Wool and alpaca are protein based natural animal fibers. They both insulate naturally. Both retain body heat even if you are soaking wet. Traditional Fisherman Sweaters kept fishermen warm. Wool is also naturally flame retardant. It will burn but more of a smoldering sort of burn and will stop when moved away from the flame. The natural quality gives depth to color and creates interesting texture. When knitting it has nice give and warms up in your hands. It makes some stitches like cables look fantastic. The drawbacks are that it can be itchy depending on the breed of wool and how it is spun and processed. Some people are allergic too it.
Once you get hand washing down it is really not that difficult to wash. I happen to have a front loading washing machine with a hand wash cycle. I can put a wool sweater in a garment bag, run it through the hand wash cycle and lay it out to dry. Even in top loader if you are careful you can probably rinse and spin a sweater in the delicate cycle if you use a garment bag.
Acrylic is petroleum based. It comes from oil. It does not breathe the way natural fibers do. So while you may be warm in something acrylic it is because it is holding in your sweat. If you are soaking wet in acrylic you will be cold and not retain any body heat. So out in the woods it won't protect you from hypothermia the way wool would. It is easy care for so people often choose it for kids things. However it actually is not safe for babies at all. It is not flame retardant, it melts when exposed to flame and can cause horrible burns. Stephanie Pearl McPhee aka the Yarn Harlot asserts that no baby should be left unattended with acrylic for this reason.
Actually lots of modern clothing with acrylic and nylon pose the same danger if you are wearing them in a fire. If your clothes catch fire you can be terribly burned by the melting of the fiber. Children's pajamas that are labeled flame retardant have been treated with chemicals.
Cotton and Linen are natural vegetable fibers, they do not insulate. They might burn in a fire but won't melt all over your skin. They can also offer interesting textures and color.
Natural fibers come from renewable resources. Acrylic does not. However acrylic is in much of our modern clothes and carpeting. I don't know anyone with wall to wall 100% wool carpet.
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