Duct Cleaning: Sucking More Money Than Dust

Duct cleaning has taken off like wildfire over the past twenty years. The advertising for it is near ubiquitous. Customers far and wide are singing its praises. And duct cleaners are making a tidy profit. So everyone is happy, right? Perhaps not. Skeptics believe the multi-billion dollar duct cleaning industry to be a multi-billion dollar scam.

Come with me and I’ll show you why you might want to be a skeptic too. I’ve chosen Chicken Little as our travel companion because his story dovetails nicely with the story of duct cleaning. If you have time for the complete story, read on. If you want to skip the fiction and get straight to the facts, scroll down to Einstein.


“The sky is falling! The sky is falling! I must go and tell the king!”

Chicken Little’s bump on the noggin is nothing compared to what you have to worry about. Advertisements for duct cleaning read like this:

“We spend as much as ninety percent of our day indoors.”
“Indoor air can be up to ten times more polluted than outside air.”
“Health organizations, scientists and medical professionals tell us that
airborne dust may be one of the most common causes of health problems.”
“Billions of dollars are spent annually to treat the symptoms of contaminated air.”

“I saw it with my own eyes, and heard it with my own ears, and part of it fell on my head!”

Then they pull out the big guns. Chicken Little had an acorn to deal with. You have much worse. Click on the doors to reveal the horror…


Not everyone appreciates my sarcasm. Nevertheless, peruse your junk mail and you’ll see pictures like those. The visual impact combined with seemingly factual quotes make for a persuasive argument. If that’s not enough, just look up at your ceiling vents. Filthy, right? Surely your central heating and air conditioning system is spewing out vile, dust laden air.

“Go home in peace and do not fear because the sky cannot fall.”

Duct cleaners aren’t frightened little chickens, but the comparison is a good one. Duct cleaners take seemingly believable “acorns of data” and try to convince you that they mean something they don’t. Let’s crack those nuts one by one.



Those "Factual" Quotes

Most ads I’ve seen don’t actually state that your air will be cleaner or your health improved by way of duct cleaning. Instead, they tell you all sorts of so-called facts about dirty air and illness. Then they tell you about duct cleaning. Then they let you make the connection between your supposedly dirty air and your ducts. What those ads rely upon is the joint effect fallacy. That’s when A is said to be caused by B, when in fact both A and B are caused by C.

Without actually saying so, duct cleaners want you to believe that dirty ducts cause dirty air. As you’ll see below, that’s almost never true. Most ducts are made of plastic and metal. Plastic and metal ducts have no inherent ability to contaminate the air. If your air is dirty, and/or if your ducts are dirty, they are almost certainly dirty for the same reason. Particulate from the outdoors, carpets, clothing, pets, etc. are the usual sources of airborne dust.

Those Hideous Pictures

The duct behind the first door above is indeed very dirty. But let me put that duct in a different light. In my twenty plus years of experience, I’ve found that the overwhelming majority of ducts that look like that are in commercial systems that get constant use. Few residential duct systems look like that. The few that do are almost always return ducts, which is the only duct in your system that might have unfiltered air flowing through it. Your supply ducts are downstream of the filter and rarely accumulate more than a thin film of dirt. This is an example of what I’m talking about. That thin film of dirt might look bad, but it’s functionally meaningless.

Add to that this bit of (what will seem to be at first, but isn’t) warped logic. We can all agree that ducts are incapable of producing dust. Therefore ducts can only collect dust. It’s tempting to think that the collected dust gets released, but then it would never collect to begin with. Ducts can only collect dust and, except for the occasional return duct as mentioned above, most ducts collect very little. To whatever extent ducts do collect dust, this much is true: The more dust your ducts collect from the air, the less dust that’s in the air. So when you see a dirty duct, take heart. Your ducts are cleaning the air for you!

The dust mite behind the second door above takes the cake. With little monsters like that running around your ducts, who can blame you for cleaning them? Fortunately it’s highly unlikely those monsters are running around your ducts. To suggest otherwise is to ignore a basic fact of biology: All living creatures require water to live. Entomologists say that dust mites need at least 70% relative humidity to survive. Turn your furnace on for a few minutes and those ducts become hotter and drier than the desert. Even running the air conditioner dries out your ducts. A significant percent of your air conditioner’s capacity is spent dehumidifying the air.

The Dirt on the Ceiling

Fastidious homeowners don’t need quotes or pictures to be sold on duct cleaning. All they have to do is look up. Dirty ceiling vents are all the proof they need that their ducts need to be cleaned. However, as you have read, not everything in the world of indoor air quality is as it appears to be. The source of the dirt on the vents isn’t always what it appears to be either. The easiest way to understand where that dirt may come from is to look at this picture. Circular currents cause deposits to form on and near the vents. In other words, the dirt on the vent may not have come from the vent. Dirt streaking and ghosting are well researched phenomena. You can read more about them here, here and here or do a search.

The Dirt In Their Vacuum

A minority of duct cleaners have courage enough to show homeowners what was removed from their ducts. I witnessed this years ago while working as an HVAC repairman for a large HVAC contractor. Said contractor bundled so-called tune-ups with their duct cleanings. In some cases I’d arrive to perform the tune-up just as the duct cleaners were showing the homeowners what was removed from their ducts. The homeowners’ response was telling.

More often than not the homeowners responded with disappointed silence as they stared at the paltry amount of dirt sitting in the big vacuum before them. Unphased, the duct cleaners dispensed well-rehearsed mumbo-jumbo to assure the homeowners of the significance of that cup of dirt. But on other occasions they saw a truly impressive amount of dirt. That made the homeowners very happy indeed. Most of those happy homeowners had floor vents.

Most homes with floor vents have debris sitting directly under the vents. It doesn’t come from the HVAC system. It falls in from the floor above. Most of it is too heavy for the big vacuum to suck up from the other side of the duct system. So the duct cleaners vacuum it out with a small shop vac – something homeowners can do for themselves – and then dump it into the big vacuum. In most cases, the vast majority of the big vacuum’s “catch” comes from the little vacuum.

As an aside, I eventually talked with all eight duct cleaners that worked for that contractor at that time. To a man they told me that they thought duct cleaning was a waste of time. Their employment as duct cleaners was simply a way of paying the bills until a job in the installation department (of the heating and air conditioning side of the company) opened up.

What about mold?

The mold in your salad may be blue, but for trial lawyers mold is gold. Duct cleaners are all too happy to sit at the foot of the table judge’s bench and wait for the crumbles to fall. Despite the payouts, and despite many searches over the course of many years, I have yet to find any proof of a connection between mold and widespread illness. Even so, it’s reasonable for those sensitive to mold to wonder if it’s growing in their ducts.

It’s just not that common. Most molds need sustained water activity that’s equivalent 70% RH to grow. The cooling coil is the only part of a properly functioning air conditioning system that may see that much sustained moisture. If mold is actually growing in there then you’re better off calling an HVAC tradesman than you are a duct cleaner. Under normal circumstances the environment inside a properly operating duct system is just too dry to sustain mold growth.

Plumbing leaks, poor ground drainage and the like are the usual causes of mold growth. Get rid of excess moisture and mold growth will almost certainly follow. Then, if you actually had mold growth in your ducts that you want completely removed, you may need to replace the affected ducts. As you’ll learn below, most duct cleaners don’t actually clean ducts very well at all. It takes a lot more than the swipe of a brush to remove those embedded buggers.

It’s ironic that the overreaction to airborne particulate may be causing the very problems people are trying to avoid. Studies both old and new show that exposure to a variety of germs at a young age strengthens the immune system. In fact, some of the lowest asthma rates are amongst farm kids who breathe enough junk to give a hypochondriac nightmares. Conversely, a lack of exposure may increase the chances of asthma and allergy problems as an adult.

What about system performance?

Years ago I was in the midst of an ethical conundrum. My employer had just started selling duct cleaning. I didn’t have to perform the service, but the boss asked me to sell it to homeowners whose equipment I repaired. He asked again when I was up for a raise. Get the picture? Not only was my raise on the line, my career was too. The largest contractors have the best career paths. Virtually all of them sell duct cleaning. For me to reject duct cleaning was to reject my then current raise and my then future job prospects. As such, I had a huge financial incentive to sell the service just as evidence was mounting against it.

It was around that time that I attended a trade show in San Francisco. The National Air Duct Cleaners Association had a booth. I asked the gentleman manning the booth if there was any evidence that cleaning ducts did anyone any good. His answer went something like this: “Oh… We don’t just focus on ducts. We also recommend coil and blower cleaning.” Much to my dismay, the NADCA rep couldn’t come up with any evidence that cleaning ducts would produce any gains in air quality, system performance, etc.

The booth boy’s answer was a clever dodge. Cleaning dirty coils and blowers rarely improves air quality, but it can improve system performance. Repairing and sealing leaky ducts also has real benefits. In fact, duct sealing is one of the few things some duct cleaners do that actually has the potential of improving air quality by reducing pressurization issues. However, those real improvements aren’t the focus of what duct cleaners do. Nor did the duct cleaning industry need to be contrived to address those real issues. Real HVAC technicians were cleaning coils and fixing ducts long before duct cleaners came along.


 The Real Science

We’ve busted the myths. Now let’s take a look at the facts as borne out by studies. We’ll follow that with some comments from respected medical professionals.

From a study sponsored by the EPA:
The results of the particle mass measurements suggest that… the airborne concentrations before and after (duct) cleaning were not substantially different…SOURCE

From a study sponsored by the Canada Mortgage and Housing Corporation:
Measurements of the dust in house air showed that there was no significant reduction after duct cleaning. Some houses showed a temporary increase in dust levels for several hours after the duct cleaning.” SOURCE

From a study of fifteen office buildings:
Duct cleaning had no measurable effect on supply air quality.” SOURCE

From a study performed by Indoor Biotechnologies:
…when the air ducts of these homes were tested, only 2 of the 27 samples showed dust mite allergen at the 0.5 microgram per gram of dust threshold of detection. An allergen level below 1 microgram per gram of dust is considered too low to cause allergic reactions. SOURCE

From the same Indoor Biotechnologies study:
This suggests that there were no mite allergens going in because the air ducts are not a good environment for mite growth… there are no mites in there.SOURCE

From the American Academy of Allergy, Asthma and Immunology (AAAAI):
There are no data to support the practice of duct cleaning as far as allergen environmental control is concerned, and an ultraviolet filter would not be helpful for aeroallergen filtration.SOURCE

From the AAAI’s 2012 practice parameter:
Duct cleaning has not been proven to reduce exposure to furry animal allergens.SOURCE

From Dr. Richard Lockey, past president of the AAAAI:
I would not recommend that someone get their air ducts cleaned.” SOURCE

What makes the EPA study especially interesting is that it was cosponsored by the National Air Duct Cleaners Association. NADCA saw to it that the very best practices were used. Nevertheless, their best wasn’t good enough. When you factor out the effect of background particulate, the air in those homes was nine percent dirtier after duct cleaning. This Washington Post story talks about that same study and more.

The Fake Science

To be fair, the study I’ve heard duct cleaners talk about the most is not fake at all. You can get a copy of it here. It appears to have been conducted by real scientists and published in the Annals of Allergy over twenty years ago. Many a duct cleaner has cited it as proof that their service is worthwhile. Ostensibly reputable trade magazines have done the same. So what’s the problem? This easily missed sentence from the study report is the problem:

“A permanent, washable electrostatic air filter was installed in each system.”

While the study may indeed be real, how it’s used to prove duct cleaning’s efficacy is not. The researchers were NOT studying the effect of duct cleaning alone, but rather the effect of duct cleaning combined with coil cleaning and improved filtration. Such a combination of measures proves nothing about any one measure alone. If any one measure was responsible for the improved air quality, common sense suggests it was the upgraded air filter.

Measuring airborne contaminants isn’t hard. If duct cleaning actually improves air quality then we’d be hip deep in studies that prove it. Instead all we have is half-baked propaganda from the nineties. NADCA, some contractors, some so-called IAQ specialists and the manufacturers of duct cleaning equipment all have a huge vested interest in proving the validity of their service. Yet in over two decades they’ve failed to do so.

What's Really Going On

The Motivation

Many HVAC contractors are barely making it. It’s understandable that they should want to offer a profitable service to their customers. In some cases a duct cleaner can generate the same revenue as an HVAC repairman while costing half as much to employ. Given that profitability, it becomes easy for bosses to ignore the facts and embrace the fiction.

The Work

Duct cleaning methods vary wildly. Companies that just hook up a truck mounted vacuum to your system are accomplishing little. Your car travels down the road at over sixty miles per hour, yet the dust clings stubbornly. So does the dust in the ducts, what little there is, unless they agitate it. They have to knock the dust loose or there won’t be much to vacuum.

That’s where the big roto-rooter looking brush comes in. Companies that use one are at least removing some of the dirt. But they have a problem: The brush is of a fixed size. It has to fit comfortably down the smallest duct it passes through. As smaller branch ducts progress into larger trunk ducts, the brush scrubs less and less of the duct walls.

Then they have another problem: Ducts often make sharp turns. As they feed the rotating brush down the duct it eventually comes to a point where the brush will no longer go in. At that point they’re done, and it’s usually well shy of the end of the duct run. Most of the time it’s just not possible to clean all of the ducts. Sometimes they don’t even clean half of them.

Then comes the worst of all potential problems: Many duct systems are not put together very well. It’s not that hard to knock a duct loose. If it’s a wire flex duct, they can also damage or even collapse the inner liner. And it’s not like they carefully inspect the interior of the ducts when they’re finished. They’re usually unaware of the damage, as are you at first.

The Result

What you could end up with is lost time out of your day, hundreds of dollars out of your pocket, possibly damaged ducts (damage that you may not become aware of for months), a partially cleaned duct system, temporarily dirtier air, unchanged or worse system performance, and a lot of smoke and mirror assurances.




Science and plain old common sense just don’t back the claims made by duct cleaners. Real science, in fact, refutes them. The odds of that ever changing are about as good as that of the sky falling. In those rare circumstances where a home’s ducts are indeed contaminated with something awful, it’s probably best to replace them entirely.

If you just like the idea of duct cleaning and are willing to risk the damage, fine. Just don’t do it believing your system will run better or that your air will be cleaner. The odds are not only against that, but they’re in favor of exactly the opposite.

Save your money. Go out to dinner with family and friends. Stop worrying about what’s going to kill you. You’re not going to get out of this life alive anyway!


Absolutely nothing in this essay should be interpreted or taken as medical advice. If you believe poor air quality is affecting your health then you should seek the advice of a real medical professional and/or air quality expert immediately.

This essay is strictly the opinion of the author. It’s intent is to focus on residential duct cleaning as it is typically advertised and implemented locally. It does not address commercial applications, specialized situations or other unusual circumstances.