Menda firm bristles are made of conductive yarn, pig hair and horse hair firm bristles are made of conductive yarn and horse hair Generally speaking, once the conductive yarn is added to the bristles, it neutralizes the possibility of static build up caused by the natural hair. - Synthetic bristles can easily become charged with static in standard humidity conditions. Natural hair usually builds static in areas of low humidity, but due to the conductive fibers in our brushes, this problem does not take effect.