What ACTUALLY Causes Cancer?

Cell phones.  Microwaves.  Diet soda.  Sugar.  Power lines.  TV screens.

Every day, we are exposed to a laundry list of modern devices and substances.  And I would bet that most of us have wondered what the effects of all those substances are.  The list above is are things that are usually found in news headlines, right before the part that screams "CAUSES CANCER!!"

When you stub your toe, there is no question what the cause was.  But cancer takes years, even decades, to develop.  We carry our cell phones every day, use microwaves to heat our food, and drink our diet sodas at work.  And as humans, we like to attribute things to a cause.  So when someone hears about a diagnosis of brain cancer, it is natural to wonder what really happened over all those years of holding a cell phone to your head.

So, what causes cancer?  There are two ways to answer that question - one way which lends itself to fear mongering, and one way which lends itself to rational public health decisions.  I'll let you guess which one I prefer.

Read more after the break.

Carcinogenic

Let's say I've just created a new energy drink - BRAWNDO.  Brawndo has what your body needs to kick ass and take names.  But Dr. Nancy Wussypants at Emory has a problem with Brawndo.  She thinks it has a chemical in it that causes cancer.  Here's the tricky part - cancer takes years to develop, and even then we can never know for sure what caused a particular person's cancer.  So Dr. Wussypants can't just have some poor undergrad drink Brawndo and wait to see what turns up.  Instead, she uses what is called the Ames Test.  In a nutshell, this test looks to see if a substance can cause mutations to DNA.  If mutations occur, then it is inferred that the substance is carcinogenic - meaning it can cause cancer.  And as it turns out, my Brawndo contains bismethylacrylaphosphate, which not only activates the brain's ass-kicking center but was also found to cause point mutations in bacterial DNA.  Headline - Brawndo Causes Cancer!

It is impossible to prove an absolute negative.  There is no way to say with 100% certainty that Brawndo cannot cause cancer.  When we look at our question from this angle - "can this substance cause cancer?" - we are skewing ourselves towards false positives.  We also may spend to much time worrying about common carcinogens that are extremely weak.  Even Bruce Ames, the inventor of the test, cautioned against the "hysteria over tiny traces of chemicals that may or may not cause cancer", that "completely drives out the major risks you should be aware of."

Epidemiology

Carcinogens cause cancer, but a person's cancer has no individual cause. This statement may seem quite confusing, but the meaning becomes apparent when we understand cancer through the concept of risk. Let's say we set out to study the cancer-causing effects of smoking tobacco. We recruit 30 volunteers: 15 smokers and 15 non-smokers. Then, we wait. After many years, we look at the number of volunteers in both groups who have contracted cancer. Since cancer is extremely common, 5 of our non-smokers have contracted the disease. However, we notice that 10 of our smokers have developed cancer. If we choose our volunteers very carefully, we can assume that members of the two groups have an equal chance of developing cancer, except for the effect of smoking. Using this, we can say that smoking caused cancer in 5 of our smokers, since we would have expected only 5 of them to develop cancer if they hadn't smoked. We can't say for sure which of our smokers developed cancer as a direct result of smoking. But we can say that smoking increased the risk of developing cancer, and that smoking led to 5 excess cancers.

Using studies like these, we can analyze the number of cancers caused by different factors. These studies are not perfect, and so the results reported are not exactly accurate. But they are good estimates, and help to put into context the broader question of what causes cancer.  Moreover, these studies tell us what ACTUALLY causes cancer.  In other words, they tell us what the causes of cancer are most likely.

An organization called Cancer Research UK (CRUK) has analyzed studies like these in detail, and published a very nice figure that summarizes the results.  Some of the main causes of cancer are things we all are familiar with - smoking, HPV, or excessive tanning.  But many people don't realize that a huge number of cancers are attributed to lifestyle factors like diet and exercise.  Click on the picture below for a larger version, or Click Here for a higher quality pdf version that you can zoom and read easier.

In the figure, the larger the circle, the greater proportion of cancers are due to that cause.  Also, each cancer site is broken down by its own individual causes.  The CRUK figure focuses on preventable cancers, so you may notice that it ignores things like hereditary factors like BRCA genes or familial mutations.  

You may ask why cell phones aren't on this list.  Or diet soda.  Or living near power lines.  Some studies have shown that these things have the potential to cause cancer.  But no study has ever been able to demonstrate any significant population of people whose risk of cancer was increased due to these things.  Think of it this way - hundreds of millions, if not billions, of people use cell phones.  If there was a great danger of cancer from these devices, then we would have seen cancer rates skyrocket when cellphones came into use.  In reality, we don't see any such effect (relevant XKCD comic).

We may never be able to prove that cell phones do not cause cancer.  But we can say that IF cell phones cause cancer, the effect is so small that it is unable to be measured. In other words, any cancer risk due to cell phones is dwarfed by our risk due to smoking, being overweight, drinking alcohol, staying in the sun too long, and not eating enough fruits and veggies.  In the end, these are the things that ACTUALLY cause cancer.