Mechanical obstruction of skin follicles with helmets or chinstraps can worsen pre-existing acne. Enlisted airmen's chinstraps and visors are of plain black leather or polymer material. The first helmets had thin chinstraps that would break with normal use. Some of the chinstraps were later used on the modern day kevlar para helmet. Then in 1979, the helmet with thicker chinstraps appeared. Subsequent designs reduced the size of the cap and introduced chinstraps and buttons. All commissioned officers' chinstraps are also of plain black leather or polymer material. Nylon chinstraps were introduced in the U.S. military in the 1980s . Another difference of the M1C was the chinstraps (this was seen on the M2). The gentoos have thus seen 7500% population growth since 1974, and the chinstraps 2700%.