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Does This Cause Cancer?

“Scientists find link between X and cancer.” There’s a good chance you’ve seen something like this recently in the news headlines. Or maybe you’ve received it in an e-mail, 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 it 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 it really means for you. Do you need to stop using your cell phone to be safe? Or stop eating meat?

Here we’ll talk about what you can do to better sift through the information you may see. We’ll go over some of the questions you should ask when you see news about something causing cancer. We won’t discuss specific things that might cause cancer, but we’ll try to give you some tools you can use to help decide if the information you see is something you may need to act on.

Ask the right questions

When we hear something might cause cancer our first reaction is usually to think, “I need to stay away from that!” But it’s often 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 done 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.

Asking the right questions can help you better understand the information and what it might mean for you.

What is the source of the information?

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

Major news sources generally strive to provide accurate, unbiased information. They tend to have reporters and journalists who have experience covering health-related issues and who understand what is important. They can often put new information into context. Smaller news organizations may 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 room to express the nuances or uncertainty of study findings. Even full articles can have space limits that may 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 or other organization. Some groups promoting new findings (such as industry trade groups, environmental groups, animal rights groups, groups that promote specific diets, etc.) may have a particular point of view or an agenda that might affect their ability to provide unbiased information. This does not always mean their information is invalid or not worthwhile. In fact, in many cases they may be the first to report on something that might prove to be an important issue. Still, if you’re not very familiar with the source of the information, do some research. A quick web search can often reveal a lot about a group’s makeup and goals. 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 other sources of information for many people. Many cancer-related emails spread alarming information and ask people to forward them on to friends. The original source of these emails is often impossible to find, and the actual content of the emails may 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. 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. If you can find out, having a sense of 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 determine if something causes cancer. Tests done in the lab on cell cultures or on animals can often give an idea about whether something might cause cancer, but it’s not always clear if the results from lab studies will apply to people. For example, the effects seen in the lab using very high doses of a substance may not be the same at much lower doses, or the effects of a substance when it is inhaled may not be the same as when it is 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 is 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 may 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 may not have a scientific or medical background, so they may 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, a study may find that a 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 this finding as chemical X raising 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 may have been a fluke, or that somehow they don’t really represent what is going on. Other researchers will need to confirm or refute the findings with their own experiments. Most often, it’s only once a body of 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 we might need to act on them. 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 agencies (national and international) review the available evidence to try to determine the cancer-causing potential of different substances.

When the evidence is conclusive that an exposure causes cancer, it is 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 may 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 and whether or not they cause cancer. But it’s not possible to study all of 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 30 years, the IARC has evaluated the cancer-causing potential of more than 900 likely candidates, 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 list 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 http://ntp.niehs.nih.gov/go/roc12.

Environmental Protection Agency (EPA): The US EPA maintains the Integrated Risk Information System (IRIS), an electronic database that contains information 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 viewed online at www.epa.gov/iris.

Other agencies and groups: Other federal agencies, such as the National Institute for Occupational Safety and Health (NIOSH) and the Food and Drug Administration (FDA) may comment on whether a substance or exposure may 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 do not maintain their own lists of substances or exposures that cause cancer. Instead, they look to expert organizations such as the NTP and IARC, who serve this mission. The ACS and NCI do 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 may take several months (or longer) before it is 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 can have more than one name. This can make it hard to find a particular substance if a list does not use the most common term.

So, just because a substance doesn’t show up on one of the lists may 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 at times.

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 appears 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, we’re 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 do not raise the risk of all types of cancer.

This may 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 are much more likely to get breast cancer in the first place.

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

Carcinogens do not 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 may matter. Some carcinogens may raise cancer risk after only a very small exposure, while others might require intense exposure over many years before there is 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 is 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 about 20 times more likely to develop lung cancer than men who don’t smoke. Most other known carcinogens do not raise cancer risk by nearly this much. That’s not to say they shouldn’t be avoided, but it may be something to keep in mind when weighing the risks. As noted above, the type and extent of exposure may also play a role here.

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

Your baseline risk of some cancers is higher than others to start with. For example, for most people the lifetime risk of colorectal cancer is much higher than their risk of bile duct cancer. So something that doubles your risk of bile duct cancer is likely to have less of an impact on 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 does not necessarily mean that it can or should be avoided at all costs. In fact, this may not even be possible. Sunlight is a good example. It is 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 is not 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 such as this may clearly outweigh the possible risks, even if they include cancer.

What is 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, such as our 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. Another third of cancers are linked to body weight, diet, and physical activity.

We may 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 also still very important.

Sometimes it’s hard to know what to be worried about. We are often more likely to be concerned about factors over which we feel we have less control, such as pollution, even if they 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 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 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 is not always straightforward. If you find yourself asking this question about something, take the following 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 already 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.

To learn more

More information from your American Cancer Society

The following related information may also be helpful to you. These materials may be read on our web site or ordered from our toll-free number, at 1-800-227-2345.

Cancer Clusters

Known and Probable Human Carcinogens

International organizations and Web sites*

In addition to the American Cancer Society, other sources of information include:

National Cancer Institute (NCI)
Toll-free number: 1-800-422-6237 (1-800-4-CANCER)
Web site: www.cancer.gov

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

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

Environmental Protection Agency (EPA)
Web site: www.epa.gov

Food and Drug Administration (FDA)
Web site: www.fda.gov

National Institute for Occupational Safety and Health (NIOSH)
Web site: 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 2012. Atlanta, Ga: American Cancer Society; 2012.

National Cancer Institute. Cancer Risk: Understanding the Puzzle. Accessed at http://understandingrisk.cancer.gov on September 21, 2012.

Last Medical Review: 11/12/2012
Last Revised: 11/12/2012