I ran across this in Wikipedia. A study published in 1993 found that polyester underwear reduced sperm count and sperm motility in male dogs. Similar studies have shown similar results in humans and rats. The cause is not known but is believed to be due to an electrostatic field created by the fabric. Notes ^ Shafik, Ahmed, "Polyester textiles and infertility", Ahmed Shafik Foundation for Science. Retrieved 10 August 2010.