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Air quality is often overlooked when it comes to the air you actually breathe inside your car. When it comes to cars and air quality, the first thing you think of is usually going to be related to vehicle emissions and the environment. Ultimately your vehicle’s climate control system is one of the most important systems in your car. You’re affected by it virtually anytime you’re behind the wheel. The quality of the air you breathe is important, especially as related to potential distractions and secondary health issues related to dust, mold, and other environmental contaminants. In this article, we cover what exactly a cabin air filter does, how often to change the cabin air filter, and some of the best cabin air filters on the market. 

Despite this, the HVAC system remains one of the most overlooked parts of a car when it comes to routine service and maintenance. In this article, we’ll review the basics of how your AC system works, what a cabin filter is, and how it has evolved over the years, as well as some of the best ways you can maintain your car’s climate control system for peak performance.


How does car air conditioning work? 

Just like the HVAC system in your house or apartment, the climate control system of your car is designed to heat or cool the environment, and to provide fresh air to the passengers of the vehicle. Virtually every vehicle has two choices for passenger airflow: fresh air or recirculated air. 

climate controls in a car

Recirculated air means that the air already inside the cabin is utilized as the primary source of air for your HVAC system. This is especially useful for the “max” setting for air conditioning, because the air being used is already close to the desired temperature, making easier work for the AC system. 


If your system is set to fresh air, your vehicle’s climate control system pulls fresh air from a vent under the hood. Typically this is located near the engine bay, either up in the rain tray at the base of the windshield or otherwise separated by some sort of physical barrier from the dirt, oil, and heat of the engine. 


There are multiple benefits to this general location. One, the air intake vent is physically protected from the outside air and debris. This position near the base of the windshield is also an area of high air pressure, which means that there is always a plentiful supply of fresh air for the system to use.

climate control components in heaterbox diagram

Once the air has entered through the air inlet, it is directed to the heater box. The heater box contains more or less all the major components of the climate control system, such as the blower motor, cabin air filter, AC evaporator, heater core, flaps to select fresh or recycled air, as well as flaps to divert air to different vents in the car. 

how a cabin air filter works

The cabin air filter does its thing first, removing physical particles from the air such as dirt, mold spores, and other particulate matter. At the same time, the activated charcoal section, if it’s equipped with one, absorbs chemicals and other harmful or odor-causing contaminants. 

VW GTI blower motor diagram

From there, the air is sent through the blower motor, which dictates how much air is being pulled from the fresh air vent, as well as the force of the air coming out of the vents inside the car. Power going to the fan is controlled by the blower motor resistor, which is normally located close to the fan to take advantage of the airflow to keep the resistor from getting too hot. 

auto air conditioning diagram]

Air then passes over a heat exchanger, the AC evaporator. If you have the AC turned on or the system is set to defrost, a refrigerant, typically R134A, has been compressed and supercooled by the compressor and condenser before being sent to the evaporator. Incoming air is cooled as it is forced over the evaporator core and as it cools, moisture is removed from the air. 

AC evaporator diagram

This drier air is more effective at keep the passenger compartment cool, as well as dissipating moisture and condensation from the windshield and other interior surfaces. 

automotive defroster airflow diagram

If you’ve ever wondered why there’s often a puddle of water underneath your car in the summer, it’s because of the evaporator pulling moisture out of incoming air. All cars with an AC system have a drain that dumps the condensation from the air cooling process outside of the heater box, usually somewhere around the middle of the car. 

heater core diagram

If the climate control is set to the heat setting, the air is forced across a second heat exchanger called the heater core. This works the same way as an engine radiator, but with a different goal. With the engine radiator, ambient temperature air is used to cool the engine coolant being passed through the radiator in order to manage the heat generated by the engine. 

how a cooling system and heater core works diagram

The heater core uses the hot engine coolant to warm the air being sent through it to warm up the passenger compartment.

Automotive hvac airflow diagram

After leaving the heater core or evaporator circuits, the air is then directed by a final set of flaps to whatever interior vents are selected on the climate controls, such as the footwells, dash vents, and defrosters. 

A cabin air filter is an integral part of all modern vehicle climate control systems. They’re visually similar to engine air filters, and like engine air filters, cabin air filters are sized based on the volume of the air they are required to filter for efficient operation.


What does a cabin air filter do? 

micronAir cabin air filter by Corteco

A cabin air filter’s purpose is the same as any air filter in that it is designed to clean and filter air before some other component or element. The HVAC filter on a furnace or air conditioner at home is filtering air before it is taken into the system for either heating or cooling but is operating on a closed system, and not really open to the outside air and elements. 

How a cabin air filter works

A cabin air filter is working in the same capacity, filtering air before it is processed and introduced into the air ventilation system inside the car.  The difference is that the cabin air filter is dealing with potentially much more harmful contaminants in the air since your car is often in the direct presence of dangerous air pollution. 

15_cabin air filter 3

These dangerous particles can include chemicals such as sulfur dioxides, nitrogen dioxide, and unburned hydrocarbons, as well as particulate matter from the exhausts of other cars and trucks. Exhaust from manufacturing or industrial output, as well general environmental particles like dirt, dust, and pollen are other contaminants that a cabin air filter is tasked with removing. 


Who came up with cabin air filters? 

The basic concept of a cabin air filter first appeared back in the 1950s. The chauffeur for former Freudenberg CEO Hans Freudenberg had noticed that his clothes were becoming noticeably dirty from the air coming in from outside the vehicle. He had the original idea of placing a non-woven piece of fabric over the air vents to prevent the excessive dirt from making its way into the passenger compartment. This simple idea of placing a handkerchief over the top of an air vent was developed by Freudenberg into the modern cabin air filter that we know today. 


A cabin air filter is made out of pleated paper material, often with an activated carbon element. The pleated design increases the available filter surface area, and therefore air filtration capacity of the filter.  The activated carbon element absorbs contaminants and impurities via chemical absorption. This includes physically harmful elements, such as ozone, as well as the smell of smoke or other unpleasant odors. 

1989 Mercedes SL 500 side profile

It was not until 1989 that a vehicle manufacturer incorporated this concept into a production vehicle. The 1989 Mercedes-Benz SL roadster was the first vehicle to come standard with a cabin air filter from Freudenburg. Other higher-end German marques such as BMW, Audi, and VW followed suit with optional cabin air filters on various models through their line-ups. The first mass-market car to receive a cabin air filter as standard was the 1991 Opel Astra, a European market compact car, similar to the VW Golf or Jetta

1991 OPEL Astra hatchback

Throughout the 1990s, cabin air filters became more and more common across multiple makes and models, but it wasn’t until the early 2000s that nearly every car in the USA was delivered with one from the factory. Some cars during the period were even manufactured with a location for a cabin air filter but they weren’t installed from the factory. If you own an older car from the late 90s or early 2000s, you may be surprised if you check for a filter and find the spot empty. If so, you can always add one now and experience the health benefits of a cabin air filter. 

Volvo IAQS_Interior_Air_Quality_System

Volvo has taken this concept even further with its Interior Air Quality System. The IAQS uses sensors to actively monitor the quality of air coming into the vehicle, and will automatically close-off the outside air vent if it reads too high a level of carbon monoxide, nitrogen oxides, or hydrocarbons. 

Mercedes-Benz Fragrance Refills

On the opposite end of the spectrum, manufacturers such as Mercedes, and Audi, are developing in-cabin fragrance technology. These are literally systems that have replaceable scent cartridges that allow the driver to choose a fragrance to be added into the cabin air for a new individualized in-car air quality experience. 

evolution of micronAir cabin air filters

Freudenberg, as the developer and originator of the cabin air filter and supplier to many automotive OEMs, has continued to improve and innovate within the category. Through their micronAir brand, a division of Corteco, itself a division of Freudenburg, they introduced the first charcoal activated combifilter in 1995. Continual updates in both filtering and activation mediums have resulted in the newest cabin air filters being able to remove even the smallest physical particles and irritants, filtering out particulates down to ≥2 microns. 

A micron, also known as a micrometer is a unit of measurement equal to one-thousandth of a millimeter, or one thousand nanometers. Here are some examples of different things of varying micron measurement to give you an idea of just how small ≥2 microns really is. 

micrometer range and examples of what a cabin filter can catch

  • Skin cells 30μm
  • Red blood cells 8μm
  • Pollen 90μm
  • Flu .1μm (100 nanometers for reference)
  • Staph 1μm
  • E. Coli 2μm
  • Spider web silk 5μm
  • Human hair strand 20-40μm
  • Dust mites 5-20μm

micronAir Gas shield cabin air filter explained

In 2019, 30 years after the first cabin air filter was introduced, micronAir launched their Gas Shield activated carbon filters. These offer sophisticated activated carbon materials that are targeted and regionally customized for protection against extremely harmful air pollutants such as nitrogen, ammonia, ozone, aldehydes, and sulfur dioxides.  These latest Gas Shield cabin air filters provide the maximum in anti-allergen, anti-viral, and anti-microbial protection. 


What can you do to maintain your car’s air conditioning system?

There are several simple things you can do to ensure your car’s climate control system is performing at its best. First, you should inspect and replace your cabin air filter regularly. Secondly, if your car is more than a few years old, you may want to perform a deep clean of the evaporator and other system components. Lastly, you should make sure that your AC system’s refrigerant is up to spec and performing as it should. 


How often to change a cabin air filter?

You should inspect and replace your cabin air filter annually, or about every 12,000 miles. Where you live and the kind of environments in which you typically drive will change this significantly. You should change your filter more frequently if you live in an area with extremely high humidity or an excessive amount of dust particulate, like if you have a dirt driveway or spend a lot of time off-road. 


How do you know when to replace your cabin air filter?

dirty cabin air filter

It is a fairly easy and straightforward process to inspect and replace the cabin air filter in most vehicles. The cabin air filter is typically installed in the heater box which is under the dashboard typically near the firewall or the passenger side footwell area. Some larger vehicles such as Audi A8, Mercedes S Class, or BMW 7 series may have two filters. Gaining access to the filter will depend on your vehicle, but it is usually fairly easy and can be done with basic hand tools. We have plenty of DIYs available for those looking to tackle these kinds of jobs at home. 

Once you’ve removed the filter, give it a quick visual inspection for dirt and debris. If your filter shows any signs of mold, a significant amount of dirt or a has an obviously soiled filter element, a rat’s nest, or a mouse-house, it should be replaced immediately. If the filter looks more or less brand new, gently open the external air side of the pleats to inspect the filter material. If everything looks clean, you can reinstall the filter and check it again in a few months, or maybe at your next oil change.  Keep in mind that just because the filer looks clean that doesn’t always mean that it’s truly clean and free of microscopic contaminants.


Does replacing your cabin air filter matter?

Generally, the performance of your vehicle’s HVAC system decreases dramatically without regular maintenance. The increasing pressure drop of air moving through a clogged filter might reduce the fresh air supply to the vehicles’ passenger compartment, and cause the blower motor to work harder to overcome the restriction. 

In terms of filtering efficiency, once the filter is clogged with debris, its ability to capture smaller, and more likely more harmful to your health, particles are reduced. In the case of an activated carbon filter, the capacity of the layer responsible for capturing harmful gasses and unpleasant odors can be reached, disabling the additional functionality of the filter. Again just because it looks clean doesn’t mean that it’s still working exactly as it should. 

Ultimately, replacing your cabin air filter is something both cheap and easy to maintain on your car. It also gives you peace of mind that you and your passengers are receiving properly filtered air in a world constantly filled with additional harmful pollutants. 


How do you deep clean your AC system? 

Most professional automotive repair shops have specialized equipment for deep-cleans of a vehicle’s climate control system. These machines are typically large and expensive, but an at-home solution is available for DIYers. LIQUI MOLY has been at the forefront of developing these systems for 20 years, and offers an AC system cleaning kit that you can use to do it yourself at home. 

How the LIQUI MOLY air conditioner cleaner works

With the LIQUI MOLY kit, a special two-part cleaning solution is sprayed directly on the evaporator with a specialized spray gun, killing off mold and bacteria that may have accumulated over years of use. The best time to do this is when you’re doing your annual cabin air filter replacement, saving you some time and the effort that it takes to access this part of the system. 


The importance of keeping your AC refrigerant charged 

R134A is the life-blood of your car’s air conditioning system. It is what allows the system to perform at its peak on the hottest days of the year. Because the system operates on a specific pressure level, if your AC system is low on refrigerant, it may not operate efficiently or even at all. Your AC components also rely on a lubricity that’s added into the refrigerant charge to keep switches and components operating smoothly. 

R134A does have a bit of a shelf life, and so some refrigerant loss over time is not unusual. If your air conditioning isn’t blowing cold and you’ve done the maintenance we’ve mentioned above it may be time to seek professional help. Servicing the charge on an AC system requires special tools and a very specific process as far as evacuating and refilling the R134A level and pressure. This kind of refrigerant is nothing you want you, or the environment, to be unnecessarily exposed to, so it's best to leave it to professionals with the right tools and machines. 

Hopefully, with this article, we’ve provided some insight into one of the more commonly overlooked but important systems of your car. Performing just a few simple services such as regularly inspecting and replacing your cabin air filter can go a long way to ensuring maximum performance and the cleanest, most healthy air for you and your passengers when you drive your car. 

author image
Written by :
Nathan Brown

FCP Euro's Event Director by day, writer and contributor by night, and wanna-be race car driver on the weekends. Nathan has been working in the VW and Audi performance aftermarket for nearly two decades, and dabbled with Porsche and BMW. He also used to write under the pen-name of Alex Rogan for magazines like Eurotuner, Performance VW, Total 911, and European Car. He has a Cornflower Blue Rabbit Edition GTI daily driver which is surprisingly still mostly stock, and a Mk5 GTI track car which is mostly not. ••• Instagram: @njbrown55

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