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The word “carcinogen” tends to get people’s attention. We see headlines about carcinogenic foods, chemicals, environmental exposures, and everyday products, and it can be easy to assume that anything labeled a carcinogen is essentially a guarantee of cancer. Fortunately, biology is not quite that simple. A carcinogen is not something that automatically gives you cancer. It is an exposure that has the potential to cause cancer, and the actual risk can vary considerably depending on the amount and duration of exposure, how the exposure occurs, and individual factors.
What Is a Carcinogen?
According to the National Human Genome Research Institute, a carcinogen is a substance, organism, or agent capable of causing cancer. Carcinogens can occur naturally, such as ultraviolet radiation from sunlight and certain viruses, or they can be generated by human activity, such as cigarette smoke and automobile exhaust. Many carcinogens contribute to cancer by damaging DNA and creating mutations. Others can promote cancer through different mechanisms, including chronic inflammation or increased cellular proliferation. Common examples include tobacco smoke, asbestos, ultraviolet radiation, certain infections, alcohol, radon, and processed meat. The Cleveland Clinic divides carcinogens into three broad categories: physical, chemical, and biological. Most importantly, exposure to a carcinogen does not mean you will develop cancer.
How Does Something Become a “Known Carcinogen”?
You may have noticed that carcinogens are often placed into categories such as “known,” “probable,” or “possible.” These classifications can sound like a ranking of how dangerous something is, but that’s not quite what they mean. The American Cancer Society explains that a known human carcinogen is an exposure for which there is sufficient or conclusive evidence that it causes cancer in humans. A probable carcinogen has strong evidence suggesting that it causes cancer, but the evidence is not yet considered conclusive. Organizations such as the International Agency for Research on Cancer (IARC) and the U.S. National Toxicology Program (NTP) independently review scientific evidence and classify substances and exposures.
The IARC classification system includes:
- 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 in humans
It is important to understand what these categories actually represent. They describe the strength of the evidence that something can cause cancer; not the size of the cancer risk. For example, tobacco smoking and processed meat are both classified by IARC as carcinogenic to humans. That does not mean they carry the same level of cancer risk.
Hazard vs. Risk: What’s the Difference?
This brings us to one of the most important concepts in understanding carcinogens: hazard is not the same thing as risk. Think about a swimming pool. A swimming pool can be hazardous because drowning is possible, but that doesn’t mean everyone who goes swimming has a high probability of drowning. Your actual risk depends on factors such as your swimming ability, supervision, and how long you spend in the water. Carcinogens work similarly. Scientists consider factors such as dose, duration, frequency, route of exposure, and individual susceptibility when determining how an exposure may affect cancer risk.
The American Cancer Society’s explanation of how carcinogens are evaluated emphasizes that scientists use a combination of laboratory research and epidemiological studies to determine whether an exposure is associated with cancer. The National Academies’ discussion of carcinogen risk assessment also highlights the importance of dose-response relationships and the uncertainty involved in estimating risk. Likewise, the U.S. Environmental Protection Agency uses quantitative risk assessment methods to estimate how particular levels of exposure may translate into cancer risk. So instead of simply asking, “Is this carcinogenic?”, a better question is: “How much exposure is associated with how much additional risk?”
The Processed Meat Example
Processed meat is a great example of how confusing cancer statistics can become. You have probably seen a headline along the lines of “Eating processed meat increases your risk of colorectal cancer by 18%!” That sounds pretty alarming. And the number isn’t made up. The World Health Organization reports that an analysis of 10 studies estimated that eating 50 grams of processed meat every day was associated with an approximately 18% increase in colorectal cancer risk.
But that 18% is a relative risk increase, not an 18-percentage-point increase in your chance of developing cancer. For example, if a hypothetical group of 100 people has a 5% baseline lifetime colorectal cancer risk, an 18% relative increase would raise that risk to approximately 5.9%. That’s an increase of about 0.9 percentage points, not 18 percentage points. The distinction between relative and absolute risk is important when interpreting medical headlines because a percentage can sound dramatic without telling you how large the underlying risk actually is.
Of course, that does not mean processed meat is harmless. IARC classified consumption of processed meat as carcinogenic to humans based on sufficient evidence of an association with colorectal cancer, and the risk also increases with greater consumption. The MD Anderson Cancer Center recommends limiting processed meat and explains that foods such as bacon, hot dogs, deli meats, sausage, salami, and similar products fall into this category. The goal isn’t to panic over a single hot dog. It is to understand how repeated exposure over time can influence overall cancer risk and make reasonable decisions accordingly.
What Else Is on the Carcinogen List?
Once you start looking at carcinogen classifications, the list can seem surprising. IARC Group 1, for example, includes substances and exposures such as tobacco smoking, alcoholic beverages, asbestos, ultraviolet radiation, certain viral infections, and processed meat. That list can create some strange comparisons. Alcohol and tobacco are both Group 1 carcinogens, but that classification does not mean having one alcoholic drink carries the same cancer risk as smoking a cigarette.
The Compound Interest breakdown of IARC’s carcinogen classifications provides a helpful visual explanation of this concept and highlights some of the surprising substances and exposures that appear across the different categories. Again, the classification tells us about the strength of the evidence for carcinogenicity, not a simple “danger score.”
Don’t Let the Headlines Do the Thinking for You
Cancer is complicated, and complicated science does not always make for compelling headlines. “Study finds substance associated with small increase in cancer risk” isn’t nearly as attention-grabbing as “Scientists Discover Common Household Product CAUSES CANCER!” But the second headline may leave out most of the information you actually need.
When you encounter a cancer-related claim, ask: Was the study performed in humans or animals? What dose was used? How long were people exposed? Was the increase in risk relative or absolute? Was the evidence strong enough to establish causation, or was it simply an association? These questions can dramatically change how we interpret the same piece of information.
This is also why it’s important to be skeptical of claims that certain everyday products or foods “cause cancer” simply because a chemical has been detected in them. The presence of a chemical does not automatically tell us that the exposure is large enough to produce a meaningful increase in cancer risk. As discussed by Compass Oncology, available evidence does not support many popular cancer claims in the simplistic way they are often presented online.
What Can You Actually Do?
Understanding carcinogens isn’t about becoming afraid of everything around you. You cannot eliminate every carcinogen from your environment, but you can often reduce exposure to some of the better-established risk factors. That might mean avoiding tobacco products and secondhand smoke, limiting or avoiding alcohol, protecting your skin from excessive ultraviolet radiation, reducing regular consumption of processed meat, following appropriate vaccination recommendations for cancer-associated infections, paying attention to occupational exposures when working with hazardous substances, and following recommended cancer screening guidelines.
Most importantly, don’t let the word “carcinogen” make you panic. Cancer risk is influenced by a combination of genetics, environment, lifestyle, age, exposures, and many other biological factors. The goal isn’t to live in a world without carcinogens. The goal is to understand which exposures matter, how much they matter, and what you can realistically do about them.
Sources
- https://www.genome.gov/genetics-glossary/Carcinogen
- https://my.clevelandclinic.org/health/articles/25081-carcinogens
- https://www.cancer.org/cancer/risk-prevention/understanding-cancer-risk/known-and-probable-human-carcinogens.html
- https://www.cancer.org/cancer/risk-prevention/understanding-cancer-risk/determining-if-something-is-a-carcinogen.html
- https://www.ncbi.nlm.nih.gov/books/NBK232623/
- https://www.epa.gov/fera/risk-assessment-carcinogenic-effects
- https://www.who.int/news-room/questions-and-answers/item/cancer-carcinogenicity-of-the-consumption-of-red-meat-and-processed-meat
- https://www.mdanderson.org/cancerwise/processed-meat-and-cancer-what-you-need-to-know.h00-159778812.html
- https://www.compoundchem.com/2023/07/14/carcinogens/
- https://www.compassoncology.com/blog/5-common-cancer-myths-debunked
