By Joseph D. Younger

Changing your car’s cabin air filter can make a world of difference.

How clean is the air in your car?

The answer matters, especially if you count yourself among the 50 million Americans who suffer from allergies or the 20 million who have asthma. Respiratory problems notwithstanding, you’re probably already sick of diesel exhaust, soot, smoke, and who-knows-what other foul-smelling stuff you regularly suck into your lungs on Carolina roadways. 

And for a geographic area prone to pollen that can trigger allergies, a car with clean air can be a sanctuary.

According to a recent national survey by Dohring, a market research firm, 85 percent of the drivers expressed concern about air quality inside their vehicles.
The solution to your clean-air worries may actually be right in front of your face, without even knowing it. If you drive a vehicle made after the millennium, chances are it has cabin filtration as part of its heating, ventilation, and air conditioning (HVAC) system.

“About 30 million vehicles in North America currently have cabin air filters,” says Rich White, vice-president of the Automotive Industry Aftermarket Association. “However, most people have never heard of a cabin air filter and don’t know whether their vehicle is equipped with one.”  Knowing how the filter works and how to perform easy, do-it-yourself maintenance can help eliminate allergens and pollutants, shut out bad smells, and generally help you breathe better while driving.

Where to Find Your Filter
Between 40 and 45 percent of the new cars sold on the U.S. market come with cabin filtration systems. That compares to almost 90 percent in Europe and about 70 percent in Japan, so a newer import is more likely to have one than a domestic model. 

Check the owner’s manual. The term “air filter” in the index usually refers to the engine air filter. Look under “cabin air filter,” “cabin filtration,” or “pollen filter” instead.

Can’t find a reference in the owner’s manual?

Then ask your dealer’s service department whether your particular model has a cabin air filter, and, if so, ask where it is.

On most cars, you’ll find the filter’s access panel on the passenger’s side, either under the dash, beneath the glove box, or in the glove box itself. You might need to look under the hood—again, on the passenger’s side—just across the firewall from the glove box.

“Some cars actually have two filters, depending on the capacity of the HVAC system and the available space,” says Dave Lester, general manager of MicronAir cabin air filtration, which supplies original-equipment cabin filters for two out every three cars that have them. You’ll most likely find the second filter either right alongside the first or in a comparable location on the driver’s side.

Replacing and Upgrading
Carmakers recommend replacing the cabin air filter every year or every 15,000 miles. Unfortunately, many drivers never replace their filters because they don’t even know they have one. And some normal maintenance checklists don’t include cabin filter replacement.

A persistent bad odor might signal your filter needs replacing.

“One tell-tale sign is reduced airflow when you turn on the air conditioning or heat,” notes Lester. “That’s because the filter has trapped so much dust and other contaminants that it’s restricting airflow into the cabin.”

Some upscale cars include a warning light on the dash to remind you a change is needed.

Filters fall into two general categories.  Particle filters remove dust, pollen and other microscopic specks suspended in the air. Usually, particle filters have at least one layer to trap coarse particles and an electrostatically charged layer to catch really tiny stuff. So-called combination filters add a layer of activated charcoal to trap odors and noxious gases.

Combination Filters
A combination filter makes a noticeable difference, especially in big-city, bumper-to-bumper traffic.

“Studies show that pollutants can collect inside the vehicle at concentrations ten times higher than the air outside,” notes Lester.

With the windows up and the A/C on, your car’s HVAC system sucks in exhaust from vehicles directly ahead and concentrates them inside the cabin, as if you were driving through a tunnel.

A combination filter can trap most of those gases, including nitrogen oxides, sulfur dioxides and other smog-forming pollutants. But don’t think of it as a gas mask for your car. Even the best activated charcoal won’t filter out carbon monoxide or carbon dioxide, two common auto emissions. 

If your current system uses only a particle filter, you can probably find an aftermarket combination filter to fit.

“Because combination filters have an extra layer with charcoal and the filtration media are somewhat thicker, you might experience a slight reduction in airflow,” explains Lester. “But if you can live with that then, yes, you can definitely upgrade.”

Comparison Shopping
When it comes to choosing a replacement filter, the automotive industry doesn’t make comparison-shopping easy. Unlike home furnace filters, cabin air filters don’t have MERV ratings (Minimum Efficiency Report Value, a standard measuring the filter’s ability to trap particles as small as 3 microns) or 3M MPR numbers (Micro Particle Performance Ratings, which indicate the efficiency of trapping particles under 1 micron). 

However, Freudenberg Nonwovens, maker of MicronAir filters, intends to include MERV ratings on its packaging, perhaps leading other suppliers to follow suit.
In the absence of standards for comparison, you have to judge a filter by its material. Avoid inexpensive cellulose (paper) filters.

“The performance of cellulose degrades with water,” notes Lester. “Depending on the location of the vehicle’s air inlet, water can be blown into the system and wet the filter.” Besides, cellulose serves as a good medium for mold and bacterial growth—the last thing that an allergy or asthma sufferer wants.

Like engine air filters, cabin air filters come in all shapes and sizes, so you have to find the precise one to fit your car. Once you do, however, the rest is pretty easy—certainly easier than changing the accordion-type filters on some new home furnaces.  You’ll find instructions for your make and model in the owner’s manual, on the box, or on the Web (try www.micronair.com and click on the “Product” button).

You’ll feel—and smell—the difference a new filter makes.