2.1 The smell and the filter media

Cat W/C Air Filtration System · 13 Oct 2026

The first research topic is what the smell is made of and what catches it. I gave ChatGPT Pro the plan and asked it to start on filter media and processes for both the urine and the faeces smells. It worked for about 16 minutes and came back with a long answer and 20 sources. This is what it found, what I make of it, and which parts I checked.

The short version: the filter needs two gas media, not one. Plain activated carbon takes the broad mix of smells, and a separate medium made for ammonia takes the ammonia. And my baffle idea doesn't work the way I thought it would.

I had the smell wrong

In post 1 I said litter tray smell is mostly ammonia. That's only part of it.

Urine gives two different smells. As it ages, bacteria break the urea down into ammonia, and that's the sharp smell. Cats also excrete felinine, an amino acid, and sulphur compounds form from it. The main one is 3-mercapto-3-methylbutan-1-ol, MMB for short. Intact males have far more of it in their urine than females or neutered males. Both of mine are spayed females, so I'd expect less MMB here than in a house with a tom. Published figures still only give me a list of targets, not the load from my two cats.

Faeces are a different mix again: indole, skatole, p-cresol and organic acids. In one study of cat urine and faeces, the phenolic and nitrogen compounds mattered most to how bad people said it smelled. So the faeces stage isn't just a hydrogen sulphide problem either.

The point that changes the design is that the amount of a gas and the strength of its smell are different things. Some of these compounds smell strongly at trace levels. A filter can take out most of the gas by mass and leave the room smelling, and taking out a small amount of the right compound can make a big difference. When I test, I'll log urine and faeces as separate events, not roll them into one "air quality" number.

Two ways to catch a gas

There are two mechanisms. In physical adsorption, molecules stick to the inside surfaces of the medium. In chemisorption, a chemical in the medium reacts with the gas and holds on to it. Plain activated carbon mostly does the first. Impregnated media add a chemical for a target gas.

Why two stages

NASA tested phosphoric-acid-treated carbons, including Ammonasorb II and Chemsorb 1425, for spacecraft air. The acid treatment improved the ammonia capture but cut the carbon's capacity for other VOCs. One impregnated carbon can't do both jobs well, so the general carbon stays as its own stage. The EPA's technical summary says the same from the other side: plain activated carbon isn't especially effective against ammonia or hydrogen sulphide. Separate stages also mean I only replace the one that's run out.

NASA also found that other VOCs in the air cut Ammonasorb II's ammonia capacity. That suggests putting the plain carbon first, so it takes the VOCs before they reach the ammonia medium. It's my reading of the result, not something NASA tested, so the order is one of the things the test cassette has to try.

There's a naming trap here. Carbon impregnated for bases catches ammonia. Carbon impregnated for acids catches hydrogen sulphide. "Impregnated carbon" on its own doesn't say which, so I'll buy by the target gas.

Humidity

I'd assumed damp air makes carbon work worse. For the acid-treated ammonia media it's the other way round. Research on phosphoric-acid sorbents found that adding moisture to the air raised their ammonia capture, and that drying the sorbent for longer when it was made cut it. Camfil gives 40 to 90% relative humidity as the operating range for its medium for bases. So the humidity in the baseline isn't just a number to log. It helps choose the medium.

Zeolite

The plan lumped zeolite in with the carbons. It's a mineral, not a carbon. Clinoptilolite does adsorb ammonia gas in published tests, but one study of one zeolite says little about a random bag of granules. I'd want data for gaseous ammonia at room humidity before treating it as a substitute for a medium made for the job.

HEPA doesn't touch the smell

HEPA catches particles, not gas molecules. A better HEPA grade won't make up for weak gas media. The US EPA makes the same split, and its guide says a carbon filter works when there's a large amount of material in it, the thicker the better. A thin layer with carbon on the label won't do much. The particle filter goes in front of the gas media to keep litter dust off them, with something after them to catch any dust the media shed.

What changes in the design

The "carbon" layer in the plan becomes a gas media section with separate cartridges:

• One cartridge of plain carbon and one of the ammonia medium, each replaceable on its own.
• The order and the depth can change between tests.
• I weigh what goes into each one.
• There's space for a third medium later, without a new case.

So the first thing to build is a test cassette I can swap media in and out of, before the final case.

Honeycomb against granules is a packaging question as much as a chemistry one. What matters is how much sorbent each holds, the pressure loss and the removal, and granules give me more freedom while I'm experimenting. FÖRNUFTIG's gas filter stays as something to compare against. IKEA describes it as activated carbon for household smells and reports a formaldehyde test, but gives nothing on ammonia or MMB.

Contact time, and the end of my baffles

The useful number is the empty-bed contact time: the volume of the packed bed over the airflow.

$$\text{EBCT} = \frac{V_\text{bed}}{Q}$$

The bed volume includes the gaps between the granules, so this isn't the actual time a molecule spends in the gaps. It's a way to compare beds.

ChatGPT's example is a bed 220 × 220 × 60 mm:

$$V_\text{bed} = 0.22\ \text{m} \times 0.22\ \text{m} \times 0.06\ \text{m} = 0.0029\ \text{m}^3 = 2.9\ \text{L}$$

At the plan's guess of 36 m³/h, which is 10 L/s:

$$\text{EBCT} = \frac{2.9\ \text{L}}{10\ \text{L/s}} = 0.29\ \text{s}$$

Split between two media, each gets about 0.145 s, not 0.29 s each. Camfil quotes 0.1 to 0.2 s for its cylindrical molecular filters and 0.5 to 2.5 s for its deep-bed units, so a split bed would sit at the low end. That's a reference, not a design rule. The 220 × 220 × 60 mm bed is a space to experiment in, not a size I've worked out.

Now the baffles. In post 1 I wanted to fold the air's path so it spends longer in contact with the carbon. But contact time is bed volume over airflow. A longer empty path adds no media, so it adds no capacity. And packing the same carbon into a longer, narrower path keeps the contact time the same while the fan has to work harder. The baffles might still help spread the air evenly over the media and stop it going round them, but a well-sealed cartridge does more for that than a maze. The baffle experiment changes from "does folding help" to "does all the air go through the media".

Moving air isn't cleaning it

For each gas, the clean air the unit delivers is the airflow times the share of that gas it removes in one pass, $\eta$:

$$Q_{\text{clean}} = Q \times \eta$$

There's no reason $\eta$ is the same for ammonia, MMB and p-cresol.

In a well-mixed room with nothing still giving off smell, the concentration $C$ falls from its start value $C_0$ as:

$$C(t) = C_0\,e^{-Q \eta t / V}$$

With the plan's guesses, $V = 3.6$ m³ and $Q = 36$ m³/h, after 15 minutes $Qt/V = 2.5$. A perfect filter ($\eta = 1$) leaves $e^{-2.5} = 0.08$, so 92% has gone. At $\eta = 0.8$ it leaves $e^{-2} = 0.14$, so about 86% has gone.

Those are model numbers, not smell. The tray keeps giving off smell, and what you smell depends on which compounds are left, not on a percentage. The 15-minute target in the plan is something to test, not something 10 air changes an hour guarantees. And I'll choose the fan only once I've got pressure-loss figures for the actual filter stack.

Out of version one

• Ozone. The EPA says ozone at levels that are safe to breathe does little to clean indoor air, and at levels high enough to work, it's harmful. The cats breathe that air.
• UV photocatalytic oxidation, plasma and ionisers. Each can make its own by-products. The EPA lists formaldehyde, acetaldehyde, nitrogen dioxide and carbon monoxide from photocatalytic units, ozone and formaldehyde from plasma, and ozone from ionisers. Any of them would need its own testing. They're not a shortcut round choosing the right media.
• Liquid scrubbers, biological filters and regenerating media at home. All more to maintain. Version one should show whether sealed, replaceable media solve the problem first.

Cleaning the room isn't keeping the smell in

A unit that takes air from the room and puts it back into the same room cleans that air, but it can't pull air in from outside the room. It doesn't stop air leaving under the door. Extracting to outdoors would, and it's a different design. So two targets in the plan become two separate tests: the room clears quickly, and no smell gets out through the door.

Testing without fooling myself

Two of my planned sensors can't tell me what I thought they could.

• Pressure across the filters shows dust clogging, not gas media running out. Gas media can lose their capacity without any change in pressure. The pressure sensor tells me when to change the particle filter, not the carbon. Manufacturers check gas media by sampling the gas or analysing the media.
• A VOC sensor like Sensirion's reports a VOC Index relative to a baseline it learns over the past 24 hours, mapped to 100. Back to 100 means back to what the sensor has got used to, not back to clean. And two sensors, before and after the filter, each learn their own baseline, so dividing one by the other doesn't give a removal percentage.

The experiment, in order:

1. Baseline. Log temperature, humidity, particles and VOC, with a note of when the trays are used and cleaned, and a sniff test at set times. Keep the litter and the cleaning the same throughout.
2. Compare at the same airflow. Plain carbon first, then carbon plus the ammonia medium. Same measured airflow, not the same fan PWM, because the cartridges differ in resistance.
3. Separate the events and the age. Compare urine and faeces events separately, and repeat once the media have been in use for a while. A good first day says nothing about how long they last.
4. Measure ammonia where it settles a question. A good ammonia sensor would show whether the ammonia stage works, not whether the whole smell has gone. The cats' normal use is the test gas.

What I'm asking suppliers for

The useful figure is breakthrough: when a medium that starts well begins letting too much through. I'll ask for breakthrough data with the test concentration, humidity, temperature, airflow or face velocity, bed depth, pressure loss and the point they call breakthrough, not a headline capacity percentage.

The shortlist for data and samples:

Whether either company sells a sensible amount to one person is the next question, and that belongs to the sourcing topic.

What I checked

I had Claude check twelve of the claims against the sources ChatGPT cited, reading the papers and data sheets themselves. I worked through the arithmetic myself.

These held up: NASA's trade-off between ammonia and VOC capacity, Camfil's separate media for bases and for acids and the 40 to 90% humidity range, the contact times, Sensirion's 24-hour baseline mapped to 100, the felinine and MMB chemistry, the faeces compounds, Purakol and Puracarb AM, the EPA on ozone and on putting sorbents after the particle filter, the FÖRNUFTIG filter's lack of ammonia data, and clinoptilolite adsorbing ammonia.

These needed correcting, and the post above uses the corrected versions:

• ChatGPT cited Camfil's blends as the alternative to two stages. They're carbon mixed with alumina, for ozone and corrosive gases, not ammonia.
• The moisture paper's "drying" was drying the sorbent while it was made, not dry air.
• ChatGPT gave one list of by-products for photocatalytic units, plasma and ionisers together. The EPA lists different ones for each.
• The EPA doesn't say "appreciable" sorbent. It says "sufficient", and its guide says the thicker the better.
• The McGill article it cited for cat urine chemistry only covers urea turning into ammonia. It doesn't mention felinine or MMB, and it blames steroids for the smell of male urine, which the papers don't support. The felinine and MMB points come from a 2024 paper, which cites them from earlier work.

The rest, including the ion-exchange resin, the permanganate media and the pressure sensor's blind spot, comes from ChatGPT's answer and wasn't checked claim by claim.

Still open

• How much of each medium, how deep, at what airflow, and how often to replace it, in this room.
• Whether a third medium for the sulphur compounds earns its resistance and cost.
• Whether a recirculating unit keeps the smell in the room well enough, or the room needs extraction too.

Sources

Papers and reports:

• L. Zhang et al., Semiochemicals from Domestic Cat Urine and Feces Reduce Use of Scratching Surfaces, Animals, 2024.
• C. Banik et al., Simultaneous Chemical and Sensory Analysis of Domestic Cat Urine and Feces with Headspace Solid-Phase Microextraction and GC-MS-Olfactometry, Separations, 2021.
• O. Monje et al., Analytical Methods and Testbeds for Characterizing Adsorbents and Catalysts for Atmosphere Revitalization of Crewed Spacecraft, NASA/TM-20210026041, 2021.
• Hsieh, Tsai and Chang, Effects of Moisture on NH3 Capture Using Activated Carbon and Acidic Porous Polymer Modified by Impregnation with H3PO4, Materials, 2022.
• H. Asilian et al., Removal of Ammonia from Air, using Three Iranian Natural Zeolites, Iranian Journal of Public Health, 2004.
• US EPA, Residential Air Cleaners: A Technical Summary, 3rd edition, 2018.
• US EPA, Guide to Air Cleaners in the Home, 2nd edition, 2018.
• US EPA, Ozone Generators that are Sold as Air Cleaners.
• J. Schwarcz, Why does cat urine smell so bad?, McGill Office for Science and Society, 2017.

Manufacturers:

• Camfil, impregnated carbon for bases, impregnated carbon for acids, blends and molecular filters.
• Purafil, Purakol and molecular media, including Puracarb AM.
• Sensirion, VOC Index info note.
• IKEA, FÖRNUFTIG filter for gas cleaning.
• Hencolin, AMC chemical filters, on checking the life of gas media.