Loose cotton shirts are one of the most comfortable types of clothing in a hot, dry climate -- but when it comes to hiking in that climate, cotton is a questionable choice at best. All it takes is getting drenched by a sudden rainstorm or your own sweat, temperatures drop for the night (or in the shade), and suddenly you're courting hypothermia.

If you're in a cooler climate, cotton goes straight from "not such a great idea" to "potentially deadly." (Here's why.

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So what can I wear instead?

You have two primary choices in today's array of technical fabrics and gear: Synthetic fabrics (including polypropylene and a number of trademarked textiles like Patagonia's Capilene) or merino wool.

Both synthetics and merino wool do a great job of wicking -- drawing the moisture of rain/snow/sweat/condensation away from your skin, keeping you dry and chafe-free. And both fabrics do what cotton can't, keeping you warm even when wet through.

Despite those similarities, synthetics do two things that wool can't: They dry quickly and they stink. For some folks the stink happens almost as soon as sweat touches fabric; I've heard this blamed on quirks of personal body chemistry. But for anybody, the accumulated stink of heavy use can eventually work its way into the synthetic fabric and just. Won't. Come. Out.

Polypro(pylene), in particular, is notorious for its stink-holding capabilities. More modern synthetics have come a long way, with manufacturers embedding microbial treatments in their fabrics to help cut down on the pit-stink -- but for some folks it's still an issue.

You can also purchase wool/synthetic blends that combine the best of both fabrics: They dry faster thanks to the synthetic content, and stink less thanks to the wool.

Wool vs. Synthetic

When it comes to base layers, I vastly prefer merino wool over synthetics. Here's why:

1. Merino wool is much harder to stink up.
2. Wool is a renewable resource -- it comes from sheep, not oil.
3. Wool feels softer and more supple against the skin. (Modern merino wool is virtually itch-free, thanks to fine fibers of a regular length.)
4. Wool helps keep you cool in warm temperatures, although opinions are pretty sharply divided about how well it performs in truly hot conditions.
5. I find that wool also breathes much better when I'm active, helping me not to overheat.

Of course, as always, there's no single right answer -- and on truly cold winter days I still like the thick pile of a heavyweight fleece as a midlayer. (All that fluff traps air that's warmed by your body heat and works as an extra layer of insulation.) But there's almost always going to be some wool in there too.