Re: Sharp Air Purifier Washable Carbon Filter Sweet Odor

A Reader writes:

Hi Ed,

I have been struggling in vain to eliminate persistent odors emitted by my new (Spring, 2008) Sharp FPP-60CX air purifier.

It was not exposed to extreme chemical loads (used in a bedroom, no paint fumes, etc).

I have been unable to wash the carbon filter such that the odors are eliminated.

I purchased 2 sets of carbon/HEPA filters, since the carbon filter was not available separately shortly after I purchased the machine.

Each brand new carbon filter began emitting odors within about 3 months after installation (I followed manufacturer instructions carefully, changing as indicated or before, washing-- even overnight soaking--in different detergents, etc., thinking I was smelling the offgassing of perfumes).

I have attempted to eliminate the sweet odor for over a year, with no success, and am concerned that I am re-contaminating or adding contamination to the air.

The odor has persisted even in different settings and even after letting in run in "fresh" air outdoors.

The only thing that temporarily helps is my Biozone air purifier, which I run only in transient contaminant situations due to ozone output.

After several hours of ozone, the Sharp smells better for a few hours..

I phoned Sharp and got all the expected unhelpful responses (by the time I realized that the problem was not something I had done, the original warranty had expired, so "repair" would cost me more).

The carbon filter is not available for separate purchase, so that means $80 every 3 months to make the machine usable.

Sooo... I am writing to let you know, since my purchase was made based on your review.

I would certainly not recommend this machine to anyone with a budget.

It has so many great features, but I bought it to get rid of particulates and gases in my environment, not add them.

One of the major selling points was the washable carbon filter.

For my purposes, unless I can solve the odor (off-gassing chemicals) problem, I have spent nearly $400 for nothing.

Please don't downplay this issue in your review (yes, I did read your response to the "sweet odor" email).

I have had other air cleaners with carbon filters stop trapping odor particles and smell funky, but (in my circumstances) not create apparently new contaminants.

I suspect the difference is in the technology.

If molecules are altered in the "plasma" process and new substances created by recombining, then that could perhaps explain why the odor emitted is unlike any odor that entered the filter.

Why it will not wash out of a "washable" filter is a mystery (is there some special solution necessary to clean it?).

I am also writing to ask if you have attempted to make or substitute a cheaper carbon filter for the Sharp (I read your section on pre-filters).

Any suggestions for salvaging this sizable investment would be most welcome.

Meanwhile, I must assign a minus rating to this promising machine.

Please let others know about this before they make the same mistake.

Thanks for listening.

A Reader

Ed's Reply

Hey Reader;

You have encountered a common problem among air purifier users: chemical desorption (off-gassing) from activated carbon filters.

What I propose is an ongoing experiment.

I don't expect an immediate and complete solution to an industry-wide cost issue, but we can work on this problem together.

Carbon filters do not trap gas phase pollutants in the manner HEPA restrains particles.

Gas phase pollutants - molecular in size, invisible next to the smallest submicron particle - are held by ionic attraction.

Activated carbon has very tiny pores where single layers of molecules hang like bats in a cave. Better quality carbon has more pores.

Not all pollutant molecules are held tightly, those with odd polarity (positive and negative electrical charge distribution) escape more readily than non-polar competitors.

There is no mechanism for determining when a carbon filter is saturated, - and therefore becoming less efficient, even outgassing - other than the user's sense of smell.

A used carbon filter, depending on where it has been run, can be odorless or smell like a landfill on fire. It can contain a toxin cocktail or be relatively innocuous.

The Sharp Plasmacluster FPP60CX is now long discontinued, filters are disappearing, and price gouging on the remaining replacements has begun.

Sharp Plasmacluster models FPN40CX, FPN60CX, FPR45CX, FPR65CX, FPP40CX, FPP60CX, KCC150U, KCC100U, and KCC70U, were sold with washable carbon filters.

Other washable carbon filter users report the sweet smell after washing, notably in the bestselling RabbitAir BioGS 1.0.

Winix Plasmawave also has some washable carbon odor filters.

There are other purifiers, notably the E. L. Fousts, which allow users to refill their canisters with bulk activated carbon, greatly lowering costs.

So a wide range of users should be interested in this subject - anyone with experience using these washables is invited to contribute.

I think it is premature to blame just the Sharp, and though the Plasmacluster technology has safety and application issues, I don't think it is to blame for the sweet odor.

My Sharp Plasmacluster washable carbon bag is now over 4 years old, it does have a mild unpleasant odor, but it smells more like diesel exhaust and petrochemical refinery emissions than over-ripe fruit.

This, plus your experience with the ozone temporarily reducing the smell, suggests that some environmental factor is to blame, rather than the filter itself.

I think the new contaminants are being produced by the ozone machine, and accumulating in the Sharp's carbon filter.

Most Likely Cause: Acetaldehyde

I consider Acetaldehyde (AH) a very important topic for those with serious allergies, and imperative for those with chronic fatigue/MCS.

Acetaldehyde ("ass-sit-ul-al-de-hide"), in dilute concentrations, has a sweet, almost syrupy, odor. AH is present in ripe fruits, especially apples.

AH is the first metabolite of alcohol, and a principal mediator, not just of alcoholism, but of multiple addictions.

Formaldehyde, so common in our air and homes, readily oxidizes to acetaldehyde. This is especially the case where saturation ozone has been used.

Aldehydes in general are penetratingly aromatic, and frequently used in perfumes.

The chemical sensitive (MCS) individual often finds aldehyde containing scented products intolerable, with perfumes perhaps the worst offender.

Acetaldehyde is everywhere.

AH is the product of incomplete combustion.

Woodstove smoke, natural gas appliances, furnaces, and water heaters, car exhaust, and tobacco smoke are the most common sources.

People who spend time commuting in dense traffic or live near major traffic arteries, as well as anyone who has ever been any kind of drinker, are seriously at risk for acetaldehyde accumulation.

The switch to alcohol-based fuels ("ethanol") has increased the acetaldehyde concentration in urban air.

In Brazil. where the E85 alt-fuel push is almost a decade ahead, AH concentrations reach 45 parts per million on major streets.

Although ethanol-burning cars produce less of some wastes, they emit 20 times the acetaldehyde.

Oxidizing air cleaners, especially those producing sustained ozone concentrations, are believed to increase indoor acetaldehyde concentrations above levels found outside.

Partial oxidation of higher molecular weight volatile organic compounds (VOCs) to simpler aldehydes can indeed happen with Plasmacluster oxidation.

This would be especially true where plug in or sprayed "air fresheners" have been repeatedly deployed with a plasma-ion air purifier running.

And/or with natural gas appliances running, which I do NOT recommend.

The Plasmaclusters would encounter, and partially oxidize, these intrusive airborne chemicals.

Aldehyde based fragrances also permeate drywall, drapes, HVAC ducts, and furniture, where ozone would seep.

But my belief in the relative safety of Sharp's engineering was based on the fact that plasma ions exist too briefly to penetrate, for instance, leather furniture, fiberglass insulation, foam cushions, mattresses/pillows, terpenes in wood flooring and furniture, common plastics, or dry cleaned clothing hanging in closets.

In my Houston apartment bedroom, there was a walk-in closet.

It contained my wife's wardrobe, much of which is over 40 years old.

The stench of detergents and cleaning chemicals, some obviously of near ancient vintage, is overwhelming in there.

In that same bedroom, my Sharp FPN-60cx ran 24/7 for over 6 years.

Its oxidizers have not reached the closet, only 13 feet away.

You'd have to put the Sharp in the closet, which I'd like to do but fear it might fade colors on her favorites. (I spend the majority of my time at my rural cabin.)

Ozone, even from an Oreck or Breeze, not to mention a Biozone, would have permeated that closet and converted many of those chemicals to aldehydes.

One reason for my initial enthusiasm for the Sharp Plasmaclusters was their relative safety over ozone.

Recall that in 2005, when the original FPN-60cx review was written, ozone spewing Ionic Breezes and Orecks were best sellers, the two sharing around 70% of air purifier sales.

Consumer Reports ranked the ozone-prone electrostatic Friedrich C90A number one in '05.

While activated carbon adsorption is effective against many VOCs, small highly polar molecules like Acetaldehyde are weakly held, and will outgass.

This is one reason why the sweet smell goes away with time.

Although many air purifier vendors include acetaldehyde on their lists of chemicals adsorbable by activated carbon filters, independent lists generally indicate only fair to poor retention.

One of my IQAir purifiers, after running 3 years near an open window in uptown Houston, developed the sweet odor.

I removed the carbon filter (V5 cell) and set up that IQ in my rural loft cabin.

I placed the used IQAir V5 cell in an upstairs room where it routinely gets very hot in the Texas summer.

Your e-mail prompted me to get it down. To my surprise, the sweet odor is gone!

Not all VOC filters are created equal.

Many premium brands (Sharps, Rabbits, and Winix are mid-priced Asian value leaders, not premiums), such as IQAir, Austin Air, AllerAir, Aireox, E. L. Foust, and others, have reinforced their carbon with activated alumina, potassium permanganate, and/or potassium iodide.

They add these oxidizers specifically for improved aldehyde removal.

Acetaldehyde is also generated within the human body.

It is the cause of the morning-after hangover, and the main waste product when Candida yeast die-off following a carbohydrate-heavy meal.

Ok, team, let's tackle this washable carbon filter odor problem!

Possible Remedies for Washable Carbon Filter Odors

So what I propose is to try oxidizing deodorizing remedies on my high-mileage Sharp carbon bag.

The bag and its contents will be subjected to attempts at recharge/recycling, and reused if possible.

After study, folk remedies, including candidate agents vinegar, baking soda, zeolite, and 24 hour salt bath, are excluded.

Industrial options are impractical and unsafe - do NOT try acid bath, alcohols, anhydrous ammonia, amines, phenols, ketones, acid anhydrides, hydrogen cyanide, hydrogen sulfide, and sodium hydroxide!

Activated carbon is created from charcoal by exposure to oxygen (oxidation).

So why not TRY to rejuvenate the 4-year old washable filter and just keep using it indefinitely?

Saturated carbon is regenerated on an industrial scale using heat - 700 to 800 degrees Celsius.

Oxidizers, solar ultraviolet energy, and heat, which speeds up oxidation and volatilizes chemicals, are my chosen tools.

As you found out, regenerating (oxidizing) the carbon with ozone from the Biozone, oxidation of carbon surfaces decreases the acetaldehyde odor temporarily.

Acetaldehyde is oxidized to acetic acid, and many more VOCs from that modern Houston air are present, so we need strong oxidizers.

Let's start with some readily available oxygen releasers, hydrogen peroxide (H2O2) is the most likely candidate.

So I start with a filter bag wash in Oxiclean (without the blue fragrance crystals), which releases hydrogen peroxide, a powerful oxidant.

It foams and fizzes violently, releasing lots of gunk and a nasty stink.


After drying, the filter smells WORSE than before washing.

When the bag dried, it had a "burnt" odor, which filled the room when run in the purifier.

Rethinking the experiment, I decide to cut open the polyester/rayon bag to eliminate it as an odor source.

I use scissors to cut off 3/8 inch, just the stitching along one edge.

I dump the carbon pellets in a Pyrex bowl.  washable carbon bag and granules

I want to find out if the Polypropylene-Polyester bag out-gasses.

I wash the bag, now empty, in a mixture of baking soda, hydrogen peroxide, and Seventh Generation free and clear detergent.

It rinses up odor-free and goes out in the garden to dry in the sun.

So the bag is innocent.

I read that acetaldehyde vaporizes above its boiling point of about 70 degrees (Fahrenheit), so I decide to try the "bake-off" method with batch one of the carbon.

I warm my kitchen oven to 125 and put a bakelite bowl with about 5 ounces of carbon pellets in.

Ten minutes later I open the oven door.

A blast of hot air hits my face, as nasty as fresh car exhaust.

The horrible smell fills the kitchen as I rush the batch outside.

Forget doing this project indoors.

Who knew just how much nasty toxic stuff could reside inside those little black carbon thingys.

So now I go to a backlot shed and retrieve a microwave I don't use (I do not microwave food), and set it on my outdoor work bench.

I put the bowl in for 2 minutes.

It comes out scorched, still smelling but not as bad.

Now the microwave stinks of the awful odor.

Sure glad we went outside!

By this time I am starting to feel the effects of breathing the results of our experiment.

My nose is running and throat is sore.

Now I repeatedly wash all the carbon in ordinary hydrogen peroxide, it lets off a fizz, but less violent with each rinse.

I let it stew for about 20 minutes each time, until no more tiny bubbles are surfacing.

After 24 hours, added oxidant still causes bubbling, but the peroxide is definitely purging the carbon pellets.

Pretty obviously, "mild detergent" wasn't going to get this job done.

The bowl of washable carbon, along with the now exonerated polyester Plasmacluster bag, are set to sun on my garden boardwalk.

It is necessary to add peroxide repeatedly - if there are tiny bubbles and white vapor rising from the surface, keep adding.

Every few hours, I drain the container.

Then I microwave the pellets again, and start over.

After 3 days and numerous cycles of H2O2, there is still a slight odor, but the smell is dramatically reduced.

The recycling process described above is cheap - a little Oxiclean, free and clear detergent, baking soda, and about $5 worth of peroxide.

This experiment ended, after numerous washings and hours in the microwave, with the Sharp Plasmacluster recycling program carbon pellets still exuding the nasty odor when heated, although at a much lower level.

I used maybe 10 bucks worth of peroxide, and breathed half of Houston's annual air pollution, before giving up.

Refilling the washed bag just hasta be easier!

Refill the Sharp Carbon Bag

The simplest user-controlled solution to the recurring Plasmacluster filter set expense or washout labor is to refill the bag with fresh adsorbent.

Refillable carbon filters are common in industrial applications

Just cut off the seam as shown above, deposit old granules in dumpster, wash and dry empty bag, use funnel to refill, clip with clothespins to keep closed, sew up.

Once the first cut is made, subsequent bag refills need only cut the user-applied stitch, so the bag only shrinks by the initial 3/8 inch.

Granules sorted through a grate are better than random sizes, which, like the aquarium filter carbon mentioned below, will contain powder.

With the bulk granules, one can restore the Sharp bag for about $10 - 15 in half an hour.

I ordered bulk activated carbon from E.L.Foust.

Foust sent 3 pounds, plus small samples of their other refill products, with free shipping (on orders of 3 lbs or more), at a total price 0f $19.32US delivered.

Foust's price worked out to a mere $6.44 per pound.

Joe Muchow at E.L. Foust is a good communicator, eager to help.

Foust specializes in refillable air purifiers designed for Multiple Chemical Sensitivity (MCS).

I used the Foust-supplied carbon to refill the Plasmacluster bag, one chamber at a time, clipping filled pockets closed with clothespins.

The photo below shows 5 of the 7 chambers filled. plasmacluster bag refill

I used about 2 pounds of carbon to refill the bag, an economical $12.88 worth.

The whole process, including hand sewing the bag closed, would take about 45 minutes, maybe half an hour after learning it once.

The refilled bag is now installed and running in my Sharp. There is no odor.

NOTE: 08/28/11

An astute user supplied the following info;

"I once bought a carbon/potassium permanganate mix from them (Foust) to fill in my Sharp's carbon bag.

Being used to washing Sharp's carbon pad, I washed the mix, but it kept giving off a purple color after lots off cycles, at the end of which, the permanganate pellets turned white.

Turns out you're not supposed to wash the permanganate!

I actually burned my finger badly when I held a couple of wet pellets while washing. I ended up ruining the whole mix. I guess DIY always have these risks."

Do It Yourself Carbon Filter

I have often contemplated the homemade carbon replacement filter project, so your post was the encouragement I needed to try.

There are reasons besides just cost to think of building your own washable carbon replacement filter.

Several RabbitAir users have reported early model carbon honeycombs damaged in shipment from third party vendors.

RabbitAir has resolved the issue with aluminum reinforced filter frames, but the incident illustrates a need.

Many purifier owners are left holding the bag as models are discontinued and vendors drop replacement media.

It is easy to build a carbon bag like the Sharps'.

Rectangular honeycomb type washable charcoal filters, as found in Mitsubishi-based RabbitAirs, will be more difficult.

Earlier Sharp 60 series models had a plastic support grid for the carbon bag, later products used straps to hold it up.

My bag will be filled full enough to fit tightly behind the snap-in prefilter without either support.

So I retrieve a leftover piece of readily-available-at-WalMart fiberglass no-see-um size window screen, and lay it on the Plasmacluster to cut to size.

When filled about 60%, it fits well in the purifier.

There are grades of activated carbon. The more expensive will have greater surface area, thus longer life expectancy.

Pet supply stores sell aquarium pharmaceutical grade activated carbon.

So it's off to WalMart where the fish dept sells carbon for fish tanks.

Sharp's washable carbon is pelleted, the aquarium carbon is not, so there will be lots of powder to wash out.

But the total bill is $18.

I sniff the fresh fish-tank carbon - absolutely odorless.

But it is a mixture of chunks of varying size, including some powder.

Note this experiment is entirely preliminary, users try this with powdery carbon at their own risk.

HEPA filters could have their lives shortened by carbon powder infiltration.

Now I hand sew the rectangle of odorless fiberglass into a bag with 4 pockets.

This takes less than 1 hour - a sewing machine could do it in 2 minutes.

Using a funnel, I fill the pockets with aquarium carbon, then sew the top edge shut.

Proceeding to the garden, I wash the new filter with a hose and bucket repeatedly to remove fine black powder.

After numerous rinsing, the water is still getting black, just not as fast.

Pelleted carbon would clearly be better for this project.

By this time I am hearing Johnny Cash singing the old Merle Travis tune Dark as the Dungeon;

Come and listen you fellers, so young and so fine,
seek not your fortune way down in the mines.
It will form as a habit and seep in your soul,
'Till the stream of your blood runs as black as the coal.

I don't use the Sharp plastic filter separator, as the new bag barely fits, held by the Sharp snap-in prefilter screen.

The bag must be filled just full enough to barely fit.

A homemade filter could also be partially filled with Zeolite Granules, mixed with the carbon, for even lower costs.

I put a layer of silk cloth between the new washable carbon bag and the HEPA to catch the inevitable carbon particle leakage.

Here is the finished product, a four chamber homemade washable activated carbon filter.

You can see the white silk cloth around the edges of the new bag. homemade washable carbon filter

The DIY project bag ran 10 days in my FPN60cx, there was zero odor. The silk barrier did reduce airflow some, but caught the particles that leaked from the bag successfully.

If starting over, I would fill this bag with air-purifier supplier (E.L. Foust) carbon, which is larger and less dusty.

The carbon eventually purchased from online suppliers linked above was better and cheaper than the pet department bought aquarium type.

Again, while I consider the DIY bag a success, I'd rather just use the old Sharp bag and refill it as described above.

What this could be is a model for doityerselfers with other models without carbon filters.

Any rectangular air purifier with space in front of the HEPA filter might work....try this preliminary example entirely at your own risk...


First, carbon filter regeneration is not for the chemically sensitive, and should be done outdoors.

My (expendable) microwave stinks inside, good thing I'd never put food in there.

But the amazing amount of noxious outgassing that the Plasmacluster washable carbon filter bag produced over a 10 day period tells me that these need to be cleaned, more effectively than just washing with mild detergent, on a regular basis.

"Carbon granules have an enormous internal surface area" has taken on new meaning.

Replicating Sharp's bag was really easy, and could be done for many other air cleaners with enough space in front of a rectangular HEPA.

But just refilling the bag was almost as easy as washing it!

Also, carbon and zeolite can be set out in bags, placed in HVAC ducts, on car dashboards, ect, with no air purifier at all.

Why not try adding these low cost chemsorbers to your Sharp installation?

I have modified my original assessment of the Sharp Plasmaclusters, but they are still pretty good air cleaners.

Competition in the form of RabbitAirs built Japan and Korea, and Winix and Coway from Korea, has cut Sharp's lead.

The big problem with Sharp is not the technology, with over 10 million Plasmaclusters sold worldwide.

It is the US marketing strategy, which relied on vacuum shops and big box retail, without a stateside distributor/dealer network.

You noticed the "unhelpful" customer service.

I notice the lack of support for discontinued models.

In the course of preparing this page, I completely disassembled my FP-N60cx.

I removed the nonfunctional Plasmacluster ion emitter, it had stopped making the pinging noise, and went looking for the part.

Surprise, not only are the filters, sold only in pairs, disappearing, but I could find NO parts available on the web to repair a Plasmacluster ion emitter.

(oops, after reassembly, it is still pinging, never mind)

Realize that this is extremely common in the current business climate.

Others, such as Honeywell, have standardized all products around a common set of filters and a single trim-to-fit prefilter.

This is where a US importer with experience in the business, RabbitAir for example, can shine.

The Bunny doesn't build purifiers, it imports Mitsubishis and Coways, rebranding them.

While rebranding Asian exports is very common, RabbitAir provides decent product support, without too much of the "lost in translation" common with Asian goods in general.

Despite everything, I still think the Sharp Air Purifiers are the ones to beat in the mid-price air cleaner category.

I have run three 24/7 for over 9 years, finally buying a HEPA for my son's FPP40x recently.

I expect to continue using the Sharps for years.

I also think we have established that "ozone-and-carbon-don't-mix."

Can't wait for my next email, this is sooooo much fun!

Good luck,

Ed =======================================

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