Unless the hat is worn so tight as to cut off circulation to the hair follicles, wearing a hat does not cause hair to fall out. Instead, hats can contribute positively by blocking the sun and preventing sunburn.
Wearing hat for too long especially in hot and humid climate, however, can contribute to the excretion and accumulation of sebum and oil on your hair. Sebum is a waxy substance that is secreted by the sebaceous glands in your skin onto hair follicles. Overtime, sebum can react with cholesterol to create a hardened sebum plug that cuts off oxygen circulation to the hair follicles and cause it to fall out. Sebum plug can also accumulate dirt and become a breeding ground for bacteria and contributing to hair loss.
Source: retane.com
Wearing hat for too long especially in hot and humid climate, however, can contribute to the excretion and accumulation of sebum and oil on your hair. Sebum is a waxy substance that is secreted by the sebaceous glands in your skin onto hair follicles. Overtime, sebum can react with cholesterol to create a hardened sebum plug that cuts off oxygen circulation to the hair follicles and cause it to fall out. Sebum plug can also accumulate dirt and become a breeding ground for bacteria and contributing to hair loss.
Source: retane.com