A good question and any answer depends on the specific work and lifestyle.
Here is what the CDC says:
Based on well-documented associations between occupational exposures and cancer, it is estimated that approximately 20,000 cancer deaths and 40,000 new cases of cancer each year in the U.S. are attributable to occupation.
Millions of U.S. workers are exposed to substances that have tested as carcinogens in animal studies. However, less than 2% of chemicals in commerce have been tested for carcinogenicity.
Cancer is a group of different diseases that have the same feature, the uncontrolled growth and spread of abnormal cells. Each different type of cancer may have its own set of causes. Many factors play a role in the development of cancer. The importance of these factors is different for different types of cancer. 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
• Family history of cancer
• Diet and personal habits such as cigarette smoking and alcohol consumption
• The presence of certain medical conditions
• Exposure to cancer-causing agents in the environment
• Exposure to cancer-causing agents in the workplace
In many cases, these factors may act together or in sequence to cause cancer.
From a percentage aspect it has been estimated that from about 5 to 15% of cancers are either due to exposure to chemicals at work or significantly contributed by work exposure, which may be from living or inanimate exposures. Health care workers, for example, are at risk of primary liver cancer related to Hepatitis B and C. Much depends on your specific kind of work.
The 5-15% range above, however, is not firm. You will see many internet sites, especially law firm sites, that place the percentages much higher. The range overall is represents an approximation, and lung cancer rates are likely to be on the high side, offset by lower rates for other organs. The risk of cancer is affected by not only exposures, but by lifestyle and genetic factors, a fact seen clearly when identical twins are encountered with one developing a specific cancer and the other twin escaping the cancer.
Haz-Map, a service of the National Library of Medicine, provides a slightly different estimate:
In developed countries, 30% of cancers are attributed to diet or nutrition, 16% to tobacco, 8% to infections, 5% to occupational exposures, 2% to environmental pollution, and 39% to other causes. [Global Cancer Facts & Figures 2007, p. 4] "Of course, the major exposure to carcinogens is not through variations in the external environment (e.g., air, water, radiation) so much as in differences in lifestyle (e.g., reproduction, diet, tobacco use).
So nobody is really sure how much cancer is from work and it obviously has to very according to occupation. Fairly obviously, those occupations that require handling of known cancer-causing chemicals are the most risky. It is not reassuring to discover that very few industrial chemicals have been studied specifically for their potential to cause cancer. As a result, the list of known cancer causing agents (carcinogens) is surprisingly short. It is compiled by the U.S. National Toxicology Program and periodically revised. IARC, the International Agency for Research on Cancer, has virtually the same list of suspect and known cancer-causing chemicals.
Smokers suffer an inordinate amount of work-related cancer as the effects may not only add to the risk when combined with work exposures, but actually be synergistic, or multiplied. Perhaps the best example of this is lung cancer in asbestos workers. Either smoking or asbestos (either alone) significantly raises lung cancer risk, but combining both raises lung cancer risk at least 30 times. Smoking remains the single most preventable cause of premature death. It has multiple adverse and dangerous effects on all organs of our bodies and is linked also to kidney and bladder cancer. It seems to promote the growth of colon polyps that if unchecked, may lead to colon cancer.
A finding of an unexpected increase in breast cancer in night workers seemed inexplicable for a time when first reported several years ago. Now the most likely answer seems to be that night workers are exposed to sunlight less than day workers. The link here is most likely Vitamin D, which is needed for vitamin D to be produced in the skin with sun exposure. This then suggests that a lack of Vitamin D itself is the culprit. Some studies suggest the risk of other internal cancers also rises in night workers, but more work has to be done to settle the issue, but once again, tampering with the normal rhythms of life can result in some very unexpected outcomes.
That not all cancers are listed by the government on the list of suspect or known carcinogens is shown by reports of increased cancers such as lymphomas in farmers that use a lot of pesticides. Also, hair dyes are similarly suspect for lymphomas. Most likely some chemicals promote, but do not outright cause cancer. But one thing we do know from studies over the past 50 years is that is the leading cause of premature death, ie, something over which we have control.
Preventing cancer is an effort of both avoiding cancer causing or cancer promoting chemicals on one hand and including in our diets those foods that prevent cancer. An internet search for cancer-preventing foods will show a substantial list of beneficial foods and spices, but another search for the worst pesticide foods may encourage you to either have your own garden or consider organic sources for the worst offenders. Thoroughly washing will help reduce pesticide residues. Most people who search out these best foods will almost automatically be using less fat in their diets, another problem area for cancer.
Since hepatitis B and C are associated with later liver cancer, avoiding risky lifestyles associated with hepatitis is important. Healthcare workers can reduce their risk of cancer from hepatitis with vaccine. A new vaccine for sexually transmitted papillovirus (HPV vaccine) has the potential of striking down cervical cancer. making a significant dent in the incidence of prostate cancer.
Just following the above will dramatically lower both your cancer and heart disease risk, the main causes of premature death.