • Change holder;
  • Window opener;
  • Cup holder;
  • Seat belt; and
  • Food spills.

The results
Who had the “germiest” automobile? The results of the testing revealed that a married female in Florida had the automobile with the most germs.

The cleanest automobile? A single male living in Arizona.

It was also revealed that married individuals and females have automobile interiors with the most bacteria/germs while single individuals and males who had the cleanest automobiles.

Of the cities tested, Tampa, FL, had vehicles with the most bacteria/germs and Tucson, AZ, had the lowest bacterial numbers. Car owners in Tampa had 10 times more bacteria in their automobile interiors than those in Tucson.

The germiest sites in the interior
Measurements were also taken on which sites of the interior had the most bacteria/germs. The radio knob had the least amount of bacteria/germs. Food spills had the highest number of bacteria, as would be expected. A great point to make with customers that food spills do, in fact, grow bacteria and should be removed immediately to prevent bacteria from growing.

One interesting discovery: The dashboard had the second highest number of bacteria. It was summarized that this was due to the movement of air over the dashboard. Air is drawn in from the dashboard and bacteria may be landing on the dashboard as the air is drawn into the air circulation system. Another possibility is that this is the warmest spot in the automobile, in most cases with direct sunlight shining on the dashboard most of the day.

The types of vehicles
More bacteria were found in vans and SUVs than in passenger cars, which probably reflects the greater number of people that ride in the larger vehicles and also the occurrence of children in the larger vehicles. As would be expected, greater numbers of bacteria were found in those automobiles that carried children on a regular basis.

The bacteria, S. aureus (see sidebar) was found on the steering wheel more frequently than on any other site in the vehicle. This is logical since the steering wheel is in contact with the hands more than any other interior site.

The study also determined the percentage of automobiles in which both S. aureus and MRSA (Methicillin resistant Staphylococcus aureus) (see sidebar) were found. MRSA was isolated from a car seat and steering wheel. MRSA was found in two percent of the vehicles tested.

Molds found in vehicle interiors
The following is a list of molds that were identified in the automobiles tested and the number of times they were isolated. Members of the genus Aspergillus (see sidebar) were the most common molds identified.

Genus of molds found
in vehicle interiors
No. of
Times ID
Aspergillus 37
Ulocladium 5
Alternaria 5
Penicillium 4
Geotrichum 2
Chrysosporium 2
Trichoderma 1
Aureobasidium 1
Geomyces 1
Chrysonilia 1

The isolation of molds was the highest in Chicago and the lowest in Florida. For example, the occurrence of molds was 15 times higher in Chicago than in Tampa. Florida also had the highest annual rainfall and mean annual average temperature. Thus, bacterial numbers are probably related to a combination of high humidity and temperatures.

The authors of the study referenced in this article have granted their permission to use the results of the project for this article. If you are interested in a copy of the results, please contact the author at

R.L. “Bud” Abraham is president of Detail Plus Car Appearance Systems, Portland, OR, and a nearly 40-year member of the car care industry. He is also a member of the Western Carwash Association Board of Directors.

Abraham can be contacted at