Two different studies, and they get mixed up constantly
There are two separate pieces of research behind the "your car is filthy" headlines, and they measured different things in different ways. Worth untangling, because the real one has a more useful answer than the viral one.
The actual academic study
Researchers at Aston University's College of Health and Life Sciences, led by microbiologist Dr. Jonathan Cox, swabbed five used cars, cultured the samples, and counted bacterial colonies by zone. The trunk came out worst by far — 1,425 — followed by the driver's seat (649), gear stick (407), back seat (323), and dashboard (317). The steering wheel actually came out cleanest of the six zones tested, at 146. Every single zone still beat the average toilet seat. Dr. Cox's own line on it: "you would never even think about eating off your toilet seat" — and by this measure, most of the car qualifies too. E. coli, antibiotic-resistant Pseudomonas, and Staph aureus (the strain associated with MRSA) all turned up somewhere in the cars tested.
The viral steering-wheel number — a different study entirely
The widely shared claim that a steering wheel is "4 times dirtier than a toilet seat" comes from a separate CarRentals.com survey, not from Aston University — it's a commercial test, not a peer-reviewed one. It puts the steering wheel at roughly 629 colony-forming units per square centimeter — 4 times a public toilet seat, 6 times a smartphone screen, twice a elevator button. Worth knowing as "a company's own testing," not as academic research, and worth not mixing its numbers with Aston's.
What this actually means for you
Set aside which number belongs to which study — the shared conclusion is the same: ordinary daily use puts real bacterial load into a car's interior, and the trunk in particular (groceries, gym bags, shoes, whatever gets tossed in and forgotten) tends to be the worst offender, not the steering wheel most people worry about.
Car seats and seat belts: the zones people skip
Child car seats are a genuine higher-risk zone — fabric in constant contact with skin, saliva, and food residue creates exactly the environment bacteria need. Washing the seat cover regularly doesn't reach the frame or the harness underneath it. Seat belts are the other blind spot: they're in constant contact with skin, clothing, and sweat, and almost nobody cleans them because people worry about damaging the webbing.
How to actually address it without damaging anything
The risk with aggressive disinfecting — high-concentration alcohol or bleach-based products — is that it can discolor and dry out leather and some plastics just as effectively as it kills bacteria. The right approach uses products matched to the specific material (leather, vinyl, fabric) rather than one blanket disinfectant everywhere, plus steam where it's appropriate — it disinfects physically, with less chemical load than a spray-everything approach.
Frequently asked questions
Is it true that a steering wheel is dirtier than a toilet seat?
By one commercial study's measure, yes — though a separate academic study (Aston University) actually found the steering wheel to be the cleanest zone they tested, with the trunk far dirtier. Both studies agree the whole interior beats an average toilet seat.
How often should I disinfect the interior if a kid rides in the car?
Regular wipe-downs of high-contact surfaces between full details is reasonable, with a proper deep clean — including the car seat frame and harness, not just the cover — every few months.
Are antibacterial wipes safe on a leather steering wheel?
Many aren't — high alcohol content can dry and crack leather over repeated use. A leather-safe cleaner does the job without that trade-off.
Get past what a wipe-down actually reaches
Our Interior Reset covers the zones people skip — the trunk, the seat belts, the car seat frame — with materials-appropriate products, not a one-size disinfectant.