I'm probably late to the party here but I wanted to make note of
the NYT's recent
on water safety. (þ Melanie Schoenberg).
While there's certainly some stuff here one might be distressed
about, the article is written in such a way that it's pretty
hard to evaluate how serious the issue actually is.
The article seems to make three major factual claims:
- The Safe Water Drinking Act only regulates a small fraction
of the potentially hazardous chemicals potentially found in drinking
- Many municipal water systems contain chemicals at levels which,
while legal, may be unsafe (e.g., are above EPA safe levels).
- People are getting sick from this.
I don't doubt that the first of these is true: according to the
article, 60,000 plus chemicals are used within the US (I'm actually
surprised it's this low, since the PDR has over 4000 drugs and
claims to have 3.5 million
data sheets), and it's not clear how you would plausibly
analyze all of these, let alone determine permissible levels for
each of these. I'm not saying this is desirable, but it's not
necessarily a disaster either. Ultimately, you can either
have an "default accept" or "default deny" policy here;
given how sensitive modern analytic techniques are,
if your policy is "default deny" you're going to spend a lot
of time removing trace concentrations of harmless chemicals
from your water supply. On the other hand if it's "default accept"
you're going to end up with a lot of chemicals in your water
that you don't really know are safe.
Given the first point, the second isn't surprising either. With that
said, I'm not sure that the Times is really representing the situation
that accurately. For instance, here's the report
for Palo Alto, where I live. The Times reports "1 contaminant below
legal limits, but above health guidelines", with the contaminant being
alpha particle activity at a mean rate of 4.56 pCi/L. Let's see if we
can put this in perspective. Assume humans are made entirely of water
and rescale into kg, so we have 4e-12 Ci/kg of human body mass. A Ci
is 37e+9 disintegrations/s so multiplying out we have .148
disintegrations/kg/s. If we assume that all the alpha particles are
from U-238, and the alpha particles are being emitted at 4.270 MeV (~
7e-13 J), then we get 1e-13 J/kg/s. If we assume that all of these are
absorbed (not crazy since alpha particles have a very short path in
the body) then we're getting 1e-13 Grays/s or 2e-12 Sv/s (multiply by
the 20 Q factor for alpha particles) or .03 mSv/year. For comparison,
the background level of radiation is 2.4 mSv/year. Obviously this
isn't something you should be that thrilled about, but it's not clear
to me that a 1% increase in your radiation dose is that bad either.
Given that, why does the NYT list this as above the health level?
The answer seems to be that their safe value for alpha
particles is zero (the legal limit is 15 pCi/L): the maximum
level of alpha particle activity in neighboring
is 2.56 pCi/L, but it's still listed as having 5 "above health"
samples (Chicago had one reading of .88 pCi/L and is also listed
as a positive).
This all makes me wonder if something is wrong here and the
NYT is showing false positives. Of course, when you're processing
a lot of data it's easy to
make mistakes—assuming this is a mistake.
It could be that I'm confused or that it's just the alpha
particle threshold that's too low.
the times to ask them for a copy of the raw data, but I haven't
heard anything yet.
This brings us to the final point: the Times
All told, more than 62 million Americans have been exposed since 2004
to drinking water that did not meet at least one commonly used
government health guideline intended to help protect people from
cancer or serious disease, according to an analysis by The Times of
more than 19 million drinking-water test results from the District of
Columbia and the 45 states that made data available.
And independent studies in such journals as Reviews of Environmental
Contamination and Toxicology; Environmental Health Perspectives;
American Journal of Public Health; and Archives of Environmental and
Occupational Health, as well as reports published by the National
Academy of Sciences, suggest that millions of Americans become sick
each year from drinking contaminated water, with maladies from upset
stomachs to cancer and birth defects.
This seems to conflate a bunch of issues. There seems to be a lot
of variance in the data, with some tests showing positive results
and some negative results (or low levels) for the same toxin
even in the same area.
different to drink water with a toxin in it once than it is to
drink it ever day for 10 years. I spent a couple days in
Boston in 2007, but I'm not overly concerned about the fact
that I might have been exposed to twice the legal limit of
haloacetic acids in the two to four liters of water I
drank while I was there.
More generally, while one positive test
may qualify as an exposure, it's not clear what that means
as far as the real level of risk people are incurring.
And of course there's a difference between cumulative toxins
(e.g., arsenic) and acute toxins (e.g., e. coli).
Speaking of e. coli, "maladies from upset stomachs to cancer and
birth defects" covers a lot of territory; it's one thing if
a sewer system occasionally fails to remove all the bacteria
from the water supply (not that that's good) and another if
it delivers hot and cold running cyanide from the tap.
Obviously, when you read this article you're supposed to be scared,
but the way the article is written (and the opaque data presentation)
doesn't make me feel like I have enough data to know if I should be or not.
P.S. San Francisco really does have
great water. Almost
good enough to make up for destroying