This article covers the sources of mold growth in vehicles – AC system, high humidity storage, spills, leaks, and design malfunctions – and how to prevent them.
I go into detail on the four methods used to clean the HVAC system out, as well as methods to clean mold and cross-contamination in the vehicle as a whole.
The AC is usually the most difficult area in the vehicle to manage microbial growth. While there are certain makes and models of vehicles where this is more of an issue, to some degree it’s a problem in all vehicles due to the inherent design of the AC system.
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Preventing Mold in Vehicles
Preventing Mold in the AC and Heating System (HVAC)
The fungi and bacteria grow mostly in the evaporator core and immediate surrounds, due to humidity and moisture that forms when AC is in use. Mold can form in the ductwork as well.
To help reduce mold formation in the AC system, run the fan for the last 10 min after using the AC. (Defrost is also AC, so you need to run the fan for 10 minutes after defrost as well). This helps the evaporator dry out.
Using recirculate can worsen the problem in some situations. Bringing in fresh air (if the outdoor air has drier absolute humidity) as opposed to using recirculate can help the evaporator to dry out.
Another thing you can do is to avoid parking in areas where a lot of debris can enter the air intake.
Similarly, when driving through very dusty areas, you are bringing in dust and spores through the air intake. You may want to turn off the HVAC system completely in those conditions.
Maintenence on the vehicle should include changing the cabin air filter as needed (when it’s dirty).
I would include one of the four HVAC cleaning methods below as a preventative (before you start smelling or reacting to mold) if you do use the AC system regularly.
For those Extremely Sensitive to Mold in HVAC – How do you Turn off the AC?
Many folks who are extremely sensitive to mold do not use the AC system in their vehicle at all. If you refrain from using this right from the start with a new car, you will avoid most of the problems here.
If you want to avoid AC use altogether, using just the heating system should be fine. But, keep in mind many new vehicle models have automatic climate control which will use both heating and cooling under many different conditions (not just when defrosting). If you want to turn off the AC, look for a vehicle where this can be done – new cars have a way to disconnect the AC compressor. Ask a mechanic how to do this.
If you have already used the vehicle’s AC or you bought a used vehicle that has some funk in the HVAC system, then even the heating setting will circulate the fungi and bacteria throughout the vehicle.
Buying a vehicle with heated seats is a good idea in case you become sensitized to the vented HVAC and need to turn it off for a while (and try and clean it). Some very sensitive folks have had to turn it off permanently if they have not had success with the cleaning methods.
One other hack is that you can externally mount the HVAC system to get easy access to the evaporator. This is a very unusual hack. You can find photos of it in the group Mold Avoiders on the Road.
Alternative Ways to Stay Cool in the Car
Cars that Come with Cooling Seats
Some of these built-in systems blow unrefrigerated air through the cushions. And some use refrigerated air that is tied into the main AC systems. The unrefrigerated air system may be safe in terms of mold prevention.
Safer Cooling Seats (or Heating)
You can also add your own cooling and heating seats to avoid the use of your car’s HVAC system.
Cooling seats that you add yourself work simply by adding a fan that blows unconditioned air which helps to cool you down.
Some use fans and those can be affordable. (Though they might be made of vinyl).
The heated seats use coils, just like a heated blanket or heated jacket. Some models do both.
Then there are gel seats, I like the idea of the ones you put in the freezer first, then you bring it out to the car with you. This is a kids version. If you know an adult version let me know, I think it’s a good idea. You will want to dry those off though, when they come out of the freezer they are probably going to condensate.
The active vests provide more cooling but are more expensive.
They were life-saving for me until I healed from heat intolerance.
Preventing Mold in the Car (as a Whole)
Why Mold Grows Inside Vehicles
There are a few ways your vehicle can become moldy inside on the seats and upholstery. Sometimes just leaving a car to sit in a very humid place for long periods of time can cause mold to start growing on the seats.
More common though, is a spill or window left open that wasn’t dealt with fast enough.
Other sources of water damage are less common:
- Debris accumulating in the air intake areas and connected drains (keep your air intake area clean, and change the filter). The cowl intake and drain area can collect debris. Those areas are connected to body cavities that can harbor microbial growth. Thus structural mold is possible in a vehicle, though this is less likely than mold growth in the HVAC and inside the vehicle.
- Rainwater that has gotten into the HVAC ducts and car interior in some past Ford models – causing hidden pooling of water.
Ways to Prevent Mold Growth in Vehicles
Prevention here is key. Once mold is actively growing and has made it through to the foam in the seats, it’s a huge problem that is hard to fix.
When storing your vehicle for long periods of time in a constantly humid climate, it’s best to leave it in a sunny spot and leave it with a dehumidifier. Climate-controlled indoor storage is of course ideal.
Though generally, a vehicle should be able to withstand a good amount of humidity while being stored, really high humidity for long periods has been a problem. I left a truck in 70-95% humidity for a couple of months and came back to mold growth. No known water intrusion.
Many others in the Pacific North West or in semi-tropical and tropical areas have left vehicles sitting to come back to visible mold growing (and not from leaks).
Both active (electricity-powered), and passive dehumidifiers will help. You can also monitor the outdoor humidity.
I would check on the vehicle (or have someone check on it), both to empty the dehumidifiers and possibly to dry the vehicle out actively with heat and airing every few days if necessary.
When you spill something or leave the window open, just do everything you can to dry it out within a day, which includes the above: usually sun and air, or heat and air. Add dehumidification only if it’s going too slowly.
Decon/Clean Mold or Cross-Contamination in a Car
DISCLAIMER: Some of the cleaning methods discussed in this article, which include ozone, chlorine dioxide, hydrogen peroxide, and quaternary ammonia, require knowledge of safety precautions for handling the materials, and executing these methods. Please take full responsibility for your health and wellbeing and make sure you understand the risks, precautions, and procedures – not all of which are explained here in full (please research them before proceeding or consult with an expert). Full disclaimer here.
If you do have some light surface mold in the car or cross-contamination, I would start with a deep clean.
If you had a big spill that didn’t dry out, or you suspect someone else did in your vehicle, you might want to open the seat covers to check on the foam. Some have zippers. If they don’t you would have to cut them open to check.
Mold in the foam cannot be properly remediated other than with replacement.
Deep Clean your Vehicle
You can remove light surface mold or cross-contamination by steam cleaning or shampooing the seats.
Make sure it is dried out quickly after that. Car detailing shops can do this. Just make sure you know which products they are using, as some might not be acceptable, toxin-wise.
A mechanic I talked to near Vancouver BC, does a shampoo of the seats with high temp extraction, then an enzyme spray on the seats instead of ozone to reduce odors. I use natural enzymes to break down odors.
DIY HEPA Sandwich for a Car
If it’s not so bad as to warrant a steam clean or extractor shampoo, use a HEPA vacuum, which you can do yourself. Vacuum the seats, carpet, and upholstery.
For a vehicle, you want a rather flexible hose. I would use a shop vac. I’ve used one before with the HEPA filter and HEPA filter bags.
You can then wipe down all surfaces with quats or the cleaning product of your choice. A proper “HEPA sandwich” clean would involve another round of vacuuming.
Having your car or truck professionally detailed can help, as they do a very thorough clean, though I haven’t seen them do HEPA vacuuming.
Using Ozone on Cross Contamination
If those methods don’t cut it, and you are still sensing mold in the vehicle, you will want to use the methods below.
Many people do use ozone and it does work, but it leaves behind oxidation, which smells quite terrible.
Depending on how long you ozonated it for, it can take a while to die down.
I don’t agree with some who say that you will damage the wiring easily, you would have to use extremely high levels for a very long time to do damage like that.
Many mold-sensitive folks have ozonated vehicles at very high concentrations for 24 hours, many times. I have done a couple of 24-hour treatments on my truck.
Car dealerships use this all the time as well (but normally in much shorter treatments, like 1 hour). Many people only do 2-hour treatments for mold, which should not cause damage to the plastics, and would only cause minimal oxidation odor after.
There is more info below on pumping ozone through the HVAC. If you don’t need to pump it through the HVAC, just see my general post on ozone and run it inside the vehicle. I use many of the little ones from Amazon (like this one).
Please read through the precautions thoroughly, since ozone is a dangerous gas that can be deadly.
Another oxidization method, though it is a weaker oxidizer than ozone, is to use chlorine dioxide gas. That method is explained below, in its own section.
Other Fogging Methods for Mold
If you have another decon method that has worked well for you in decontaminating your house from mold, it’s likely you can use that here – like thieves oil fogging, Concrobium fogging, or using quats.
Silver Hydrogen Peroxide though, is likely too much oxidation for most vehicles, though it can be used in the empty part of metal cargo vans.
Chlorine Dioxide Gas to Treat Mold
Chlorine Dioxide, like ozone is an oxidizer. Like ozone, there is evidence that it breaks down mold and mycotoxins.
Although it’s weaker than ozone, you should take the same safety precautions outlined here. It is dangerous to breathe in and it’s explosive in certain concentrations.
You may not need to be so far away from it like with high doses of ozone, which is an upside. It’s also cheaper and easier.
Read the safety instructions carefully on the product and make sure you understand them. Never use a chlorine dioxide product in occupied spaces.
If you want to make it yourself, please take full responsibility for your safety and wellbeing. Here are the instructions:
How to make chlorine dioxide gas treatment for the home or car: 6.5 oz (roughly 3/4 cup) Oxine plus 3-4 tsp citric acid in glass or heavy plastic container, one for each room. Leave it for 3 hours for a shock treatment or 24 hours for a deep treatment.
You can make a smaller batch for smaller spaces with the same ratio. It takes about a minute for the reaction to take place, the liquid will turn yellow, and for about 3 hours it will release chlorine dioxide as a gas. Take similar precautions as with ozone for airing it out and re-entering.
Four Methods for Cleaning Mold from the AC (HVAC) System of the Car
1. Take it to a Dealership – Have them Clean out the System
Skill Level – Easy. Results – Can work, depending on which methods they use.
There are a number of different cleaning methods used by automotive repair shops to clean out the AC system. Some use ozone and even run it through the HVAC in the same way as described in the next section on ozone.
There are also treatments that are similar to #4 on this list, introducing a product into the system to clean the evaporator.
One mechanic explains: “one is a mist treatment, using a product which foams up. You drill a hole in the case and then introduce the mist to the evaporator core and let it sit, give it time to work, and then flush it out.” (source).
Some of these might have fragrance of other toxic additives but others would be acceptable for many folks who are sensitive to chemicals.
2. Using Ozone Through the HVAC
Skill Level – Intermediate. Results – works on most mold and bacteria.
This is a technique used by some car detailers and dealerships and mold-sensitive folks.
The simplest way to do it is to run an ozone machine in the car while the vehicle is running so that the ozone is pulled in through the air intake. Most people would run this through for a couple of hours. Depending on the amount of ozone and the time you run it for, you may need a window open to provide oxygen.
If you are running the machine in the car I would use a small machine like the ones on Amazon for under 100$.
Instead of running the vehicle, you could alternatively, hook up the battery to a battery charger to run through the HVAC settings that way.
For any serious contamination or if you want to blast this more directly through the HVAC, mold avoiders tend to use an external ozone machine that pumps in the gas, giving the machine full access to oxygen outside. Bioblaster is the most popular one.
If you are in the Facebook group Mold Avoiders, you will be able to see an example of a Bioblaster hooked up right to the air intake here.
That machine is overkill for me. But others may need this. I would have no problem running the smaller ones on Amazon that I have used many times and pumping those through the HVAC system with tubing from the outside.
To get it through all the different parts of the HVAC system, the ozone needs to run through it on all settings (heat, AC, recirculate). Run it through each setting for at least 20 min. The next section explains more about how to access the air intake.
This is tricky because of how far away you need to be from the ozone. Let it air out each time before coming close enough to the vehicle to change the settings. Again, ozone precautions are here.
3. Spray Through the Air Intakes
Skill Level – Beginner to Intermediate. Results – May or may not work.
In this method, hydrogen peroxide or quats are sprayed through the air intake.
Some very mold-sensitive folks have had success with 10% hydrogen peroxide sprayed into the air intake with these video instructions (not with toxic products like scented Lysol). Seeing the process in a video really helps to make sense of it.
You will be using 29, 30 or 35% peroxide (depending on what you can source) and diluting it down to 10% (here is the dilution table). Please research and take full responsibility for handling and spraying this concentration of hydrogen peroxide.
Some people found this worked better with quats (a type of ammonia). This ACDelco product was recommended.
4. Greg Muske’s Quats Cleaning Strategy
Skill Level – Very Advanced. Results – Likely to work if you do it right.
Greg Muske from Biotoxin Journey has a detailed and more complex method of accessing the entire HVAC system and runnings quats (i.e. quaternary ammonium) through it.
You need some mechanical expertise for this method as it does involve drilling into specific parts of the system, which differ depending on your make and model.
You need someone who can understand the heater core diagram for your car, which you can look up online. You’re going to locate the evaporator core. Then drench the two cores by running about one gallon of quats solution into 3 access holes. Followed by a water rinse.
Greg also misted quats through the ductwork via the vents and the opening for the cabin filter directly above the blower (after removing the filter), and air inlet holes located under the hood next to the firewall. Unlike when cleaning the cores, this was not rinsed out. It was dried out by running the heater and fan on high.
Greg then cleaned out the rest of the car by using a steam cleaner on the carpet with Thieves Oil Cleaner. (You can use the cleaner of your choice). All other surfaces in the vehicle were wiped down with quats.
Full instructions and a video can be found here: Clean Driving Machine. This section was printed with permission from Greg Muske.
Corinne Segura is a Building Biologist with 7 years of experience helping people create healthy homes.
This post was written with input from two engineers, a mechanic, and a car detailer.
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