Carcinogens and Me Part II: Understanding the Changes in Your Body

"This just in.  BCB may cause cancer."

Okay, so maybe there's no such thing as BCB, but how many times have you seen a headline like that?  "...may cause cancer"  "possible carcinogen..."  What on earth is that supposed to mean?  After seeing a few friends got all worked up about some (nonscientist) TV personality saying "there's no evidence that secondhand smoke causes cancer" I figured that would be a good example to help us wade through all this.

The first question is, how do you test a statement like 'secondhand smoke causes cancer'?  Easy!  You lock 100 people in one room with normal air for 8 hours a day and lock 100 people in another room with 5 chain smokers for 8 hours a day.  20 years later, see who's not dead.  Scientific? Yes.  Ethical?  Maybe not.  So what's a scientist to do?  Well here's a few more common ways people go about it:

Method 1: population studies.  In these, they track X people for Y number of years who are exposed to some amounts of substance Z.  These studies, properly interpreted, are definitely the silver bullet, "conclusive evidence" on any topic because they take just about everything into account.

Method 2: the Ames test.  No people, no mice, just cells.  Researchers take a bunch of bacteria and make a single mutation that makes the bacteria unable to make histidine, one of the 20ish common amino acids or protein building blocks.  With even one amino acid gone, cells can't make working proteins (try turning in an essay without the letter 'T', see what grade you get).  The scientists grow the bacteria in something that has the amino acid in it, so the bacteria can live without making it themselves.  Then, they take these cells and grow them in 2 cultures: one with the suspected carcinogen, and without.  Neither have histidine (the amino acid they can't make).  A "mutagen" will cause a lot of random changes in the DNA, some of which (and we're dealing with billions of cells here) happen to mutate the broken part of the bacteria's DNA back to its working form, so the bacteria can make their own histidine again, and you see colonies pop up all over your petri dish.  A "safe" chemical doesn't do anything, so you might see a few colonies from spontaneous DNA changes, but not too many.  So basically, lots of colonies = scrambled DNA, and that means the chemical you're testing is "mutagenic," which is a fancy word that means "does karate on your DNA."  And what does random DNA damage do to a guy?  If you forgot, check my last post.

So what's the bottom line?  "Probable carcinogen" or "may cause cancer" are phrases scientists use to say "this stuff messes up your DNA."  Are there problems with the Ames test?  Sure.  They're using bacteria, and humans aren't bacteria.  Obviously population studies are better.  The problem is that they take a long, long time to do.  Cancer studies are especially difficult, since people generally develop environmentally-induced cancer much later in life, so a solid, definitive study, started now, wouldn't be done for another 50-60 years.  So what does it all mean?  It means I'd think twice before eating a steak blacker than my wife's mascara.