disinfection. Cloudy, heavily contaminated
water requires more iodine to compensate for
binding of the disinfectant by organic com-
pounds; however, doubling the iodine concen-
tration to 16 mg/l or doubling the contact time
is sufficient. If the water has been filtered,
such precautions are probably unnecessary,
but no studies have been performed.
MASKING IODINE'S TASTE
If water has been filtered and then disin-
fected with 0.5 to 1.0 mg/l of iodine, such a
small concentration cannot be tasted. If larger
quantities are used and individuals find the
iodine taste objectionable, several methods for
masking its taste are available. Because such
procedures inactivate the iodine, they must
not be used before enough time has elapsed
for the microorganisms to be destroyed.
Artificial flavorings added to hide the taste
usually contain ascorbic acid, which reacts
with iodine and impairs its antimicrobial ac-
tivity. Potable Aqua
®
is now supplied with
ascorbic acid tablets--to be added after disin-
fection is complete--to eliminate the iodine
taste. The iodine can be converted to tasteless
sodium iodide with an equal weight of sodium
thiosulfate. The water can be filtered through
activated charcoal, which, by adsorption,
physically removes the iodine (and some
odors, inorganic materials, and microorgan-
isms, but not enough to make the water suit-
able for consumption.)
In clear water, the rate at which microor-
ganisms are destroyed by halogens is depen-
dant on contact time and the iodine concentra-
tion. If time is available for more prolonged
disinfection, lower concentrations of iodine
can be used. One-half the standard concentra-
tion of iodine is equally effective as a disin-
fectant if allowed to act for twice the usual
time; one-fourth the standard concentration is
an effective disinfectant if allowed to act for
four times the usual time. Even lower iodine
concentrations could be used, but less than 2.0
mg/l usually cannot be tasted. (Some indi-
viduals prefer a barely detectable trace of io-
dine as assurance the water has been disin-
fected.)
Persons with known thyroid dysfunction
should determine how they react to water dis-
infected with iodine at home before relying on
iodine water disinfection in the wilderness or
while travelling. The uncommon individuals
who are allergic to iodine, including iodine-
containing compounds in radiographic con-
trast media, and those with thyroid dysfunc-
tion who react adversely must not use iodine
for water disinfection. For such individuals a
filtration system to physically remove bacte-
ria, parasites, and parasitic cysts followed by
chlorine to kill viruses offers a reliable alter-
native.
ACUTE IODINE TOXICITY
Some years ago several publications
claimed the iodine used for water disinfection
could be dangerously toxic. The skull and
crossed bones on tincture of iodine is familiar.
However, iodine is only weakly poisonous.
The third edition of Goodman and Gilman`s
Textbook of Pharmacology states, "...that io-
dine is highly toxic ... is a popular fallacy."
The lethal dose is 2 to 3 g, but survival after
ingestion of 10 g has been reported. Iodine in
such large quantities is a strong gastrointes-
tinal irritant and causes immediate vomiting,
which eliminates most of the iodine. That re-
maining in the gastrointestinal tract is largely
neutralized by the intestinal contents. (The
immediate treatment for iodine poisoning is
administration of starchy food.)
Accidental iodine poisoning is rare; al-
most all fatalities are suicidal, but successful
