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www.ehow.com/facts_5003064_what-synthetic-hair-made.html

Fake hair women use to create different looks when they don't have enough hair or when they don't want to change the original color or style of their natural hair.

Synthetic hair is typically man-made fibers [like a plastic]  that cannot have heat close to them. Unlike the human hair that in most cases can be permed, colored, and ironed. Raquel Welch has a wonderful line of human hair extensions as well as synthetic hair, and wigs made of both. Some people want the extensions for length and fullness, and others prefer the wigs for different occassions, or thinning [loss of] hair. Synthetic hair that is already curled [shaped to specific style] is quick to incorporate with your own hair, while human hair although usually more adaptable for changes you want to make to incorporate into your own hair, takes a little more work. Most human hair has a life span of two years or  more to hold its look. But Synthetic if worn a lot will wear out much quicker.