Not all the email I get is from cranks and creationist loons. Sometimes I get sincere questions. In today's edition of "Ask Mr Science Guy!", Hank Fox asks,

I was thinking recently about the fact that wax collects in one's ears, and suddenly thought to be amazed that some part of the HUMAN body produces actual WAX. Weird. Like having something like honeybee cells in your ear.

And then I started to think about what sorts of other ... exudates the human exterior produces. Mucus, possibly several different types (does the nose itself produce more than one type?). Oils, possibly several different types. That something-or-other that hardens into your fingernails. Saliva, if you wanted to count our frequently-open mouth as sort-of exterior. What else?

Of course I know something about this subject, having taught physiology for a few years. My years of experience have also led me to notice that it is always the guys who ask about disgusting secretions. Why is that?

Anyway, fingernails (and hair) are not secretions. They are composed of interlocked, dead cells packed internally with high concentrations of the protein keratin. So let's forget about those, and concentrate on the really yucky stuff instead: ear wax and another important kind of goo, smegma.

First of all, it's not at all unusual that we would secrete a wax. What's the difference between an oil, a fat, and a wax? Nothing but the melting point. All are esters (the products of condensation reactions between carboxylic acids and alcohols) with an aliphatic chain of carbon molecules. The length of the chain determines the volatility of the molecule; short chains are more fluid, long chains more solid. Something like olive oil will have shorter chains than something like beeswax, but all are fundamentally similar. They are all classified as lipids.

So earwax isn't that unusual—it's a compound on a continuum of perfectly normal lipid products produced by cells.

So what, exactly, is in earwax, or cerumen? Here's where it gets ugly. It's a combination of things:

• Desquamated keratinocytes. Dead skin cells, in other words, that have peeled off of the epithelia lining the ear canal.
• Sebum. This is an oily substance produced by the sebaceous glands that are scattered over most of your body. If you don't wash your hair for a few days, you know that oily, greasy substance that builds up? That's sebum.
• Various waxes. The dense, waxy part of cerumen is a secretion from specialized glands in the ear canal, the ceruminous glands.

All of these combine into a greasy paste that helps protect the passageway into the ear from invaders. I know I wouldn't want to set foot in it.

For a more detailed analysis of the chemical composition of ear wax, one can do a little chromatography. About half of the dry weight of ear wax is lipid, and it consists of:

• Cholesterol and cholesterol esters: 32.5%
• Fatty acids: 22.7%
• Ceramides: 18.6%
• Squalene: 6.4%
• Triacylglycerols: 3.0%

Everyone with a little biology or chemistry background will recognize these as quite ordinary products of cellular metabolism. Also, these particular compounds are found in similar concentrations in another place: the stratum corneum, or outer layer, of your skin, where the fats and waxes and oils are secreted in a layer that surrounds the cells, providing waterproofing and lubrication.

While rummaging around in the files, I also found an older paper (from 1947) that analyzes another similar substance: smegma. As you might expect from the fact that it is also a waxy, oily secretion from skin cells, it is also about half lipid, and consists of:

• Cholesterol and cholesterol esters: 18%
• Fatty acids: 71%

This paper is notable for a couple of things. It tells us where to get a supply of smegma.

Smegma is best obtained from dead horses in rendering plants or from anesthetized animals in a department of veterinary surgery.

That's good to know; I wouldn't want to make the error of trying to collect smegma from live, conscious horses.

The other distinctive thing about the paper is that it is one of the more disgusting experiments I've read about. The authors were testing the potential carcinogenic effects of smegma, and the experiment involved making up slurries of smegma and smearing it or injecting it into folds of skin on mice, and assessing their health. It had to have been a big job, slathering 400 mice with smegma every week, and treating another 400 control mice with ear wax.

This study found an increased frequency of various cancers in the treated mice: 57 smegma-smeared mice developed various kinds of cancer, versus only 12 of the controls. Before everyone gets all worked up into the circumcision debate, though, I'll mention that the paper is one of many that have tested this kind of thing, they acknowledge that other researchers have seen no carcinogenic effect, and that more modern papers suggest that there are no special carcinogenic properties of smegma. The paper shows another curious result, that the authors didn't even discuss:

There was no significant difference in the survival rates of treated and control mice up to the 400th day of life: 85 and 88 per cent, respectively, after 200 days; 74 and 80 percent after 300 days; 65 and 57 percent after 400 days. After 500 days, 47 percent of those treated with smegma were alive as compared with 30 percent of the controls. From the 600th day on, there was a marked difference (26 and 6 percent, respectively), and on the 700th day, the survival rates were 12 and 1½ per cent.

Personally, I think the smegmated mice were just so pissed off that they kept going out of infuriated spite.

By the way, Hank also asked about mucus, but I think I'll save the discussion about snot for another day. Right now, it's time for me to go to lunch.

Bortz JT, Wertz PW, Downing DT (1990) Composition of cerumen lipids. J Am Acad Dermatol 23(5):845-9.

Plaut A, Kohn-Speyer AC (1947) The carcinogenic action of smegma. Science 105(2728):391-392.