Rachel's Environment & Health News
August 4, 2005

The Urban Legend of Precaution and Cholera in Peru

By Peter Montague and Tim Montague

Anyone who is paying attention to the "precautionary principle" has
heard the story that precaution caused a cholera epidemic in Peru in
1991. The story isn't true -- and it was revealed as untrue in 1992 --
but that hasn't stopped it from being told, retold, and told again. It
has now taken on the characteristics of an urban legend with a life
all its own.[1]

The Precautionary Principle

As our readers know, the precautionary principle is a modern way of
making decisions to minimize harm, which can be described in five

(1) Set a goal by an open, participatory process;

(2) Examine all reasonable ways of achieving the goal, with the
expectation that the least harmful way will be chosen;

(3) In the face of uncertainty, shift the burden of proof, giving the
benefit of the doubt to nature, public health, and community well-

(4) Monitor results, heed early warnings and take further action, as
needed, to prevent harm;

(5) Throughout, give a real "say" to -- and honor the knowledge of --
the people who will be affected by the decisions.

Cholera in Peru in 1991

An epidemic of cholera occurred in Peru in 1991. Cholera is not a
regular visitor to Peru -- the last such epidemic had occurred in
1867.[2] Nevertheless, cholera is a powerful killer disease, well-
known to public health specialists, and no one takes it lightly.
Cholera is characterized by high fever, awful abdominal cramps, and
diarrhea so bad that it can readily kill its victim by dehydration
unless the lost fluids are replaced.

Cholera can be prevented by putting disinfectants into drinking water
supplies. Chlorine is the most common disinfectant added to municipal
water supplies to kill bacteria including the Vibrio cholerae bacteria
that can cause cholera. But chlorinating water creates chlorination
byproducts in the water, called trihalomethanes, which can cause
cancer. In the U.S., with a population approaching 300 million,
chlorination byproducts cause an estimated 700 cases of cancer each
year.[3] Cholera, on the other hand, can kill thousands quickly if an
epidemic gets started.

The Urban Legend of Precaution Causing Cholera in Peru

The "precaution caused cholera in Peru" legend goes like this: In
1991, as a precautionary measure, health officials in Peru stopped
chlorinating their drinking water to avoid the danger of cancer from
chlorination byproducts -- thus causing the deaths of thousands of
Peruvians from an outbreak of waterborne cholera. As the legend has
it, this was a case of "risk assessment gone wrong."[4] In other
words, Peruvian public health officials balanced two hazards and made
a bad choice: they turned off their chlorination systems to avoid a
small problem, thus creating a much larger problem. That is how the
legend goes.

Opponents of the precautionary principle -- mainly friends of the
chemical industry -- seized upon the "cholera in Peru" legend and
began to tell it and retell it at every opportunity.

In 2001, Henry I. Miller, a professor at Stanford University --and a
relentless opponent of the precautionary principle --recounted the
"cholera in Peru" legend as follows:

"By the late 1980s, radical environmentalists were attempting to
convince water authorities around the world that carcinogenic
byproducts from drinking-water chlorination might constitute a
potential danger. Mired in a budget crisis, officials in the Peruvian
government spun such allegations into the basis for discontinuing
chlorination of drinking water in much of their country. That move
contributed to the acceleration and spread of Latin America's
1991-1996 cholera epidemic, which killed at least 11,000 of its more
than 1.3 million sufferers."[5]

In 2004, C.T. "Kip" Howlett, director of the U.S. Chlorine Chemistry
Council, re-told the "cholera in Peru" legend this way:

"In Peru, in the early 1990s, public health officials responded to an
antichlorine campaign by stopping proper chlorination of their
drinking water. The results were predictable and horrific. Within
months, a cholera epidemic swept through the country, eventually
causing 1.3 million cases of illness and 13,000 deaths."[6]

It turns out that the "cholera in Peru" story is just plain wrong, and
scientists have known it was wrong since 1992. Yet scholars like Henry
Miller and Kip Howlett -- and dozens of others who are seeking ways to
discredit the precautionary principle -- have continued to repeat it.
As we shall see, one can only conclude that either these scholars are
intentionally repeating a falsehood, or their scholarly methods are
woefully deficient.

The "cholera in Peru" story got its start from a one-page "news" item
that appeared in Nature magazine (the British equivalent of our
Science magazine) in November 1991.[4] The report said (incorrectly,
it turns out), "During the 1980s local water officials, citing the EPA
studies of chlorine's cancer potential, decided to stop chlorinating
many of Lima's wells." Lima is Peru's capital and largest city.
Nature's reporter went on to declare this "a sobering case of risk
assessment gone wrong" because the danger of cancer from chlorination
byproducts is low but the danger of cholera from drinking water is
high if the water contains Vibrio cholerae bacteria.

Eight months later, U.S. and Peruvian scientists debunked this
incorrect report when they published a study of the Peruvian cholera
epidemic in the British medical journal, Lancet.[7] Lancet is the
British equivalent of the New England Journal of Medicine in this
country, or perhaps the Journal of the American Medical Association.
In any case, Lancet is a scholarly powerhouse in the medical world, so
articles published there gain wide readership.

The main author of the 1992 Lancet study was David L. Swerdlow, now at
Emory University in Atlanta. Lancet chose to highlight the Swerdlow
study with a companion editorial.[3] The editorial began with this
statement: "The report by Swerdlow and colleagues in this issue should
put an end to two rumours about the cholera epidemic that began in
1991 in Peru." It went on: "The second rumour, reported in a news item
in Nature last year, is that lack of chlorination of many water
supplies in Peru was a deliberate decision by the authorities, and was
based on studies by the US Environmental Protection Agency showing
that chlorine may create a slight cancer risk by reacting with organic
matter in water to form trihalomethanes."

In other words, Lancet said in no uncertain terms in 1992 that the
cholera outbreak did not result from a deliberate decision by health
officials. Yet Henry Miller says in 2001 the epidemic was caused by
Peruvian officials "discontinuing" chlorination. and Kip Howlett says
in 2004 the cholera epidemic was caused by Peruvian officials
"stopping proper chlorination."

Professor Miller further embellished the legend by claiming that
Peruvian officials had stopped chlorinating water "in much of their
country" although the original news item in Nature had said only that
authorities had "decided to stop chlorinating many of Lima's wells."
Dr. Miller's scholarship created a pile of rich organic fertilizer to
help the legend thrive.

The Swerdlow study revealed that the causes of the cholera epidemic in
Peru were far more complex than Miller and Howlett would have us
believe. Here are some of the causes the Swerdlow study identified:

(a) the absence of water chlorination systems (or other water
disinfection systems) in most of Peru;

(b) people tapping into water lines clandestinely to run a pipe into
their homes, closing any leaks by stuffing paper and plastic into the
cracks, thus providing ways for bacteria to enter the water-
distribution system;

(c) local farmers illegally tapping into sewage lines to irrigate
their vegetable crops (cabbage, lettuce, carrots) with raw sewage;

(d) low water pressure or no water pressure (when the pumps are turned
off, or electrical systems fail), allowing contamination to siphon
back into water pipes;

(e) the absence of sewage systems in many communities;

(f) a majority of families storing water in barrels because water
delivery systems are intermittent or non-existent in poor
neighborhoods, with many people immersing their hands and arms in the
barrels to draw water, thus spreading bacteria to their families.

Even though the false story of cholera in Peru was debunked in Lancet
in 1992, the legend suited the needs of those who oppose the
precautionary principle, and it took on a life of its own. As we have
seen, leading scholars and ideological opponents of precaution have
fabricated new fictitious elements of the story in the retelling.

In June, a new study of the "precaution caused cholera in Peru" story
was published in the journal Risk Analysis.[8] In it, authors Joel
Tickner and Tami Gouveia-Vigeant have made substantial efforts to
discover whether any Peruvian officials actually stopped chlorinating
water -- for precautionary purposes or for any other reasons. They
could find no evidence that such a thing occurred.

The new study adds considerable new information to our understanding
of the conditions that gave rise to the cholera epidemic in Peru in
1991. It may have started with contaminated fish, which are often
eaten raw in Peru. And it seems that global warming may have created
conditions in Peruvian coastal waters that allowed the Vibrio cholerae
bacteria to thrive and proliferate.

Once cholera took hold in the population, poor sanitation was no doubt
the major cause of the epidemic. For example, a hospital in the city
of Iquitos, where thousands of cholera patients were treated, dumped
its untreated sewage into the Belen river upstream from the city's
drinking water intake. (Some U.S. cities have similarly non-sensical
relationships between sewer pipes and water intake pipes, but they can
afford to disinfect their water, killing the bacteria thus

Tickner and Gouveia-Vigeant learned that the head of the Lima water
treatment system was not worried about chlorination byproducts
compared to the risks of cholera, and he never stopped chlorinating
Lima's water. Indeed, Peruvian authorities actively encouraged
chlorination of water systems, as well as the use of chlorine pills by
individuals to disinfect their drinking water. The Peruvian cholera
epidemic of 1991 -- which eventually spread to 19 other countries in
Latin America -- was caused by an inadequate public health
infrastructure that proved unable to control a known hazard --
microbial contamination of water supplies.

But What if the Legend Were True?

Finally, let's ask ourselves: if Peruvian officials HAD stopped
chlorinating their water supplies to prevent cancer from chlorination
byproducts, would this demonstrate that the precautionary principle is
a bad decision-making technique?

Not at all. It would demonstrate that humans are capable of making
mistakes no matter what decision-making technique they employ. The
question is, Can your decision-making technique detect and correct
errors? The precautionary approach can do both these things.

Even if someone makes a bad decision using the precautionary approach,
it is still a good idea to try to avoid harm by setting goals,
examining all available alternatives for reaching the goals, and
choosing the least-harmful way. If you made a misjudgment in selecting
the least-harmful way, vigilant monitoring and follow-up would reveal
your error and you would naturally revisit your original decision.

In sum, if you mistakenly turned off your water chlorination system to
avoid a danger from cancer, as soon as you observed cholera spreading,
you would turn your water chlorination system back on -- revealing
that the precautionary approach is robust in the face of human error,
and a sensible way to make decisions under conditions of uncertainty.


[1] For a definition of an urban legend, see this site.

[2] Imogen Evans, "Cholera on the Rocks," Lancet Vol. 341, No. 8840
(Jan. 30, 1993), pg. 300.

[3] Lancet Editors, "Of Cabbages and Chlorine: Cholera in Peru,"
Lancet Vol. 340, No. 8810 (July 4, 1992), pg. 20.

[4] Christopher Anderson, "Cholera epidemic traced to risk
miscalculation," Nature Vol. 354 (Nov. 28, 1991), pg. 255.

[5] Gregory Conko and Henry I. Miller, "Precaution (Of A Sort)
Without Principle," Priorities for Health Vol. 13, No. 3 (Nov. 1,
2001), unpaginated. Also available here: Competitive Enterprise

[6] William Schulz, "The many faces of chlorine; Howlett and Collins
square off about one of the most evocative chemicals," Chemical &
Engineering News Vol. 82, No. 42 (Oct. 18, 2004), pgs. 40-45.

[7] David L. Swerdlow and others, "Waterborne Transmission of
Epidemic Cholera in Trujillo, Peru: Lessons for a Continent at Risk,"
Lancet Vol. 340 No. 8810 (July 4, 1992), pgs. 28-33.

[8] Joel Tickner and Tami Gouveia-Vigeant, "The 1991 Cholera
Epidemic in Peru: Not a case of Precaution Gone Awry," Risk Analysis
Vol. 25, No. 3 (June, 2005), pgs. 495-502.