Understanding Cancer Causes

“Scientists find link between X and cancer.” There’s a good chance you’ve seen something like this in the news headlines. Or maybe you’ve received it in an email, text, or through social media from a well-meaning friend. These days it seems X can be just about anything – antiperspirants, cell phones, hair dyes, pesticides, power lines, plastic bottles, and eating red meat are a few that often make the news or circulate as rumors. But which ones do you really need to worry about? Some of them? All of them?

The news can be confusing, even frightening, at times. Many people fear cancer more than any other disease, so hearing that something might cause cancer can bring out strong emotions, especially if it’s something you’re exposed to on a regular basis.

Often it’s hard to get the full story. And it’s not always easy to figure out what the information really means for you. Do you need to stop using your cell phone to be safe? Or stop eating meat?

Specific things that might cause cancer won’t be covered here. Instead, we’ll talk about what questions you should ask when you see news about something that could cause cancer. Asking questions can help you better understand the information and decide if it is something you might need to act on.

Ask the right questions

When we hear something might cause cancer our first reaction is often to think that we need to avoid it at all costs. But often it’s not that simple.

Studying cancer (and what causes it) is complex. Study findings are usually not black and white – they need to be viewed in the context of other research on the subject. Even then, many times answers aren’t clear. Sometimes there just isn’t enough information to say for sure how the findings might apply to you. Media reports can sometimes cloud the issue even further if they don’t present study results in the proper context.

What’s the source of the information?

It’s important to consider where the information is coming from.

Major news sources generally try to provide accurate, unbiased information. They tend to have reporters and journalists who have experience covering health-related issues and who understand what’s important. They can often put new information into context. Smaller news organizations might not have as much experience and may have a harder time conveying the information in the right context.

But even when the news comes from a source you trust, headlines often try to boil down a complex topic into something short, simple, and catchy. They may not have the room (or the time) to express the nuances or uncertainty of study findings. Even full articles can have space limits that might mean leaving out important details. Often the whole story can’t be told in a short article or a 60-second news clip.

Sometimes what looks like a news story is really a press release directly from a university or medical journal or from a medical group or other organization. Some groups promoting new findings (such as industry trade groups, environmental groups, animal rights groups, groups that promote specific diets, etc.) might have a particular point of view or an agenda that could affect their ability to provide unbiased information. 

This doesn’t always mean their information isn’t valid or worthwhile. In fact, they often may be the first to report on something that might turn out to be an important issue. Still, if you’re not very familiar with the source of the information, do some research. A quick Internet search can often reveal a lot about who’s in a group and what their goals are. Try to look at their information objectively to see if they might be emphasizing certain points that are more in line with their goals.

Chain emails, texts, blog posts, and social media are increasingly common sources of information for many people. For example, many cancer-related emails spread alarming information and ask people to forward them on to friends. The original source of the email is often impossible to find, and the actual content of the email can change over time, so this type of information often needs to be checked against more reliable sources, such as those listed at the end of this document.

Again, it’s important to look at where the information is really coming from. There’s a lot of information out there competing for your attention. Sometimes the headlines that are a bit more sensational are the ones that win out.

What did the research actually find?

Getting past the headlines (or article, news story, email, etc.) and to the heart of the matter isn’t always easy. Here are some important questions you should ask:

First, is the news actually based on new research? Often it is, especially if it’s being reported in reliable news media outlets. But sometimes it’s not clear where the information actually came from. Rumors passed around in chain emails are often a good example of this. If a source is cited, can it be verified?

Who conducted the research? Who paid for it? Most cancer studies are done by researchers at universities, medical centers, or government agencies. Some research is done by other groups, such as advocacy organizations. Funding for studies can come from federal or state funds, non-profit groups, or other interests. Finding out where the study was done and who funded it can give you a better idea of how trustworthy it might be. 

What kind of research was it? Scientists use different types of studies when trying to figure out if something causes cancer. Results of tests done in the lab on cell cultures or on animals can often suggest something might or might not cause cancer, but it’s not always clear if the results will be the same in people. For example, the effects seen in the lab using very high doses of a substance might not be the same at much lower doses, or the effects of a substance when it’s inhaled may not be the same as when it’s applied to the skin. Also, the bodies of lab animals and humans don’t always process substances in the same way.

Other studies look at large groups of people to try to figure out which factors might be linked to cancer. These studies may be more likely to provide useful information, but they also have their limits. It’s often hard to separate out a single exposure as a possible cause of cancer. People are exposed to many substances at any given time, including those they encounter at work, school, or home; in the food they eat; and in the air they breathe. It’s very unlikely they know exactly what they’ve been exposed to or that they would be able to remember all of their exposures if asked by a researcher. And even if something does cause cancer, it’s usually many years (often decades) between exposure to it and the development of cancer.

Has this been found before? A single study of any type is rarely enough to prove something causes cancer. Scientists often have to combine data from several studies of both types to make this assessment (see below).

Where was the research reported or published? Usually, researchers make their findings known by publishing them in a medical journal or presenting them at a conference. Some types of research, such as government summaries of studies on a particular subject, may be published as stand-alone documents.

Generally, scientists give more weight to research published in major medical journals, as it’s been reviewed by other experts before publishing. Likewise, summary reports from government agencies or other respected groups are usually created by large committees of experts. Research reported at a medical conference is often important but has not undergone the same level of review.

It’s not always as easy to know how much to trust news coming from secondhand sources. Sometimes it could reflect the biases of the group or person who wrote it. For example, news coming from press releases generally hasn’t had any type of expert review (unless it’s referring to a published study). The same can be said for blog posts, emails, etc. These sources may or may not be valid, and you should be aware of possible biases.

What were the actual study results? As we’ve mentioned, it’s not always easy to accurately convey a new research finding in a short news summary. In fact, some people who report the news might not have a scientific or medical background, so they might sometimes have trouble putting the findings into the proper context themselves. The result may be that, intentionally or unintentionally, the news ends up sounding more important and definitive than the study findings actually are.

As an example, suppose a study finds that chemical X affects hormone levels in lab animals. Hormones are known to play a role in increasing the risk of certain cancers. Therefore it’s tempting to report that chemical X can raise cancer risk, even though this isn’t what the study found. It raises the possibility of a link, but further research would be needed to confirm this.

What about other evidence? What’s the bigger picture?

“More research is needed to understand these findings.” This statement, or something like it, can be found at the end of just about every cancer research paper. There’s a reason for this. Scientists understand that the results of a single study are rarely the last word on the subject. There’s always a chance that the findings might have been a fluke, or that somehow they don’t really represent what’s going on. Other researchers will need to confirm or refute the findings with their own experiments. Most often, it’s only after enough evidence builds up that researchers are comfortable accepting the results.

Unfortunately, this can be a slow process that can last years, sometimes even decades. This is understandably frustrating, as it can leave us without the answers we want for long periods of time. But research needs to be done this way so that we have a better understanding of what’s truly important.

Here’s an example. Let’s say a new study finds some kind of link between X and cancer, which is widely reported in the news. But many previous studies of this same issue did not find such a link. This has to be taken into account when considering whether the new findings are reliable, and if they need to be acted on. It may be that the researchers doing the new study did something slightly different that opens up a new avenue of research. Or it could be that, based on the total body of evidence, the new research isn’t likely to change what most experts think about the topic.

Who are the experts?

Determining whether or not something truly raises cancer risk usually is not easy, especially if the possible increase in risk is very small. Experts in several national and international agencies review the available evidence to try to determine the cancer-causing potential of different things we are exposed to.

When the evidence is conclusive or certain that an exposure causes cancer, it’s labeled as a carcinogen. But in many cases there simply isn’t enough information to be sure. When the available evidence is less certain, the exposure might be considered to be a probable carcinogen or a possible carcinogen. Or it may just be stated that not enough information is available to make a determination at this time.

Some people might be surprised to learn that there’s no single, comprehensive list of all chemicals that cause cancer. But it’s not possible to study the millions of chemicals (natural and man-made) people can be exposed to. It would simply be too expensive, would take too long, and in many cases it wouldn’t be necessary. Instead, scientists use what is already known about chemical structures, results from other types of lab tests, the extent of human exposure, and other factors to select the most important chemicals for testing.

International Agency for Research on Cancer (IARC): The IARC is part of the World Health Organization (WHO). Its major goal is to identify causes of cancer. In the past several decades, the IARC has evaluated the cancer-causing potential of more than 900 likely exposures, placing them into one of the following groups:

  • Group 1: Carcinogenic to humans
  • Group 2A: Probably carcinogenic to humans
  • Group 2B: Possibly carcinogenic to humans
  • Group 3: Unclassifiable as to carcinogenicity in humans
  • Group 4: Probably not carcinogenic to humans

Perhaps not surprisingly, based on how hard it can be to test possible carcinogens, most are listed as being of probable, possible, or unknown risk. Only a little over 100 are classified as “carcinogenic to humans.”

The IARC lists can be found online at http://monographs.iarc.fr/ENG/Classification/index.php.

National Toxicology Program (NTP): The NTP is formed from parts of several US government agencies, including the National Institutes of Health (NIH), the Centers for Disease Control and Prevention (CDC), and the Food and Drug Administration (FDA). The NTP updates its Report on Carcinogens (RoC) every few years.

The Report on Carcinogens identifies 2 groups of agents:

  • Known to be human carcinogens
  • Reasonably anticipated to be human carcinogens

The current version of the RoC lists about 240 substances and exposures. Unlike the IARC’s list, the RoC does not list substances that have been studied and found not to be carcinogens.

The most recent RoC can be found online at https://ntp.niehs.nih.gov/pubhealth/roc/index-1.html.

Environmental Protection Agency (EPA): The US EPA maintains the Integrated Risk Information System (IRIS), an electronic database on human health effects from exposure to certain substances in the environment. The EPA uses a rating system similar to that of IARC when describing the cancer-causing potential of a substance:

  • Group A: Carcinogenic to humans
  • Group B: Likely to be carcinogenic to humans
  • Group C: Suggestive evidence of carcinogenic potential
  • Group D: Inadequate information to assess carcinogenic potential
  • Group E: Not likely to be carcinogenic to humans

The IRIS database can be found online at www.epa.gov/iris.

Other agencies and groups: Other federal agencies, such as the CDC’s National Institute for Occupational Safety and Health (NIOSH) and the Food and Drug Administration (FDA) may comment on whether a substance or exposure might cause cancer and/or what levels of exposure to a particular substance might be considered acceptable.

Some state agencies also keep lists of known or probable carcinogens. For example, the California Environmental Protection Agency (CalEPA) maintains a list of “chemicals known to the state to cause cancer or reproductive toxicity.” (Much of this list is based on the IARC and NTP lists.)

Some organizations, such as the American Cancer Society and the US National Cancer Institute (NCI) also conduct or fund research in this area, but they don’t maintain their own lists of substances or exposures that cause cancer. Instead, they look to expert organizations such as the NTP and IARC. The ACS and NCI often offer information and guidance to the public on possible causes of cancer, especially when new findings generate a great deal of interest.

Using information from expert groups

Expert groups such as IARC and the NTP maintain the most comprehensive lists of exposures that might cause cancer and are often the most trusted word on the subject. Still, these lists have their limits in terms of how useful they are for most consumers.

First, these are not complete lists of every substance that causes or might cause cancer. Again, the experts tend to focus on substances and exposures most likely to cause cancer, but there are many others that have not been studied fully yet.

Second, the lists may not be much help on a topic that recently entered the news. Even when experts review a substance, it can take several months (or longer) before it’s added to a list. The lists are updated from time to time, but they are not likely to take very recent findings into account.

Third, the lists (and their supporting documents) are written mainly for scientists and other professionals, so they can be confusing. Even straightforward lists of chemicals can be hard to read. Many of the substances and exposures on the lists have more than one name. This can make it hard to find a particular substance if a list doesn’t use the most common term.

So, just because a substance doesn’t show up on one of the lists might not mean that it doesn’t cause cancer. There are a few possibilities:

  •  It was indeed studied and found not to cause cancer.
  • It hasn’t been thoroughly studied yet.
  • It may be on one of the lists, but under a different name.

It’s not always easy to figure out which one is the case.

Beyond the lists, the agencies publish extensive reviews that go over the evidence linking an exposure to cancer in great detail. Again, these reviews are not written for consumers, so they can be hard to understand.

If you’re having trouble finding information on whether something might cause cancer, contact a trusted group such as the American Cancer Society or the National Cancer Institute. They can tell you whether a particular exposure is on one of the lists, and may also be able to provide you with some context about what the research findings might mean for you.

What if something is found to cause cancer?

What if most or all experts agree that an exposure is linked to cancer in some way? Most often, people are going to want to avoid the exposure whenever possible. But again, there are some important questions to ask.

What exactly does it raise the risk of?

Some exposures clearly raise a person’s risk of one or more types of cancer. But even the strongest carcinogens don’t raise the risk of all types of cancer.

This could affect how important it is to you. For example, something that raises the risk of breast cancer is more likely to be a concern for women than for men, as they’re much more likely to get breast cancer in the first place.

Is the risk tied to a certain type or amount of exposure?

Carcinogens don’t cause cancer at all times, under all circumstances. Some may only be carcinogenic if a person is exposed in a certain way (for example, swallowing it as opposed to touching it). Some may only cause cancer in people who have a certain genetic makeup.

In the same way, the amount of exposure might matter. Some carcinogens may raise cancer risk after only a very small exposure, while others might require intense exposure over many years before there’s any noticeable increase in risk. For example, heavy alcohol use is linked with an increased risk of several types of cancer, but an occasional beer or glass of wine isn’t likely to affect risk.

How much does it raise my risk? What’s my risk to begin with?

Whenever possible, it’s important to try to get an idea of how much an exposure might increase your risk. As an example, tobacco smoke is a well-known carcinogen. Men who smoke are more than 20 times more likely to develop lung cancer than men who don’t smoke. Most other known carcinogens don’t raise cancer risk by nearly this much. That’s not to say they shouldn’t be avoided, but it might be something to keep in mind when weighing the risks. As noted above, the type and extent of exposure can also play a role.

It’s also important to have an idea of what your risk is to begin with. Many factors can influence your risk, including your age, gender, family history, and lifestyle factors (tobacco and alcohol use, weight, diet, physical activity level, etc.).

Your baseline risk is normally higher for some types of cancers than for others. For example, most people have a higher lifetime risk of colorectal cancer than bile duct cancer. So something that doubles your risk of bile duct cancer is less likely to affect you than something that doubles your risk of colorectal cancer. When possible, you should consider the actual amount of increased risk when deciding if you need to limit or avoid an exposure.

Can I limit or avoid exposure? If so, should I?

Even if a substance or exposure is known or suspected to cause cancer, this doesn’t necessarily mean that it can or should be avoided at all costs. In fact, this might not even be possible. Sunlight is a good example. It’s the major source of ultraviolet (UV) rays, which are known to cause skin cancer. But sunlight can also have benefits, such as helping the body make vitamin D. Completely avoiding sunlight isn’t recommended, and it would be nearly impossible to do.

Many medicines, including some hormones and drugs used to treat cancer, can also increase cancer risk. For example, tamoxifen, which is very useful in treating some breast cancers, increases the risk of certain kinds of uterine cancer. Sometimes the benefits of taking medicines like this clearly outweigh the possible risks, even if they include cancer.

What’s important to me?

We are all to some degree at risk of developing cancer. Our risks for different kinds of cancer vary based on a number of factors. Some of these, like age, gender, and genetic makeup, are beyond our control.

For other factors, there may be things we can do that could alter our risk. For example, smoking is linked to about one-third of all cancer deaths in the United States. And about 2 out of 10 cancers are linked to body weight, diet, and physical activity. These risk factors are all within our control.

We might have less choice as individuals when it comes to some other environmental factors, such as pollution and workplace exposures to cancer-causing chemicals. These are thought to be responsible for a smaller portion of cancers, but they are still very important.

Sometimes it’s hard to know what to be worried about. People are often more likely to be concerned about factors over which they feel they have less control, such as pollution, even though these things may not pose as much of a risk to us as some other things over which we do have some control. But both of these types of things can contribute to our risk of cancer. Having an idea of how much they affect our risk can help us make informed choices when it comes to changing them, avoiding them, or limiting our exposure.

The bottom line

“Does this cause cancer?” is a very important question to ask, but it can be a hard one to answer. Information about what causes cancer can be confusing and isn’t always straightforward. If you find yourself asking this question about something, take these steps:

  • Consider the source of the information. Is it reliable?
  • Consider the science behind it. Is it convincing? How well does it apply to you?
  • Get more information from trusted sources. What else is known about the subject?
  • Try to put the findings in context. How much does it affect your risk? What can you do to limit your exposure if you feel you need to?

We encourage you to learn all you can. You can always call your American Cancer Society to see if we can help you get more information about any concerns you have.

The American Cancer Society medical and editorial content team

Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Along with the American Cancer Society, other sources of information and support include:

National Cancer Institute (NCI)
Toll-free number: 1-800-422-6237 (1-800-4-CANCER)
TTY: 1-800-332-8615
Website: www.cancer.gov

International Agency for Research on Cancer (IARC)
Website: www.iarc.fr

National Toxicology Program (NTP)
Website: http://ntp.niehs.nih.gov

Environmental Protection Agency (EPA)
Website: www.epa.gov

Food and Drug Administration (FDA)
Toll-free number: 1-888-463-6332
Website: www.fda.gov

National Institute for Occupational Safety and Health (NIOSH)
Toll-free number: 1-800-232-4636
Website: www.cdc.gov/niosh

*Inclusion on this list does not imply endorsement by the American Cancer Society.

No matter who you are, we can help. Contact us anytime, day or night, for information and support. Call us at 1-800-227-2345 or visit www.cancer.org.

American Cancer Society. Cancer Facts & Figures 2014. Atlanta, Ga: American Cancer Society; 2014.

National Cancer Institute. Cancer Risk: Understanding the Puzzle. Accessed at http://understandingrisk.cancer.gov on September 21, 2012. (Content no longer available.)

Last Medical Review: November 11, 2014 Last Revised: May 11, 2017





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