It depends on the smoothness of the hair. Human and cat hair do not work. Those microscopic hairs do not open
up enough. They slide away from each other.
What does work is, sheep ( also wool yarn), alpaca, goat and dog hair.
BUT…not every breed , nor layer is suitable.
Outside hairs are made to be smooth so that rain and snow falls off. Carding mixes the outside hairs
with the easy-to-felt under layer. If you don’t do that you can create a “shepherd’s bag. They stay fluffy
on the outside. Beause those outside hairs don't felt.
I also depends from individual to individual. Some just have “bad hair” and don’t felt that
You can always mix bad felters with good felters. Some hairs and fibers don't felt at all.
Human hair is one of those hairs. Thank Gods it doesn't. It would have been a mess every time I would wash my hair. But what
you could do is mix human hair or synthetic yarn in with wool, so it will be kept in place by the felting wool.
Take a short length of yarn or fiber, from 6 inches to a couple of feet in length depending on the weight
of the fiber and the desired ball's size (small balls are easier to handle). Roll the yarn between the palms of your hand
to form a loose round clump of fiber. Add a little soap. Wet the yarn with soap under warm running water. Knead and roll the
sudsy ball of yarn between the palms of your hands. You will likely have more soap than you need so rinse it under warm running
Continue rolling and kneading your fiber ball until it begins to stick together and felt into a ball shape.
It will likely take only five minutes or less. If the fibers do not want to stick together after just a few minutes you will
know this fiber doesn't want to felt or it will require a very long washing and felting cycle to achieve a felted fabric.