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As the amount of synthetic or cotton in a yarn increases and the amount of wool decreases, the yarn loses its wool characteristics. Thus, if using a wool blend, the higher the percentage of wool in the yarn, the better.
From Ewe to You by Nancy Reid:
"The wool fiber itself consists of an inner section, the cortex, and an outer layer, the cuticle. The cortex consists of millions of spindles called cortex cells. That make up 90% of the fiber. The cuticle is made up of flat horny scales which overlap each other like shingles on a roof. The scales on each fiber can be made it interlock with the scales on other fibers. This frictional interlocking makes it possible for the fibers to be spun easily. Without it the fibers would slip past each and not form a strong thread....The scales are covered by a water repellant protective membrane....
In addition to their basic structure, there are several other inportant things to know about wool fibers. First, they have natural crimp....The crimp, plus the molecular structure of the fibers, gives them a resilency that contributes both to the quality of the fabric and its warmth....
The second important property of wool is its action with moisture. Because of the outer membrane over each scale the fibers are naturally water repellant. However the interior of each fiber is very thirsty and can absorb up to 30% of its weight in water *vapor*. This means that while droplets of water, fog, or drizzle, will bead up and not penetrate the surface, the interior of each fiber, through gaps between scales, can absorb the vapor your body gives off before it can condense and form perspiration.
Wool's resiliency and its ability to absorb water vapor are major factors in its warmth. The crimp and resilency plus the scales on each fiber means that when the fibers catch and lock together means there is a great deal of air trapped between them. Wool yarn, especially those spun by the woolen process, are very fluffy -- full of air pockets. The air trapped between the fibers and between the fibers and your skin (for the fine hairs keep a wool garment from lying tight against you) act as insulators, keeping your body heat close to you. The absorption of water vapor your body gives off, even on the coldest day, keeps your perspiration from forming droplets and cooling you by condensation. The absorption itself produces heat. Wool releases its accumulated moisture to the outside air very slowly. In contrast, synthetics do not absorb water vapor. They carry the condensed perspiration rapidly away from you down the length o their fibers by a wicking action. A wet synthetic is a cold one. Wool is warm even when wet.
Another important feature of wool is that is non-flammable. If you put a piece of wool into a direct flame it will burn, but the minute you remove it from the flame it will extinguish itself. It won't flare up in a big flash or melt and drip on you..."