epidemiology mesothelioma

Understanding Mesothelioma

Mesothelioma is a rare type of cancer caused by asbestos exposure that affects the lining surrounding internal organs in the body. There are three types of mesothelioma. The most common, accounting for about 70% of cases, is pleural mesothelioma that affects the lining surrounding the lungs and internal chest cavity. Mesothelioma can also affect the peritoneum (the lining of the abdominal cavity) and the pericardium (the lining surrounding the heart). 

Mesothelioma is not lung cancer. Lung cancer affects the inside of the actual lung, while mesothelioma affects the lining that surrounds the lungs or other internal organs. Mesothelioma is caused by exposure to asbestos, a naturally occurring compound that was used in many construction materials and consumer products in the 1940s through 1970s. People may have been exposed to asbestos at work, during military service, at home, and at school. 

Symptoms of mesothelioma are similar to those presented from other more common diseases and may not appear until decades after exposure. If you know you have been exposed to asbestos and are experiencing symptoms, seek medical care and tell your doctor about your asbestos exposure. Your doctor will conduct physical exams and tests to rule out more common illnesses like tuberculosis, and will not diagnose mesothelioma until biopsy results and pathology reports conclusively show the cancer. Treatment options will be discussed and determined with you.

epidemiology mesothelioma

Cancer Clusters

When beginning to learn about cancer clusters it is important to know the facts about cancer itself.  Cancer is the growth and spread of abnormal cells in the body and if left untreated it can often result in death.  There are more than 100 types of the disease and it can attack any part of the body.  

One out of every four adult deaths in the United States can be traced to some form of cancer and according to the National Institute of Health over 1,437,180 new cases of cancer are projected to occur in the United States in 2008. 

Cancer is the leading cause of death in children between the ages of one and 14 and more than 8,000 new cases are expected to be diagnosed this year.   

The most common forms of cancer affecting men are lung, stomach, liver, colorectal and esophagus.  The most common cancers faced by women are breast, lung, stomach, liver, colorectal and cervical.   

Tobacco smoke is the leading cause of cancer and is the single most preventable cause of the disease in the world.  It is estimated that over 30% of all cancer could be avoided by not smoking, eating a healthy diet and participating in daily exercise.   

Early detection and treatment are critical in the management of cancer and it is estimated that one third of all cases could be cured with early treatment.   

Asbestos hazards epidemiology

Asbestos Toxicology

There is some understanding of the etioloy, but the basis for mesothelioma formation is unknown.

Asbestos is inert in natural rock form. For it to be hazardous, it must be turned into fine flakes of dust that can be inhaled. So there is a risk in mining, milling, and abatement activities with friability.

Respirators make effective safety devices. However, dust and fibers go home on the clothing of workers. Self exposure, family exposure: there is a ten times increased risk of mesothelioma in women who live with asbestos workers.

Science has shown that the hazard depends on particle size.

Length of fibers: 2 microns: asbestosis

5 microns: mesothelioma, asbestosis

10 microns: bronchogenic lung cancers associated with cigarette smoking and asbestosis.


Over 3 microns: no mesothelioma

Less than 0.5 microns: mesothelioma

epidemiology occupational safety


In the United States occupational lung disease is the number one cause of on-the-job illness. This disease tops the list both in frequency and severity yet is, in almost every case, preventable. Over 20 percent of men worldwide have been exposed to some type of dangerous occupational irritant that may cause cancer and 5 to 20 percent of women have experienced similar types of exposure.

Occupational asthma is the most common occupational lung disease in the U.S. and approximately 15-23 percent of all new asthma cases are caused by occupational irritants. Individuals who already suffer from asthma may experience increasing severity of symptoms due to workplace exposures.

According to the American Lung Association the cost associated with occupational illnesses is estimated to be over $45 billion per year. Additional indirect costs may make that number climb as high as $229 billion annually.

Ethnicity of employees also seems to play a part in the statistics of occupational disease. In 2005 African American employees held approximately 30 percent of the 70,000 U.S. textile jobs. These individuals were 80 percent more likely to die from byssinosis, a chronic lung disease caused by dust produced in textile factories, than their Caucasian counterparts. Hispanics are the most likely ethnic group to work in high-risk occupations while those of Asian descent are least likely to hold a job that involves dangerous exposure.


Summary of Cancer Clusters

Cancer clusters are not common in the United States and determining the existence of such a cluster is extremely difficult. The vast array of cancers coupled with the large quantity of new cases, make cancer a very common disease.

The significance of cancer clusters should not be dismissed however there are very few proven cases in medical history. The most important information for individuals regarding cancer is healthy living, early detection and cancer screenings.

Taking the time to know and understand the controllable risk factors for cancer can help individuals prevent the disease in their lifetime.


Cancer Cluster Reporting

Each year approximately 1,000 cases of possible cancer clusters are reported.  Of these cases only 5% to 15% are investigated after the initial report.  The bulk of the cases are clearly not cancer clusters and researchers can eliminate the possibility with just a bit of research.

Out of the 5% to 15% of cases that are investigated most are still definitively classified as cancer clusters.  Epidemiologists are rarely able to provide definitive answers however a set of specific questions helps guide the initial investigation.  The following questions from the Cancer Council of New South Wales are similar to those utilized by many local and state health organizations and include:

  • Have all the cases been identified? 
  • Are all the cases of the same (or similar) type? 
  • Are both sexes affected? (If environmental factors, such as chemical contamination or radiation leakage, are suspected to be a cause then a rise in cancer amongst both sexes would be expected.) 
  • Is there statistical evidence to suggest that the number of cases exceeds what would be expected in this population? 
  • Do the cancer types have a known common cause, whether occupational or non-occupational? 
  • Did the persons diagnosed with cancer have a common occupational (or non-occupational) exposure? 
  • Are there known workplace exposures that could have contributed to the occurrence of the cancers? 
  • Did the cancers occur at an appropriate time in relation to the possible workplace exposures? (Some cancers can take many years to develop.) 
  • On the balance of probabilities, is it likely that the identified cancers occurred as a result of occupational exposures? 
  • Are there any plausible non-occupational causes for the apparent cluster? 

Many epidemiologists feel that identifying cancer clusters is so difficult because of the lack of accurate cancer reporting from state to state.  Although the federal department of the Center for Disease Control (CDC) controls the cancer data registry this department must rely on the information provided by each state.  Each state may have different reporting systems making collecting and compiling accurate information difficult.

Further complicating the process is the lack of information on the local and state level regarding environmental risk factors and the existence of possible carcinogens.  Some local governments collect insufficient information and some don’t collect any information at all. 

In recent years there have been investigations of nuclear facilities, hazardous dumping grounds, schools, offices and power lines.  None of these investigations have provided proof that carcinogens exist in the environment nor have they provided definitive evidence of cancer clusters.  Individuals in areas surrounding these sites have reported cancer clusters, however the rates are in line with the expected rate of cancer in these areas.    


Many states have a central registry of cancer statistics for the state.  This data often includes the number of newly reported cancer cases as well as death rates due to cancer.  Epidemiologists use this data to compare expected cancer rates to the actual reported rates.  This information helps in the initial decision when investigating a cancer cluster. 

Some of the most common reports of cancer clusters come from individuals who suspect occupational exposure to carcinogens.  This information is often directed to the National Institute for Occupational Safety and Health (NIOSH).  NIOSH will investigate occupational hazards and assist with determining if a cancer cluster does exist.   

Asbestos hazards epidemiology

Cases of Cancer Clusters

A well-known cancer cluster was discovered in the 1960s and continues to be studied today. The use of asbestos, a naturally occurring mineral, was prevalent in factories, shipyards and building products throughout the 1950s and 1960s. Many individuals who were exposed to asbestos on the job developed a disease known as mesothelioma. Many individuals also developed lung cancer. These diseases were directly linked to asbestos exposure and this exposure is still the largest known cause of mesothelioma in the world today.

Between 1941 and 1971 many women were exposed to a drug called diethylstilbestrol (DES). This synthetic hormone was prescribed for women to prevent miscarriage. It was also used to prevent breast engorgement during pregnancy. Over 10 million women were exposed to the hormone and the cancer cluster was discovered after many daughters developed vaginal clear cell carcinoma. This rare cancer is directly linked to exposure in the womb to DES. It is the largest known cause of vaginal clear cell carcinoma.

These two cases are examples of cancer clusters in very specific situations with clear-cut exposures and development of very rare forms of cancer. Other historical cases include the development of leukemia and lymphoma in chemical workers who were exposed to the chemical benzene, dye workers exposed to aniline compounds who developed bladder cancer and farmers who developed skin cancer after extreme exposure to the sun.

Cancer is not the only disease that can sometimes occur in clusters. Other disease clusters have also helped to identify environmental hazards like bacteria contaminated foods, and a respiratory illness now known as SARS. Epidemiologists study these types of environmental factors as well as those known to cause cancer.


Heredity and Environment in Relationship to Cancer Clusters

Heredity and environment are the main contributing factors to the development of cancer. Epidemiologists focus on these two areas when studying cancer clusters. Behavior and lifestyle also contribute to cancer risks so they are also included when studying environment.

To establish a connection between an environmental risk factor and a genetic predisposition is difficult. Scientists must study large groups of people over the course of a long period of time to understand the process of carcinogenesis (the process of normal cells becoming cancer cells). This makes establishing the existence of cancer clusters a lengthy process.

The exposure to certain carcinogenics may also not affect health for many years. In the case of asbestos exposure, it often takes decades for mesothelioma to develop after exposure to the substance. Therefore, the cancer cluster was not identified until many years after the initial exposure began.


Researchers have made some discoveries in regards to heredity and cancer. They have determined that all cancers are caused by some form of gene mutation, which causes the gene to perform abnormally.

Some of these abnormalities are present in an individual at birth. These types of gene mutations are passed down from parent to child. Although this is an uncommon source of cancer it does create “inherited susceptibility” or a predisposition to cancer. Inherited susceptibility does not mean cancer will definitely occur in an individual, however it does indicate a condition where, if other exposures and factors exist, cancer may occur.

Cancer clusters can often be suspected within families due to a higher than expected rate of disease within the family. These cases can sometimes be attributed solely to coincidence. Other times these “familial clusters” can be due to inherited susceptibility coupled with environmental factors.

Somatic alterations are changes that occur to genes throughout an individuals lifetime. These alterations are more likely than inherited susceptibility to be the cause of cancer.


Environmental exposures have often been suspected by individuals to be the cause of cancer clusters. The environmental factors that are considered by epidemiologists include the air, water and earth surrounding an individual or group. Other environmental factors that influence cancer clusters are home and workplace conditions, tobacco use, drugs and alcohol, chemical exposures, and exposure to radiation or even sunlight.

Due to varying susceptibility in individuals and the vast array of chemical and other substances in our world identifying specific causes of cancer is extremely difficult.

Workplace discoveries in regards to chemical exposures have provided positive results in the study of dangerous exposures to certain substances. Certain occupational studies have proved the toxicity of chemical carcinogens and led researchers to develop ways of reducing exposure.

Causes of Cancer Clusters

There are many risk factors for cancer in the world today. Smoking, poor diet, lack of exercise and inactivity are some of the preventable cancer risks. Carcinogens in the environment can also cause cancer. Cancer clusters, once identified as such, often may not have an underlying cause.

There are often several explanations when considering cancer clusters including coincidence, external behaviors (smoking) and miscalculation of historic incidences. When these factors do not explain the cancer cluster, a deeper, more thorough study can be conducted. These often take many years and do not always provide a clear and definitive answer.


Epidemiology and Cancer Clusters

Epidemiology is the study of the factors that cause illnesses and disease with in a population. This field of study is critical in the prevention and spread of communicable and non-communicable disease. Epidemiology is used to identify disease from the outbreak through diagnosis and treatment.

Epidemiolgists, those scientists who study the causes of disease, utilize knowledge in biology, sociology and philosophy in an effort to understand and communicate risk factors for disease.

Carcinogenesis, or the process by which normal cells become cancer cells, is studied by epidemiologists to determine the existence of cancer clusters.

Cancer clusters are rare and epidemiologists determine, through specific criteria if and when a cancer cluster exists. Many individuals will report a cancer cluster but it is the job of an epidemiologist to determine if there is an environmental cause for the incidence of cancer or if the high rate of cancer is merely coincidence.

An epidemiologist will utilize several specific criteria to decide if further investigation is necessary in regards to a cancer cluster. When one type of cancer is found in a population in large numbers this may be a warning sign of a cancer cluster. There are over 100 varieties of the disease so it is rare for one, specific type to be found in large numbers in a population.

There are rare forms of cancer and there are more common forms of the disease. If a rare form of cancer is found in high numbers within a community or group this may also signify a cancer cluster.

A third warning sign to the existence of a possible cancer cluster is when a specific age group in an area suffers from a form of cancer that does not usually affect that age group.

In the beginning stages of determining the existence of a cancer cluster, epidemiologists must first assess the health of the individuals affected. If the cancer is a result of an underlying infection or due to the spread of cancer from another area of the body, this individual is not part of a cancer cluster. The primary cancer is the only cancer considered in the study of a cancer cluster.

Epidemiologists will use biological knowledge to study the significance and causes of the particular type of cancer. The scientist will use knowledge of biological causes of disease to determine if exposure to environmental factors has the ability to cause the specific type of cancer.

In the process of deciding whether or not a cancer cluster truly exists it is important for the epidemiologist to define the borders of the geographical area. Often there are cases in outlying areas causing additional individuals to become involved in the study. These situations may create false cancer clusters.

It is very difficult to determine with absolute certainty if the number of cancer cases is higher that what would be normally expected. The use of statistical factors such as age, gender and race are utilized but lifestyle factors may cause the numbers to be different than what would normally be expected.

It is not possible in all cases to make a determination that a cancer cluster does exist. In some situations the cases of cancer are higher than what would normally be expected within the geographical area in the specified time period, however the epidemiologists cannot find an underlying cause for the disease.

In some situations the study of a cancer cluster cannot be completed due to the small amount of subject cases. The epidemiologist must have enough cases of the disease to provide accurate conclusions.

Even in situations when there are many cases, the history of the individuals involved may make specific determinations regarding exposure impossible. For example, an individual may have lived in many areas over the course of a lifetime and determining when and where exposure occurred would then be impossible.


Factors Affecting Cancer Rates

The age of a group of individuals will affect the rate of cancer development. Individuals over the age of 45 have a much greater risk of developing cancer than people younger than 45 years of age. Men and women over the age of 60 have an even greater risk of developing some form of cancer. It would stand to reason that these age groups would have more cases of cancer than other groups in younger age brackets.

Other factors that may affect the rates of cancer development in a particular group are lifestyle behaviors. Individuals with a poor diet may experience a higher cancer rate than those who eat healthfully. A diet rich in vegetables, fruit, lean meats and fish and healthy fats can help prevent cancer in some individuals.

Exercise is also an important part of keeping cancer at bay. Individuals who do not participate in any regular form of exercise are at a greater risk for developing cancer. According to the American Cancer Society one third of all cancer deaths are in some way related to nutrition, obesity, and/or physical inactivity, and could possibly be prevented.

In one study conducted by Dr. Christine Friedenreich, of Alberta Health Services-Alberta Cancer Board in Calgary, Canada, it was discovered that women with breast cancer who had exercised more than four hours per week over their lifetime had a 44 percent lower risk of dying from the disease. Other studies have found similar results with other forms of the disease.

“Greater participation in physical activity has consistently been associated with reduced risk of cancer incidence at several sites, including breast and colon cancers,” said James McClain, Ph.D., cancer prevention fellow at the National Cancer Institute.

Another enormously risky lifestyle behavior is tobacco usage. It is estimated that discontinuing the use of tobacco could prevent one third of all cancers.

These lifestyle factors may cause the incidence of cancer to increase in certain groups such as families or employees that participate in these high-risk behaviors. Cancer clusters may seem to exist however it is lifestyle behaviors, not external environmental carcinogens that cause the unusually high incidence of cancer in these groups.