I have heard this many times, sometimes worded as such:
Wearing headphones for just an hour will increase the bacteria in your ears by 700 times
I'm wondering, is this true and, if so, does it pose a threat?
The claim is false. Cecil Adams, from The Straight Dope, took a look at the claim and summarizes his findings in the following manner:
The statistic you cite is fairly accurate up through "ears," after which it goes awry. The problems are (1) the rumored scale of the phenomenon is way off and (2) the stat may not tell us too much, besides that ears are livelier places than one might imagine.
He goes on to explain that the source of the notion probably is Changes in the microbial flora of airline headset devices after their use, published in the medical journal Laryngoscope, in which the researchers evaluated the bacterial flora of twenty headsets before and after they were worn for one hour.
He notes that, while the study supports an increase in bacteria after wearing headsets, it does not support the "700 times" figure:
At the beginning of the experiment, the typical headset had 60 microorganisms on its surface; after an hour's use of the headphones by a volunteer, that number went up to 650 - roughly 11, not 700, times more.
For that figure, rather, he blames an New York Times column from 1993 by Betsy Wade: PRACTICAL TRAVELER; That Airline Pillow May Be Well Traveled.
In the article, Wade summarizes the aforementioned study in the following fashion:
Dr. Brook's new work focused on the bacteria found on the headset after use for 60 minutes. In this test, he used sterile headsets of the type most often found, ones with the smaller earphones, usually covered with gray plastic foam. The research showed a 100- to 700-fold increase in bacteria on the set, including staphylococci of three types and streptococci of two types. After this conclusion, Dr. Brook said, he does not feel comfortable wearing the headsets, and he said that people whose immune systems are compromised, including those undergoing chemotherapy, should be quite concerned.
Having read the study, Cecil Adams concludes that
Now, the Straight Dope staffers and I have turned that study upside down and can't fathom how Wade got her numbers. Whether they arose from a cryptic reading of the data, a misreading, or a proofreader's slip, it's of such that Internet factoids are born.
So there you have it.