The Start of Epidemiology

At the other conference I attended last week (which I thankfully finished transcribing my notes from yesterday) we spent two days discussing the kinetics and mode of action of perfluorinated compounds (a class of chemicals used to make non-stick surfaces and flame-retardant materials that have the disturbingly long half-life of 4 years in a human). An epidemiologist from one of the two companies involved in the manufacture of the chemicals related the following anecdote which may be familiar to those more medically/biologically inclined but was new to me.

In 1854, Dr. John Snow managed to suppress an outbreak of cholera in the Soho district of London by simply marking the locations of water pumps on a map and then marking the location of cholera cases. Most of the cases clustered around a single water pump, so he removed the handle. The outbreak subsided and, to this day, the water pump handle is a major symbol for epidemiology.

Wikipedia actually had a copy of the map used by Snow:

Snow's Notes

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2 Responses to The Start of Epidemiology

  1. bmarts says:

    Do you know why the half life is so long? Is it just because this stuff isn’t sticky so it’s not easy to push through in the usual way? Given that you seem to have some real knowledge, perhaps you can shed some light on the urban myth about how long chewing gum will stay inside a person.

    Incidentally, we’ve recently ordered some Fluorinated Ethylene Propylene. It’s one of these compounds that Dow throws under the heading “Teflon.” Hopefully it’s useful to us because it’s optically transparent (96%) and has a refractive index very close to water. So you can put it into an aqueous solution and essentially not see it. Hopefully it’s not too much of a pain in the neck to melt the stuff and shape it how we want. You can see a similar idea of a glass rod disappearing in Wesson oil , and a magic trick based on the idea.

  2. jwambaugh says:

    Nobody is actually sure why the half-life is so long in humans. Up until 1997 it was assumed that because perflourinated chemicals (PFCs) vastly prefer to stick to each other rather than anything else (which makes them so slippery and stain-resistant) that they’d just pass through people if they ever got inside. When techniques were finally developed to measure the concentrations in blood, however, PFCs were found in almost all Americans, and a lot of wildlife, including polar bears in the arctic.

    What was then realized is that particular perflourinated chemicals look almost exactly like certain fatty acids except that all the hydrogen atoms have been replaced with flourines. That means that any biological process that depends on the shape, but not the chemistry, of a fatty acid is confused by some PFC’s. What we think happens is that the chemicals are excreted to urine and bile (which ends up in feces) but that transporters designed to harvest fatty acids suck them back into the body, where they just sit since they can’t be burned for energy.

    What isn’t known is what sets the precise length scale. For instance, in female rats the clearance has a half-life of about four hours, in male rats the half-life is about four days and in humans (male or female) the half-life seems to be about four years. Recent research indicates that an interaction between kidney transporters and testosterone may explain the differences in the rats, but so far the difference with humans is a topic of ongoing research.

    As far as gum goes:
    http://www.snopes.com/oldwives/chewgum.asp

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