Mesothelioma is characterized by having a long latency period, which means that symptoms do not develop until 20-50 years after exposure to what caused it. The average age at which symptoms usually begin to appear is around 60 years old. It is estimated that about 3,000 to 4,000 cases of mesothelioma are diagnosed each years in the US. The number of cases reported each year is increasing.
Among men 55 years and older, a possible reason for the current increase in diagnosed cases is due to asbestos use in their work environment. Since the 1930's, asbestos has been used primarily in factories, shipyards, and other industrial settings. It was not until 1973 that work places began reducing asbestos exposure. The cases now being seen have developed many years after a worker's first exposure to asbestos back in the 1940s, 1950s and 1960s.