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The IARC's Cancer Classification System

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As an agency of the World Health Organization (WHO), the International Agency for Research on Cancer (IARC) is the world’s definitive leader in cancer research and discovery. To help the public understand the negative effects of various carcinogenic substances, the IARC developed a simple classification system that groups substances based on their risk level to human health.

Group 1: “Carcinogenic to humans”

Group 2A: “Probably carcinogenic to humans”

Group 2B: “Possibly carcinogenic to humans”

Group 3: “Not classifiable as to its carcinogenicity to humans”

IARC Group 1: “Carcinogenic to humans”

According to the Preamble to the IARC Monographs, agents classified under Group 1 have been found to contain “sufficient evidence of carcinogenicity in humans” and are factually cancer-causing. Some of the items below are more dangerous than others found in this list— like tobacco smoke, for example— but Group 1 substances should generally be treated with caution and consumed lightly.

Commonly-recognized Group 1 substances and factors:

Tobacco smoke
Cigarette smoke contains several Group 1 carcinogens, including arsenic, benzene, cadmium, formaldehyde (upon combustion), nickel, and other chemicals. 30% of ALL cancer deaths in the U.S. are caused by cigarettes. According to the Centers for Disease Control and Prevention (CDC), tobacco smoke causes a wide variety of cancers, including lung, mouth, throat, pancreatic, bladder, stomach, colon, liver, cervical, and kidney cancer.
Alcoholic drinks
Ethanol, which is a consumable alcohol found in all alcoholic beverages, can cause several different cancers to develop over a long period of time, including oral, larynx, colorectal, esophageal, liver, and breast cancer. If you choose to drink, sticking to 1-3 alcoholic beverages a week is your best bet to keep your cancer risk low.
Processed meats
Sausages, hotdogs, bacon, salami, chorizo, corned beef, and cold cuts can cause colorectal or stomach cancer when eaten frequently. Consuming alcohol and smoking tobacco are much more likely to cause cancer than eating processed meats, so don’t be alarmed by this classification. Limit processed meat intake to once or twice a week to keep your risk low.
Asbestos
Formerly used in boilers, sealants, pipes, cement products, foam insulation, and other building materials. Still used in brake pads, automobile clutches, and some roofing materials. Traces of asbestos have been reportedly found in various talcum (baby) powder products.
Formaldehyde
Used in glues, resins, building materials, disinfectants, foam insulation, and many personal care products, including some chemical hair straighteners. Significant exposure to formaldehyde is a known cause of nasal cancer, and strong evidence suggests that long-term exposure may also be connected to leukemia.
Solar (UV) radiation
UV radiation can cause skin cancer in multiple ways, including overexposure to the sun, as well as the usage of tanning beds, sunlamps, UV phototherapy, black-light lamps, and other complex types of lamps.

IARC Group 2A: “Probably carcinogenic to humans”

Any products classified in Group 2A do not have enough sufficient evidence to be considered be cancerous, but, according to the Preamble to the IARC Monographs, they have been found to be “possibly carcinogenic to humans” after passing at least one of these three evaluations:

“Limited evidence of carcinogenicity in humans”
“Sufficient evidence of carcinogenicity” in animal test subjects
“Strong evidence that the agent exhibits key characteristics of carcinogens

Commonly-recognized Group 2A substances and factors:

Red meat
Beef, pork, veal, goat, lamb, and mutton are all considered to be red meats, which can likely cause colorectal, pancreatic, or prostate cancer after long-time consumption. The risk level of developing cancer with red meat in your diet is fairly lower than with a diet full of processed meat, but red meat intake should still be limited.
Glyphosate
Widely-used herbicide found in Roundup weed killer and several other broad-spectrum herbicides. Long-term glyphosate exposure has been linked to various forms of blood cancer, as well as lung, kidney, and testicular cancer.
Night shift work
A disruption in your sleep cycle— which occurs often for nighttime workers— will lower your body’s melatonin levels. The National Library of Medicine suggests that melatonin might prevent tumors from growing and spreading to other cells while you sleep. Those who work at nighttime and have lower melatonin levels, therefore, may have an increased risk of developing breast cancer.
Androgenic (anabolic) steroids
Using steroids to increase testosterone production has been found to possibly cause the development of liver tumors, both benign and cancerous.

IARC Group 2B: “Possibly carcinogenic to humans”

Any products classified in Group 2B do not have enough sufficient evidence to be considered be cancerous, but, according to the Preamble to the IARC Monographs, they have been found to be “possibly carcinogenic to humans” after passing at least one of these three evaluations:

“Limited evidence of carcinogenicity in humans”
“Sufficient evidence of carcinogenicity” in animal test subjects
“Strong evidence that the agent exhibits key characteristics of carcinogens

Commonly-recognized Group 2B substances and factors:

Gasoline (and engine exhaust)
The most well-known fuel for vehicle engines. Gasoline is generally not dangerous for when humans are exposed for a short time, but benzene, a natural chemical found in gasoline, is a known carcinogen. At high levels, benzene and other harmful chemicals could cause cancer for anyone who works with gasoline.
Pickled vegetables
Vegetables pickled for a long period of time in an acidic brine (often made with vinegar) have been found with limited evidence to cause stomach cancer, even though pickled vegetables have been praised for improving gut health.
Aspartame
The sugar alternative regularly used in diet sodas such as Diet Coke, Coke Zero, Sprite Zero, Fanta Zero, Diet Barq’s Root Beer, and other drinks. Long-term, frequent consumption of aspartame has been linked to, with limited evidence, an increased risk of developing blood cancers such as leukemia and lymphoma.
Human Papillomavirus (HPV)
The most common sexually-transmitted disease in the United States. It often exhibits no symptoms, but it can sometimes cause genital and oral warts. There are around 200 types of HPV, but 12 of them are considered “high-risk” and can cause several types of cancer if they persist for several years.
Aloe vera extract (non-decolorized)
Most aloe vera products are decolorized, meaning the natural carcinogenic chemicals found in aloe vera have been filtered out. However, aloe vera products that are non-decolorized have been linked to an increased risk of developing colon cancer. However, this risk only applies to aloe vera that is consumed orally instead of applied on the skin.

IARC Group 3: “Not classifiable as to its carcinogenicity to humans”

As the Preamble to the IARC Monographs notes, items classified in Group 3 have NOT necessarily been declared as non-cancerous. The carcinogenic potential of most Group 3 agents has yet to be determined due to insufficient research, but some substances in this group have been fully evaluated as non-carcinogenic with “evidence suggesting lack of carcinogenicity.”

Commonly-recognized Group 1 substances and factors:

Polypropylene
One of the world’s most widely used plastics, found in furniture, packaging, textiles, electrical components, consumer goods, and many other products. Though polypropylene has little to no cancer risk, the substance has reportedly caused complications in certain hernia mesh models due to its poor adhesion, lack of flexibility, and tendency to shrink.
Caffeine
Consuming high amounts of caffeine has been linked to an increased risk of developing bladder or breast cancer, but no conclusive evidence has been found. In fact, there are some studies linking coffee to a decreased risk of colon cancer.
Hot drinks
Beverages like coffee and hot tea have been researched to determine if drinking very hot liquids can cause esophageal cancer due to thermal injury. No conclusive evidence has been found at this time, but the likelihood of hot drinks causing cancer is very low.
Electric fields
Extremely-low frequency (ELF) electric fields and static electric fields (in addition to static magnetic fields) have been researched to determine if there is a link between being exposed to Wi-Fi and developing cancer. Only a possible link between extremely-low frequency electromagnetic fields (EMF) and an increased number of childhood leukemia cases was discovered.

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