I came home from yesterday’s 12-miler with a stench somewhere between county landfill and EPA Superfund site. This morning I was wondering how a single day-hike could be so malodorous, so I did some googling and found this page explaining why synthetic fibers get so smelly so fast.

Now let’s talk sweat. There are two kinds. Eccrine sweat is 99 percent water, with a pinch of salt and urea added. But in times of stress, your armpits and groin release a second, oily type of sweat, apocrine, which is loaded with interesting things, including protein, carbohydrate, ammonia and fatty acids.

Bacteria, who slurp up sweat and turn it into a variety of gaggy odors, eat both kinds, but they especially like the fats in apocrine sweat.

Joining the two sweats on the skin is sebum, another fatty secretion designed to protect your skin. Because sebum melts in sweat, all these groceries migrate together across your skin and into your clothes.

Now let’s talk oxidation. Some fats from sebum and sweat stick to your clothes and oxidize there, turning yellow and smelling rancid. Your old pillowcases bear the yellowing sebum from your greasy face, says Obendorf. Likewise the yellow armpits of white shirts: oxidized fat.

Ah, that explains it. As long as we’re on the subject of filth, here’s a page of hygiene tips from a veteran long-distance hiker.

I carry a container cut from a 1-gallon milk jug when I hike. The container is about 5 inches high and weighs about an ounce. I use it and a small bandana (along with a few ounces of water) to get a sponge bath in the evenings. It is truly amazing how much trail dirt can be removed with just a few ounces of plain water and a bit of effort. My Wal-Mart Grease Pot (used as a cooking pot) fits neatly inside this container, so it doesn’t take up any room in my pack.

I also carry a 2-ounce bottle of Isopropyl alcohol and some cotton balls. When I finish with my “bath” in the evenings, I apply some alcohol to a couple of cotton balls and clean and deodorize the “obvious” areas of my body (underarms, groin area, area between my buttocks and my feet).

Not only does this reduce body odor, but I’ve found that the daily application of alcohol to the groin area prevents chafing from getting started. Occasionally, the alcohol will cause some mild burning, which tells me that I had the beginnings of chafing, but that the alcohol is doing its job.

Another tip from the same page: don’t sleep in your hiking clothes, unless you have some strange reason for wanting that smell to migrate to the inside of your sleeping bag.