“What lies behind you and what lies in front of you, pales in comparison to what lies inside of you,” Ralph Waldo Emerson said. But getting from Point A to Point B, that’s life – and what you choose to do in life may increase your chance of cancer. (Spoiler alert: Your lifestyle may increase the risk of disease; some careers and cancer seem to go hand in hand.)

You’re active, you eat right, you do what’s right in regard to expert recommendations. … And the oncologist says you have cancer. Fact is, your job may have more influence on cancer than doing all the “right” things.

Scary, isn’t it?

Did you know:

  • The International Association of Firefighters says cancer is the leading cause of death among firefighters. Thirty years ago, firefighters were most often diagnosed with asbestos-related cancers. Today, the cancers are more often leukemia, lymphoma, or myeloma.

Fire departments in Boston, Calgary, Chicago, Houston, Los Angeles, New York, San Francisco, Seattle, and Toronto all report elevated cancer rates. The most aggressive cancers were oral, digestive, respiratory and urinary.

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  • Flight attendants get more breast cancer and melanoma – and non-melanoma skin cancer, uterine, gastrointestinal, cervical, and thyroid cancers are now seen at a higher rate in flight attendants. Air cabin crews receive the highest yearly dose of ionizing radiation on the job of all U.S. workers.

Among female flight attendants, the rates of breast cancer were about 50 percent higher than in women from the general population. Cancer rates in male flight attendants were nearly 50 percent higher for melanoma and about 10 percent higher for nonmelanoma skin cancers compared with men from the general population group.

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  • A 2014 study published in the Journal of the National Cancer Institute found people who spend more hours of the day sitting have up to a 66 percent higher risk of developing certain types of cancer than those who aren’t as sedentary. Dr. Graham Colditz noted, “It’s not enough to just be active – it’s also important to sit less.”

Sedentary behavior was associated with a 24 percent greater risk of developing colon cancer, a 32 percent higher risk of endometrial cancer, and a 21 percent increased risk of lung cancer.

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And what you do when sedentary can cause health issues. In 2009, researchers examined the link between blood pressure in children and their choice of inactive pastimes, including watching TV, using the computer and reading.

Of all the forms of inactivity they examined, television-viewing was the worst. It was linked to significantly higher blood pressure – the more TV kids watched, the higher their blood pressure – and the effect held true regardless of whether a child was heavy or at a healthy weight.

“These results show that sedentary behavior, and more specifically television-viewing, is related to blood pressure independent of body fat or obesity level,” said Dr. Joey Eisenmann, one of the study’s co-authors.

Importance of circadian disruption

In October 2007, 24 scientists from 10 countries met at the International Agency for Research on Cancer (IARC), Lyon, France, to assess the carcinogenicity of shift-work, painting, and fire-fighting.

About 15-20 percent of the working population in the U.S. and Europe is engaged in shift work that involves night work, which is most prevalent (above 30 percent) in the healthcare, industrial manufacturing, mining, transport, communication, leisure, and hospitality sectors. Among the many different patterns of shift work, those including night work are the most disruptive for the circadian clock.

That study also found the incidence of breast cancer was also increased in female flight attendants, who also experience circadian disruption by frequently crossing time zones.

Thales Y. Papagiannakopoulos

“Circadian rhythms are 24-hour oscillations that control a variety of biological processes in living systems, including two hallmarks of cancer – cell division and metabolism.”

Thales Papagiannakopoulos

More than 20 studies investigated the effect of constant light, dim light at night, simulated chronic jet lag, or circadian timing of carcinogens, and most showed a major increase in tumor incidence. No clear effect was seen for light pulses at night or in constant darkness.

A similar number of studies investigated the effect of reduced nocturnal melatonin concentrations or removal of the pineal gland (where melatonin is produced) in tumor development and most showed increases in the incidence or growth of tumors.

“The impact of environmental/physiologic factors on tumorigenesis remains poorly understood. A major consequence of a modern lifestyle is the disruption of circadian rhythms,” said Thales Y. Papagiannakopoulos, Ph.D.

“Circadian rhythm disruption by shift-work is associated with greater risk for cancer development and poor prognosis, suggesting a putative tumor suppressive role for circadian rhythm homeostasis. Circadian rhythms are 24-hour oscillations that control a variety of biological processes in living systems, including two hallmarks of cancer – cell division and metabolism.”

The link between sleep deprivation and cancer

The International Agency for Research on Cancer (IARC) is part of the World Health Organization (WHO). One of its major goals is to identify the causes of cancer. The most widely used system for classifying carcinogens comes from the IARC.

Substances labeled as carcinogens may have different levels of cancer-causing potential. Some may cause cancer only after prolonged, high levels of exposure. And for any particular person, the risk of developing cancer depends on many factors, including how they are exposed to a carcinogen, the length and intensity of the exposure, and the person's genetic makeup.

In the past 30 years, the IARC has evaluated the cancer-causing potential of more than 900 likely candidates, placing them into one of the following groups:

  • Group 1: Carcinogenic to humans
  • Group 2A: Probably carcinogenic to humans
  • Group 2B: Possibly carcinogenic to humans
  • Group 3: Unclassifiable as to carcinogenicity in humans
  • Group 4: Probably not carcinogenic to humans

Environmental factors can include a wide range of exposures, such as:

  • Lifestyle factors (nutrition, tobacco use, physical activity, etc.)
  • Naturally occurring exposures (ultraviolet light, radon gas, infectious agents, etc.)
  • Medical treatments (radiation and medicines including chemotherapy, hormone drugs, drugs that suppress the immune system, etc.)
  • Workplace exposures
  • Household exposures
  • Pollution

A person's risk of developing a particular cancer is influenced by a combination of factors that interact in ways that are not fully understood. Some of the factors include:

  • Personal characteristics such as age, sex, and race
  • A family history of cancer
  • Diet and personal habits such as cigarette smoking and alcohol consumption
  • The presence of certain medical conditions or past medical treatments, including chemotherapy, radiation treatment, or some immune-system suppressing drugs.
  • Exposure to cancer-causing agents in the environment (for example, sunlight, radon gas, air pollution, and infectious agents)
  • Exposure to cancer-causing agents in the workplace

Millions of U.S. workers are exposed to substances that have been tested as carcinogens in animal studies or found to be possibly carcinogenic in human studies. However, less than 2 percent of chemical or physical agents manufactured or processed in the U.S. have been evaluated by the International Agency for Research on Cancer for carcinogenicity.

Based on well-documented associations between occupational exposures and cancer, it has been estimated that 3-6 percent of all cancers worldwide are caused by exposures to carcinogens in the workplace. Using cancer incidence numbers in the U.S., this means that in 2012 (the most recent year available), there were between 45,872 and 91,745 new cancer cases that were caused by past exposure in the workplace.

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