What Is a Carcinogen?

A Substance or Exposure That Promotes Cancer

Carcinogens are substances or exposures that can cause cancer. Examples include home and workplace chemicals, environmental or medical radiation, smoke, and even some viruses and medications.

While it's impossible to eliminate carcinogen exposure, you can and should take steps to reduce what you come in contact with.

This article explores common carcinogens, the research process involved in identifying them (and what that means for you), and steps you can take to limit your exposure.

A process of an asbestos removal
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How Carcinogens Cause Cancer

Carcinogens cause cancer by damaging DNA, what carries genetic information in your cells.

A carcinogen can directly damage DNA and cause changes called mutations. These lead to a disruption in the normal process of growth and cell division.

Other times a carcinogen may cause damage and inflammation, which results in the cells dividing more rapidly. There is always a chance that a mutation will occur when this happens, which in turn increases the chance of developing cancer.

Exposure and Your Risk

Carcinogen exposure may cause cancer, but that doesn't mean that it necessarily will. The ability of a carcinogen to cause cancer depends on several things.

The most obvious ones are the amount and length of exposure. But it also depends on your individual health and other personal factors that either raise or lower your risk of cancer.

The tendency to develop cancer may, for example, be inherited as part of your set of genes, or genome. Known as a genetic predisposition, this means that you are more likely to develop cancer under certain conditions and with certain exposures than someone without the same genetic susceptibility.

It's also important to note that cancer is most often caused by an accumulation of mutations, rather than a single insult. For this reason, a number of factors may work together to either increase or decrease the risk of developing cancer.

Even when damage to DNA occurs, your body can produce proteins that either repair damaged DNA or eliminate damaged cells before a normal cell is transformed into a cancer cell. There are tumor suppressor genes that slow down cell division and do repairs.


Many factors influence whether an exposure to a particular carcinogen will lead to cancer, including the amount and duration of the exposure and your genetic makeup.

Types of Carcinogens

Carcinogens exist in a range of indoor or outdoor environments. They include chemicals, viruses, medications, and pollutants.

Some carcinogens are linked to specific types of cancer.

Home and Workplace Chemicals

Many chemicals used in building material or products in the home or workplace may be carcinogenic.

For example, formaldehyde is a carcinogen that's used in building materials and a variety of furniture and household products. It's commonly found in composite wood products (hardwood plywood, particleboard, and medium-density fiberboard) that emit chemicals into the room. You may hear this referred to as "off-gassing."

Formaldehyde is also a byproduct of lit cigarettes and smoking.

You can reduce your level of formaldehyde exposure by purchasing composite wood products that are certified to reduce emissions, using an air conditioner and dehumidifier if you live in hot and humid area, and improving ventilation in your space (e.g., opening windows, using fans).

Old pipe insulation, attic insulation, textured ceilings, or floor tiles may contain the carcinogen asbestos. If products containing asbestos are disturbed, tiny asbestos fibers are released into the air. Asbestos can lead to a specific type of lung cancer called mesothelioma.

Environmental agencies or contractors who specialize in asbestos can evaluate materials and, if necessary, take steps to carefully remove them.

Environmental Radiation

Ultraviolet (UV) radiation from the sun is a well-known cause of skin cancer

You can protect your skin by wearing hats and protective clothing when outdoors. Also use an SPF 30 broad spectrum sunscreen, which protects you from both UVA and UVB rays, the two types of UV light linked to skin cancer.

Another common form of environmental radiation is radon. It's emitted from the normal decay of uranium in the soil and then gets trapped in homes. Radon is the second leading cause of lung cancer after smoking.

You can have your home tested for radon and install a radon mitigation system if levels are considered high.

Medical Radiation

Radiation exposure that occurs during certain diagnostic tests, such as computed tomography (CT scan), and radiation therapy used to treat cancer is carcinogenic.

For example, women who receive radiation therapy after a mastectomy, a procedure to remove a breast due to breast cancer, are at an increased risk of developing lung cancer due to the radiation that is delivered to the chest area.

However, the risks of not getting such imaging or treatment when needed are often considered greater than those associated with medical radiation.


There are several viruses linked to cancer. These affect healthy cells’ genetic makeups and can make it more likely that they will turn into cancer.

Viruses that can lead to cancer include:

There are HPV vaccines and hepatitis B vaccines that can reduce your risk of getting these viruses and the cancers linked to them. They are only effective if vaccination occurs prior to virus exposure.


Some chemotherapy and hormonal therapy drugs can raise the risk of cancer.

For example, chemotherapy drugs such as Ellence (epirubicin) and Cytoxan (cyclophosphamide), used for early stage breast cancer, can sometimes lead to leukemia.

Oral contraceptive use may increase the risk of breast cancer or cervical cancer, but they may lower the risk of endometrial, ovarian, and colorectal cancers.


Both outdoor and indoor air may contain pollutants that are carcinogenic.

Common outdoor air pollutants from industrial or power plants, engine exhaust, and fire smoke are strongly linked with lung cancer.

Indoor air pollutants can come from a variety of sources such as building materials, fire retardants, paint fumes, cleaning products, and dust. Indoor air pollutants can also lead to the development of lung cancer.

Lifestyle Factors

There are a variety of lifestyle factors that can contribute to mutations that result in cancer.

Lifestyle-related carcinogens include:

  • Smoking or exposure to secondhand smoke
  • Alcohol use
  • Obesity
  • Poor diet (e.g., eating too much processed meat and not much produce)
  • Lack of physical activity

There is also some evidence that cooking meat at high temperatures—grilling or pan frying it, for example—may create carcinogenic compounds that can then be consumed.

Cooking meats for longer times at lower temperatures and removing any charred portions before eating may help reduce your risk.


Carcinogens are in many indoor and outdoor environments. They are in some building materials, car exhaust, and the sun's rays. Certain viruses, cancer treatments, medical scans, and medications are also carcinogenic. Lifestyle-related carcinogens include smoking, drinking alcohol, and poor diet.

Identifying Carcinogens

It is not always easy to determine if a substance or an exposure is a carcinogen. All possible carcinogens likely aren't even known. There are a number of reasons for this.

Not only are there millions of possible carcinogens both in nature and industry, it’s simply not practical or ethical to test every chemical on hundreds of thousands of people to see if they get cancer.

Many studies to evaluate whether or not a substance is a carcinogen—and if so, to what extent—are done on animals using high exposures. Prior to animal testing, many of these substances are first looked at in cell cultures in a lab.

Unfortunately, the results of animal studies cannot automatically be applied to humans. The same is also true for studies that use human cells in a lab setting. Even if given the same exposure, what happens to cells in a dish may be very different than what happens in a person given the millions of chemical reactions occurring in the body all the time.

Retrospective studies look at people with cancer and their prior exposures to try to determine which factors may be linked to their disease. While these can be insightful, they can take years to complete and are not without potential bias.

More practically speaking, the testing process is expensive. It is also sometimes complicated by long latency periods, which is the time between an exposure and the development of a cancer. A good example of this is smoking. It took many years of research and millions of dollars to determine its relationship to lung cancer. 

Safety Precautions

Because not every substance that may be a carcinogen has been tested, it's important to practice discretion with any potential carcinogens you may be exposed to.

These tips can help you get a sense of what those might be and what to do if you encounter them:

  • Read labels and research ingredients if you are not familiar with them. The American Cancer Society's website has a quick-reference list, and the International Agency for Research on Cancer (IARC) Monographs and National Toxicology Program databases are good resources for doing more digging. Some home products, like certain brass cleaners, even make a note that they contain human carcinogens on their packaging.
  • Follow directions for safe handling of chemicals at home. Read the small print on containers. Some labels recommend wearing gloves. Others suggest good ventilation, wearing a face covering, or even putting on a special ventilation mask.
  • If you wouldn't eat it, use gloves to handle it. Many substances can be absorbed easily through the skin.
  • Follow recommended procedures at work when handling chemicals on the job. Employers are required to provide Material Safety Data Sheets on any chemicals you will be exposed to at work. Take the time to read these carefully.
  • Consider alternatives to substances with long lists of ingredients. For example, instead of having an abundance of commercial cleaning supplies, you can effectively clean your home using only vinegar, lemon juice, olive oil, and baking soda.


The process of identifying carcinogens is challenging for both practical and ethical reasons. It's all but guaranteed that there are carcinogens in the environment that have not yet been discovered. Do what you can to protect yourself from those that have. This includes wearing proper safety gear, making strategic changes to products you use, and more.


Carcinogens cause cancer by damaging your cell's DNA or by causing damage and inflammation that leads to mutations. Exposure to a carcinogen does not necessarily mean you will definitely go on to have cancer, but it is a factor that should not be ignored.

Exposure can come from home chemicals, environmental or medical radiation, medications, viruses, polluted air, and lifestyle choices. They are a part of daily life, so you really can't avoid them entirely. But you can take steps to protect yourself, such as wearing gloves, reading labels, and not smoking.

14 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Lynne Eldridge, MD
 Lynne Eldrige, MD, is a lung cancer physician, patient advocate, and award-winning author of "Avoiding Cancer One Day at a Time."