Infographic on the chemistry of cats. 10-30% of people are allergic to cats. Fel D 1, a protein which is the main cat allergen, is found in over 99% of homes, and is produced in greater levels by male cats. Inhaling cat allergens causes an immune response that produces histamine, which causes the symptoms of allergies. Cat urine smells bad because an amino acid in the urine, felinine, gets broken down into ammonia and sulfur-containing compounds. Cat ketone is a key odorant, which also occurs naturally in Sauvignon grapes and contributes to the odour of blackcurrants. Most cat litters are clay-based, though biodegradable and silica gel litters can also be used.
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Why are some people allergic to cats? Why does cat pee smell so bad? And why do cats love catnip? This graphic looks at some of the answers!

I’ve recently become a cat owner: meet Linus (Pawling)! He’s helped to confirm that my wife’s suspicion that she’s allergic to cats is entirely correct. He’s also allowed me to confirm that cat urine really smells quite bad, particularly outside of litter trays. Oh, and, like many cats, he loves a bit of catnip.

Meet Linus: He likes fullerenes and naps.

Of course, I instantly wanted to dig further into the chemical explanations behind these observations. In the process, I discovered that cat allergens are everywhere, it’s not just catnip that cats go crazy for, and there are at least two compounds in cat urine named after cats.

If you’re allergic to cats, you’re not alone. It’s estimated that between 10 and 30% of the general population are allergic to cats, based on surveys across a number of countries. The World Health Organisation recognises eight different cat allergens, designated Fel d 1–8. In this designation, ‘Fel d’ stands for the domestic cat’s Latin name, Felis domesticus.

Of the eight allergens, the primary allergen is Fel d 1, accounting for 60–90% of allergenic activity. Fel d 1 is a protein produced by oil-producing glands in the cat’s skin. It’s found in their skin, fur, and saliva, though its exact biological function is still unknown.

Cat dander, the tiny, microscopic bits of dead skin that cats shed, allows Fel d 1 to spread around your home wherever your cat goes. And it spreads very well. A survey of American homes found that over 99% of them contained traces of Fel d 1.

Allergy issues start when you inhale Fel d 1. In people who are allergic, this kicks off an immune response from your body. It produces antibodies against the protein. These antibodies trigger the release of histamine from mast cells, which in turn triggers the allergy symptoms of sneezing, sniffing, and so on.

There are ways of managing this if you’re allergic. Antihistamines are commonly taken for all sorts of allergies, and, as the name suggests, combat the action of histamine. Additionally, some of your cat’s attributes may influence how much Fel d 1 they shed.

Male cats produce higher levels of Fel d 1 than female cats. Neutered male cats produce levels comparable to females. The bad news? The lower levels are still sufficient to trigger allergic symptoms, so it arguably doesn’t make a great deal of difference. Though it’s often suggested that some breeds might produce less Fel d 1, there’s currently little in the way of evidence for this.

Though Fel d 1 might get up your nose, you won’t smell it. The same can’t be said for cat urine. Any worries I had about not noticing if the cat was peeing around the house were immediately dispelled the first time he did it. For non-cat owners: it reeks. Pretty bad.

There’s a chemical explanation for this too. Fresh cat urine doesn’t actually smell of much. But it doesn’t stay fresh for long. Bacterial breakdown of chemical components in the urine generates odorous compounds.

Key to this is a particular amino acid found in cat urine, appropriately named felinine. Enzymes break down felinine, producing ammonia, carbon dioxide, and a compound called 3-methyl-3-sulfanylbutan-1-ol (MMB). Ammonia is known for having a pungent odour, but the sulfur-containing MMB is a big contributor to the characteristic cat urine smell.

Felinine isn’t the only cat-themed compound involved. Another compound produced as cat urine ages is 4-methyl-4-sulfanylpentan-2-one, more commonly referred to as ‘cat ketone’. Cat ketone isn’t just found in cat urine – it also occurs naturally in Sauvignon grapes and is a key component in the smell of blackcurrants.

Cat litter absorbs your cat’s urine and some of its smell. Litters come in several different types. Clay litter, based on clays such as calcium bentonite, are popular. Biodegradable and silica gel-based litters also exist. It’s more of a problem when your cat doesn’t use its litter tray and decides to pee elsewhere in your house instead. In these cases, enzyme-based cleaners are the best bet to remove the odour and prevent your cat from peeing in the same spot again later.

Finally, why do cats love catnip? This is a topic I’ve tackled previously in a separate graphic. In short, nepetalactone, a compound found in catnip, causes its weird effect on cats. It binds to receptors in the cat’s nasal tissue, which ultimately triggers responses in the brain similar to those seen for natural sex pheromones. Olfactory fatigue sets in after a short while, so the response is only exhibited for 10–15 minutes. Catnip only affects around 70% of cats, and very young kittens don’t respond to it at all.

Catnip isn’t the only plant cats love, as it turns out. Studies have shown that there are several others that produce similar responses. These include valerian roots and leaves, silver vine, and Tatarian honeysuckle. These plants don’t contain nepetalactone but contain similar molecules, such as actinidine, which cause similar effects.

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References/further reading

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