This article covers the biggest sources of mold growth in vehicles – the AC system and heating system.
I go into detail on the four methods used to clean the AC/HVAC system out, from the easiest to most involved method.
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.
I will also talk about preventing mold in the vehicle’s AC system.
Expert Review: Luke Skaff, Electrical Engineer. The article also has input from another engineer Terry Blade, a mechanic, and a car detailer.
This post contains affiliate links, upon purchase, I earn a small commission at no extra cost to you.
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 form 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 if you are extremely sensitive to mold (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 for those extremely sensitive to mold in case you become sensitized to the vented HVAC and need to turn it off for a while (and try to clean it).
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 (if You Turn Off the AC)
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 kid’s 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.
Cooling Vests
When I used to have severe heat intolerance, I used cooling vests on my body – both active and passive vests work. The active vests provide more cooling but are more expensive.
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 or other toxic additives but others would be acceptable for many folks who are sensitive to chemicals.
2. 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 products like scented Lysol). Seeing the process in a video really helps to make sense of it.
Spray the hydrogen peroxide through each setting for 3 min. You can use this mister.
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.
3. Using Ozone Through the HVAC
Skill Level – Intermediate. Results – works on most mold and bacteria, but can backfire.
This is a technique used by some car detailers and dealerships and mold-sensitive folks. Ozone kills mold and bacteria in high concentrations but it also seems to backfire in many cases, causing toxic byproducts, and since writing this article I would no longer risk the use of ozone at all.
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 last section explained 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.
4. Greg Muske’s Quats Cleaning Strategy
Skill Level – Very Advanced. Results – Likely to work if you do it right.
Note, this strategy is only for those extremely sensitive to mold. This is much more extreme than what any non-sensitive person would need to do.
Greg Muske from Biotoxin Journey has a detailed and more complex method of accessing the entire HVAC system and running quats (i.e. quaternary ammonium) through it.
You need some mechanical expertise for this method as it involves 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 on BioToxic Journey: Clean Driving Machine. This section was printed with permission from Greg Muske.
Corinne Segura holds certificates in Building Biology, Healthier Materials and Sustainable Buildings, and more. She has 10 years of experience helping others create healthy homes.
This post was written with input from two engineers, a mechanic, and a car detailer.
Did you find this post helpful? If so you can buy me a coffee to support the research behind this blog. Thank you!
Roger
This is the craziest most expensive and work intensive page on the internet for banishing ac mold. Park outdoors. Simply take out your vent guides (in your car) and intake filters (under the hood by the windshield – consult your manual or online aid) and wipe down the surface with something like diluted bleach 10 parts water to 1 part bleach. Wear gloves. Then spray white vinegar (1 part vinegar to 3 parts water) into the ac air intake with the car doors open and the fan on high so it sucks the vinegar through the vents and kills any remaining deeper mold spores. Keep the car running and let the vehicle blow it out the vents and out the car into the fresh air. Repeat 30 seconds at a time through each air intake duct (usually their are two – one on each side of the car under the hood) After a few blasts the mold smell will be gone and the residual white vinegar will act to kill any growth still present and prevent new growth. If it comes back, repeat the process and spray more concentrated vinegar into the intake. Ozone is nasty stuff, much easier/safer working with vinegar.
Corinne
do not use vinegar for this process, it does not kill many species of common molds. I outline basically the same procedure in strategy #3 but with hydrogen peroxide which will work much better. this is not meant to be a “do every method on the list” post but choose one. My readers will also not want to use bleach as my website is primarily for people with chemical and or mold sensitivity.
Joel
Hey, Greg Muske’s video on the “Clean Driving Machine” page appears to be down with no way to contact him. Does anyone have a copy or step-by-step instructions for his method?
Corinne
the method is explained in his article which I link to
Rick Scibilia
Hi Corinne,
I have two cars that have been stored outside for a few months. One (non running) seems to have a small leak in the sunroof which caused heavy white mould inside. I have sprayed the entire interior twice with white vinegar which has destroyed the mould on the seats door trims console etc. The roof lining however is stained with spots of black mould.
What would you suggest to clean remove this bearing in mind that steam cleaners will probably cause the roof lining glue to lift.
The second car, well sealed had a few light spots of white mould forming which have need vinegar bombed and seem to have disappeared. I want to ensure that mould is not lurking in the AC system and I’m not overly convinced with the ozone treatment method – human hazards and effects on the leather, rubbers and plastics etc. What would be a safer effective alternative method? As I’m not using these cars at the moment, once a initial treatment is done (is the vinegar bombing and a steam clean enough?), I can strip the entire interiors out and expose them in the sun and detail vinegar spray. What are your thoughts? How come you don’t mention white vinegar treatment – it is supposed to be the most effective method?
Joven Ruthford
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.
Christie Sterners
Hi, Wonderful site. Plenty of useful info here. Got real useful and I am very impressed by your tips.
Kristen
If I were to use the tiny Airthereal unit inside my car (the one that doesn’t have a cord) – would you run it for longer with a window cracked or stick with the 30min increments? I’d like to get it all through the air system, but I am NOT mechanical. Even taking out the cabin filter would be a struggle for me. I’m debating just taking it to a detailer to get it done.. but the quote was $325 for that so I’d love to try a DIY version first.
Corinne
That one has a cord. There are some smaller ones that might run off the cigarette lighter but I have not tried that.
Amy Bopp
Thank you for this post! Very helpful. I had been scared of ruining the electronics in my truck with ozone, I feel more confident now to do longer treatments. I am particularly sensitive and feel I need to deep clean after ozone. I have read that quats should not be used on leather, what would you recommend for leather seats? Btw the particular mold I am fighting does well with peroxide and is helped some by adms or lemongrass (but it doesnt Last long)- ec3 does virtually nothing so I am hesitant to try thieves. Do you think peroxide will damage the leather? Thanks in advance!
Corinne
I’m not sure what percentage would damage the leather. Some of us have sprayed it on leather but it’s a big risk and can depend on the finish on the leather. It will likely ruin the dyes if it’s dyed. I would put a cover on the seats and then clean or throw out the cover when it gets contaminated.