The latency period for malignant mesothelioma is the amount of time between exposure to asbestos and the time when the disease becomes apparent clinically. Of the asbestos-related diseases, mesothelioma has the longest latency. On average, 35 to 40 years will elapse between exposure to asbestos and diagnosis of mesothelioma. This is only an average, and there have been cases of diagnosis 50 or more years after exposure, or in as little as 10 years. Malignant pleural mesothelioma (lung) most often is diagnosed between ages 40 and 69. A large proportion of mesothelioma patients diagnosed earlier than age 40 have a history of asbestos exposure during childhood.
While most mesothelioma patients are in their 40s to 60s, the disease can occur even in childhood, though it is extremely rare – only two to five percent of cases of malignant mesothelioma develop in the first two decades of life. Asbestos exposure in these cases may be environmental or may not be demonstrable at all, so these statistics may not apply to the current understanding of latency periods.
Higher exposure to asbestos may result in a shorter latency period. Patients showing a higher lung burden of asbestos – that is, a greater amount of asbestos fibers in the lungs on biopsy – may have shorter latency times. The highest asbestos burden and the shortest latency times have been shown in patients who were occupationally exposed during jobs within the asbestos and insulation industries and in shipyards.
The very long latency period of mesothelioma contributes to its poor prognosis. All cancers tend to have a better prognosis when discovered early. Mesothelioma remains “silent” for a very long time compared to most cancers. When it finally reveals itself in physical symptoms, it has likely gained a strong foothold during this latency period. By the time a diagnosis is reached, the mesothelioma may be further advanced.
Mesothelioma comes in different forms. One primary distinction made is based on the initial site of the disease. Peritoneal mesothelioma, appearing in the abdomen, has a shorter latency period on the average than pleural mesothelioma, which appears in the chest. One study showed the mean latency period for peritoneal mesothelioma to be 28 years. For pleural mesothelioma, the mean latency period was 35 years. In this study, the median duration of asbestos exposure for pleural mesothelioma was 5 years and for peritoneal mesothelioma 11 years, but the exposure times could vary widely, ranging from a few days to over 40 years (4).
A majority of mesothelioma cases arise following occupational exposure to asbestos. In most of the developed countries, occupational exposure is now regulated, but because of the long latency period, new mesothelioma cases may continue to increase for years. Peak incidence of asbestos-related diseases is expected to occur about 30 to 40 years after the years of greatest asbestos usage. In the U.S., this would have been the 1960s, and it is speculated that the U.S. may have seen peak mesothelioma rates in the years 2000-2004, when there were about 2000 cases per year. These statistics are not expected to rise during the remainder of this decade. In Europe and Australia the peaks may occur 10 to 15 years later. With such a long latency period, immediate economic gains often outweigh invisible future effects, and in third world countries with few regulations of occupational hazards, the damage may be just beginning.
It’s possible that the very long latency period itself may change the pattern of mesothelioma statistics over the next few decades. Many workers in years past simply didn’t live long enough after retirement for the 30- or 40-year latency period to catch up with them. Workers retiring at an age of 65 now have a life expectancy of 16 years compared to less than 2 years several decades ago. This in itself might result in an increase in mesothelioma statistics. Furthermore, asbestos regulation has successfully reduced the amount of exposure, so that workers who are exposed to asbestos are generally encountering a much lower level than they were years ago. Workers who were exposed to high levels of asbestos, particularly smokers, run a high risk of contracting and dying from bronchogenic carcinoma, more commonly known as lung cancer, before mesothelioma develops. With longer lives, levels of exposure to asbestos and lower levels of other cancers, new-onset mesothelioma may be expressed more often.
The latency period for mesothelioma is so long that it can sometimes be difficult to pinpoint the timeframe for asbestos exposure, particularly in cases where occupational asbestos exposure is not suspected. There may have been secondary exposure. For instance, in the days before proper precautions, wives and families of mine or shipyard workers were exposed to asbestos dust that workers brought home on their clothing. Mesothelioma can sometimes develop after only short-term exposure, and thirty or forty years later the patient may not be able to pinpoint where exposure may have occurred. Perhaps a patient worked at a summer project for several weeks during student years, renovating old buildings as part of an inner-city revitalization effort. Before asbestos danger was fully understood, there may have been very few safety precautions in place, and the amount of exposure might be enough to result in mesothelioma decades later….but might not be remembered. Thus there is a built-in difficulty in obtaining latency statistics.
Experiencing Respiratory Symptoms In the U.S., the World Trade Center disaster offers an unprecedented opportunity to track latency periods from known exposure to asbestos and other air contaminants. The health consequences to rescue, volunteer, cleanup, recovery and other personnel will be severe and widespread. Long-term epidemiological studies are underway so that, from the standpoint of medical knowledge, this disaster will have some positive contribution to the understanding of mesothelioma and other disease latency periods.