In my opinion, synthetic fabrics still outperform natural fabrics. However, many outdoor enthusiasts have found that their skin can be quite sensitive to synthetic fabrics leaving them with no alternative. When considering all of the natural fabrics available for thermal underwear and other next to skin garments, merino wool is the fabric of choice. Let's take a look at the reasons.

Reason number 1: Wool fibers retain an excellent amount of heat and it also traps air well making it an excellent insulator. Just ask your nearest sheep.

Reason number 2: Wool Fibers are coated with natural oils from the sheep helping it to repel water. As a result, the wool absorbs little water and thus dries quickly.

Reason number 3: Wool also has the ability to retain heat even when it is wet. This makes it more favorable than cotton since cotton since cotton loses heat when it gets wet.

Essentially, wool has very similar characteristics to synthetic fabrics like polyester or polypropylene making it a great natural fabric for thermal insulation.

So what is the difference in regular Wool and Merino Wool? Merino wool comes from merino sheep. The wool from merino sheep is much finer making is softer and non-itchy like the wool of most other sheep. If you've ever worn a wool sweater you know what I'm referring to when I say itchy. Merino wool, especially the type found in high end garments is cottony soft. So merino wool products are the perfect natural garment to keep you warm and comfortable without putting up with the itchy feel of regular wool.

Polypropylene is a material that few people are aware of and even fewer know how to pronounce. But more and more performance clothing manufacturers are using polypropylene as a core fabric in their products. These manufacturers tout polypropylene's ability to keep the wearer warm, dry and comfortable during high aerobic activities such as running, hiking, skiing and snowboarding just to name a few. So what would make polypropylene fabrics an alternative to well known products like Under Armour? Polypropylene, like many other synthetic fabrics on the market today is a polymer or a plastic. This means that it is very lightweight and will absorb little if any water and instead repel water. As a result, polypropylene fabrics dry extremely fast. This feature is particularly useful for next to skin products such as socks and performance underwear. When worn next to your skin, polypropylene will stay dry and helps prevent chaffing and keeping comfortable throughout your aerobic activity.

Another very useful feature of polypropylene is it's ability to retain heat. Polypropylene retains more heat than any other fabric. This means that on top of being very lightweight, thermal underwear made of polypro will be warmer than long johns made of polyester or cotton fabrics.

One of the features I like about polypro is that it typically costs much less than other performance products. I can buy several sets of polypropylene thermals for the price of just one Under Armour thermal top. So the next time you are in the market for some performance active wear, you'll want to check the label to see if it is made of polypropylene.