You likely already know that cigarettes and certain types of radiation cause cancer, but there are actually over 200 other substances in the U.S. National Institutes of Health (NIH) just-released 14th annual Report on Carcinogens (ROC) that are believed to lead to malignant tumors in human beings. And seven of those were just added.

Five of the new substances added are actually viruses that either cause cancer directly or make it easier for other cancer-causing viruses to do their deadly work. They include: Human immunodeficiency virus type 1 (HIV-1); Human T-cell lymphotropic virus type 1 (HTLV-1); Epstein-Barr virus (EBV); Kaposi sarcoma-associated herpesvirus (KSHV); Merkel cell polyomavirus (MCV). All five of these viruses are classified as "Known to be a human carcinogen," as opposed to the agency's other category of "Reasonably anticipated to be a human carcinogen."

Viruses might not commonly be thought about as cancer-causing substances, but according to the National Institutes of Health (NIH), they are a real threat.

"Given that approximately 12 percent of human cancers worldwide may be attributed to viruses, and there are no vaccines currently available for these five viruses, prevention strategies to reduce the infections that can lead to cancer are even more critical," said Linda Birnbaum, Ph.D., director of the National Institute of Environmental Health Sciences (NIEHS) and National Toxicology Program (NTP). "The listings in this report, particularly the viruses, bring attention to the important role that prevention can play in reducing the world's cancer burden."

HIV-1 made the list because it serves up a double threat. It has not only been implicated in increasing the risk of cancer, including non-melanoma skin cancer, eye cancer, and possibly lung cancer, but by weakening the immune system it allows other cancer-causing viruses to move in, which can lead to "non-Hodgkin and Hodgkin lymphomas; anogenital cancers, including penile, vaginal/vulvar, cervix, and anal; Kaposi sarcoma; and possibly oral-related cancers and liver cancer," according to the agency.

As for Epstein-Barr, the NIH says that 90 percent of all adults are already infected with the herpes-related virus but most of our immune systems hold it in check. It has, however been implicated in four types of lymphoma: "Burkitt, Hodgkin, immune-suppression-related non-Hodgkin, and nasal type extranodal NK/T-cell — and two types of epithelial cancer — nasopharyngeal cancer and some types of stomach cancer," says the agency.

In addition to the five viruses, the industrial solvent Trichloroethylene (TCE) has received a bump in status from "reasonably anticipated human carcinogen" to "known human carcinogen" due to its role in increasing kidney cancer risk.

"There are many ways people can be exposed to TCE," says the NIH. "It can be released into the air, water, and soil at places where it is produced or used. It breaks down slowly and can move readily through soil to make its way into underground drinking water sources. Because of its widespread use as a metal degreasing agent to maintain military equipment, it has been found in the groundwater at many military and Superfund sites."

The final of the seven cancer-causers are "cobalt and cobalt compounds that release cobalt ions in vivo" which join the list with "reasonably anticipated" status.

"Cobalt is a naturally occurring element used to make metal alloys and other metal compounds, such as military and industrial equipment, and rechargeable batteries," says the agency. "The highest exposure occurs in the workplace and from failed surgical implants. The listing for this metal and its compounds is based largely on studies in experimental animals."

Cobalt is also found in Vitamin B12, but because the cobalt is bound to proteins in the vitamin, it does not release ions in the body and therefore is not considered a carcinogen.

When cobalt ions are taken up in the cell, they can interfered with the DNA repair process and cause other damaging effects that could lead to cancer, although the method through which the ions can lead to tumor growth is still not thoroughly understood. Cobalt ions replace cobalt sulphate in the report, which was added to the list in 2004.

Substances are reviewed for the ROC by the NIH's National Toxicology Program (NTP) after being submitted by researchers and after a period of public commentary. The full process can be seen in the infographic below. The complete 14th annual Report on Carcinogens is available to the public as a PDF from the NTP.

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