Bacteria Study Reveals Our Car Interiors Are Dirtier Than The Average Toilet

Whether you’re taking your car for its last clean before being scrapped, or down to the car wash every other Saturday morning – there’s nothing better than a freshly cleaned car.

But are we cleaning our cars thoroughly enough? And how dirty actually is the average vehicle?

We wanted to find out, so we partnered with Dr Jonathan Cox, a Senior Lecturer in Microbiology at Aston University, and his research team, to conduct a study to reveal which areas of our cars are the dirtiest

To do so, the Aston University team sampled five used cars of varying ages to discover the areas of the car which play home to the most bacteria, reveal what bacteria can be found in our vehicles, as well as revealing whether or not our car interiors are dirtier than a toilet seat!

For each vehicle, samples were taken in the six key locations of a car’s interior, using a sterile cotton bud swab, hydrated with phosphate buffered saline. Each swab was then spread onto inoculated agar plates and incubated at 37°C for 24 hours in order to identify any bacterial growth.

For anyone with a weak stomach, it’s time to brace yourself, because the study found a lot of bacteria living within our cars… and it’s not exactly the ‘good’ bacteria either!

The Areas of our Cars with the Most Bacteria

1. Boot – 1,425 bacteria identified
2. Driver’s seat – 649 bacteria identified
3. Gearstick – 407 bacteria identified
4. Back seat – 323 bacteria identified
5. Dashboard – 317 bacteria identified
6. Steering wheel – 146 bacteria identified

Our study revealed that the boot was the most heavily bacteria-contaminated area of all the cars, with 1,425 bacteria identified within the research. All cars sampled in this study contained faecal bacteria in the boot, and in all cars, the boot significantly exceeded the bacterial contamination levels of an average toilet!

Dr Jonathan Cox says: “The boot is often where we put our groceries when we go to the supermarket – and with a reduction in the use of carrier bags, there’s a much greater potential of throwing loose items into the boot of a car to travel home. With this in mind, we should be mindful of reducing the risk of this food by making sure to sufficiently clean any fruit, vegetables or other produce before eating it.”

Shockingly, each car also contained a significantly higher total bacterial load than each of the toilets sampled, and faecal matter was found on the driver’s seat – suggesting you should think twice about driving another person’s car if they are particularly windy!

Overall, the gearstick, dashboard, driver’s seat, back seat and boot generally contained high levels of bacterial contamination with each location equalling or exceeding the bacterial contamination you’re likely to find on an average the toilet seat and flush. However, there is some good news, because in most cars, the steering wheel showed a very low level of bacterial contamination.

Dr Jonathan Cox commented: “Due to increased sanitizer use since the COVID-19 pandemic, the driver contact surfaces generally didn’t have the level of bacteria that we were expecting, meaning that areas such as the steering wheel were not as contaminated as they might have been a few years back.”

He explained “We all assume the steering wheel would be the worst for bacteria build up, due to the fact that our hands are mainly on the wheel and so we think this is the main touch point. This means that the gearstick is probably overlooked when it comes to sanitising, however our findings revealed that the gearstick is actually dirtier than a toilet flush, and in most cases dirtier than a toilet seat, meaning you should consider wiping down these areas too.”

The results also show that in general, bacterial contamination correlated with the age of the vehicle. The older cars sampled for the study exhibited higher bacteria loads than those that had been on the road for a shorter amount of time, suggesting any used cars will be in need of a deep clean before driving!

Commenting on these findings, Dr Jonathan Cox explained: “It’s been fascinating to reveal that there is in fact a correlation between the age of vehicles and the amount of the bacteria found in the car – suggesting that the longer we have a car, the dirtier it becomes, regardless of the amount of cleaning we do. This does make sense, as the more we use our cars, the more likely we are to introduce more bacteria from different sources.”

So alongside finding faecal bacteria in most cars out on the road today, what are the types of bacteria commonly found in a car’s interior, and why should they be cleaned away?

• E. coli (Escherichia coli) – A faecal coliform (aka. enterobacteriaceae) – bacteria from the gut. Most strains from the gut are harmless and we are not able to identify the specific strains. Transmission of pathogenic strains is normally faecal-oral transmission e.g. dropping some food on the heavily contaminated driver’s seat and eating it. This could cause food poisoning in some cases.
  
• Pseudomonas (Pseudomonas aeruginosa) – This is a ubiquitous organism (it is found in lots of places in the environment; soil, water, skin etc.) but is a bacterial species of increasing concern due to its infectivity to humans and animals, and some strains can’t be easily treated with antibiotics.
  
• Bacillus (Bacillus subtilis) – Can be found in the gastrointestinal tract of humans and ruminating animals. It doesn’t normally come from the environment and thankfully, doesn’t pose a threat to humans.
  
• Staph. Epi (Staphylococcus epidermidis) – Skin microbes found on everyone’s hands. They can cause infections in people who are immunocompromised.
  
• Staph. aureus (Staphylococcus aureus) – Found in the upper respiratory tract (nose mainly), but can in rare occasions be found on the skin. These are more likely to be associated with coughs and sneezes. A member of the Staph. aureus family is methicillin-resistant staphylococcus aureus (or MRSA) – a superbug that is extremely hard to treat.
  
• Rhodotorula (Rhodotorula mucilaginosa) – Environmental yeast (soil, water, milk, fruit juice etc.) – can rarely cause infections in immunocompromised people.

How Often Should we be Cleaning our Vehicles?

If the results of our study have made you want to douse your car in sanitizer, do not fear. Dr Jonathan Cox explains how we should not be too concerned by the findings of our study…

“The cars sampled were very heavily contaminated – and not just with bacteria we’d expect to find – for example staph. epidermidis, a type of skin microbe found on everyone’s hands. We did find E.coli in some areas of the car, such as the boot, which is a type of faecal bacteria and can cause food poisoning, diarrhoea and vomiting if ingested in a significant quantity. However, to identify the risks of the bacteria found, we’d need to really get deeper into the microbiology to understand each species.

“Having said all of this – and to be clear – people don’t get sick every time they get in the car. We don’t need to be overly concerned; we just should be mindful that not all dirt is visible for us to see.

“This study has shown that people could definitely make more of an effort to keep their car clean. Ultimately, it’s about mitigating the risk of getting sick. We are unlikely to eat food in the boot of our car, but often consume food in the driver’s seat. Cleaning our hands before we eat in that environment is a good idea, and knowing where microorganisms may be residing can help us to think more about our habits when we drive – I would certainly think twice before picking up a dropped wine gum and eating it assuming it’s fine!”

What Should we be Keeping in Mind When Cleaning our Vehicles?

From more regular cleans, to cleaning biologically as well as physically, Dr Jonathan Cox provides his expert tips on reducing the risk of bacterial infection from your vehicle.

• Clean the inside as much as the outside: “Something we should all be thinking about is that we should be cleaning the inside of our cars as much as we clean the outside. We often see mud splashes which reminds us to drive to the nearest car wash, but we don’t see the bacteria building up in our vehicles, and we should be cleaning the inside as much as the exterior.”

• Don’t just rely on looks: “It’s also important to be aware that there is a difference between being clean visually and being clean biologically. We generally just focus on the visual element, meaning that if something looks clean or tidy, we do not look to cleanse more deeply. If you were cleaning a kitchen sink or toilet you would clean these biologically with chemicals – it’s unlikely that the majority of car owners would clean their cars in this way. Hopefully this study will encourage people to be more mindful of their vehicle and germs and clean their cars in a more thorough way.”

• Reach for the antibac!: “A quick way to ensure your vehicle is kept clean in between deeper cleans, is to spray some antibacterial sanitiser onto a tissue and wipe over the gear stick/steering wheel. You could also consider getting the upholstery of your vehicle shampooed once in a while too. This way, you can ensure the car is clean bacterially as well as physically.”

“The bottom line is that people generally do not typically get ill from the bacteria in their cars, because we naturally do things in our everyday lives (such as handwashing) to reduce the impact. However, knowing the bacteria that can build up in our cars can be really useful in cleaning our cars properly and regularly, and reducing any risks.”

Dan Gick, Managing Director here at Scrap Car Comparison adds:

“Taking care of your car, from making sure it’s running well to keeping it clean, all work towards ensuring it has a long life and is a car you love mile after mile. The last thing you want is for your car to become a risk on the roads, as well as a risk to your health. We hope the results of this study help to highlight the importance of taking good care of your car inside and out.

It’s worth thinking about how often you clean the inside of your house and apply the same thought process to your car, especially if you tend to drive it every day”.

Methodology:

We partnered with Dr Jonathan Cox from Aston University to conduct a study which took place in December 2021. Samples were taken by swabbing location using a sterile cotton bud swab, hydrated with phosphate buffered saline. Each swab was spread onto Nutrient Agar (NA) and Violet Red Bile Glucose (VRBG) culture medium. NA supports all bacterial growth, whereas VRBG only supports the growth of Enterobacteriaceae. Inoculated agar plates were incubated at 37oC for 24 hours to enable bacterial growth. All colonies were enumerated and subject to the Gram staining method to enable presumptive identification.

Vehicles sampled:

• Car 1: Peugeot 307-SW; 17 years old; animals; bought second hand with one previous owner.
• Car 2: VW Golf; 5 years old; animals and children; bought third hand with two previous owners.
• Car 3: Ford Focus; 13 years old; animals and children; bought second hand with one previous owner.
• Car 4: Honda Jazz; 9 years old; animals and children; bought third hand with two previous owners.
• Car 5: Peugeot 308; 2 years old; bought second hand with one previous owner.

Toilets:

Two domestic toilets were chosen that had not been knowingly cleaned, but had been used in the preceding 24 hours.