Part 1: SLS and cancer
Sodium lauryl sulphate (SLS for short) has been the target of a cancer scare for more
than a decade; I remember seeing a mimeographed copy of a letter that warned about a link
between SLS and cancer long before the advent of the Information Superhighway.
Now that Xerox copies are obsolete, it's not unusual to receive an e-mail version
of the warning.
The actual text varies and mutates, like all good memes do, but the following list gives
a good summary of the "facts" that everybody should be aware of:
- SLS is found in most brands of shampoo and toothpaste;
- it is used to scrub garage floors;
- it is a known carcinogen;
- it is used as a pesticide;
- it is an irritant;
- a phone call to a shampoo manufacturer elicits the following admission:
"We know that SLS causes cancer, but there is nothing we can do because we need that substance to produce foam."
The letter usually contains a list of "good" brands that don't use SLS in their products.
Part 2: Trusted sources
Item 6) in the above list sent off-scale my hoax warning system, but the text is usually
peppered by other incongruences. I've surfed some sites to check the claims:
My opinion is that the warning is a hoax, even if it's put together more cleverly than usual:
it's a good mix of truths, half-truths and outright lies, the hallmark of all believable
Part 3: Facts and fiction
The following list addresses the claims one by one:
- SLS is indeed found in shampoos, toothpastes and several other products (including
the nimesulide that I've taken while writing this). It is indeed a
good surfactant and a strong foaming agent, and it's also cheap. Status: TRUE.
- Industrial strength cleaners contain high concentrations of SLS, because garage floors
are quite dirty and hard to clean. The percentage of SLS found in personal care products
is far lower. Status: TRUE - SORT OF.
- The FDA lists SLS as a food additive, and considers it Generally Recognized as Safe or
GRAS (21 CFR 172.882). The American Cancer Society clearly states that SLS and its
other compounds are known irritants, not known carcinogens (they also debunked the hoax in
one of their web pages). The final report on the safety assessment of SLS found in the
toxnet site concludes that in mutagenesis studies, rats fed 1.13% and 0.56% SLS in the
diet for 90 days produced no more chromosomal aberrations or clastogenic effects than
did a control diet. (Those guys are sick. I won't tell you how they tested SLS on rabbits.
Still no cancer, though.) Status: FALSE.
- I've been able to track down only one pest control product that lists SLS as its active
ingredient: "Dr. Dogkatz Critter Chaser" - a pet shampoo. Moreover, SLS employs a
non-toxic mode of action in controlling fleas and ticks (I suspect that its surfactant
properties effectively smother insects). This doesn't mean that SLS isn't used along with
pesticides to improve their effectiveness: a drop of SLS-laced parathion will have less
surface tension, and will be more readily absorbed by the unlucky bug. Other common
chemicals, such as caffeine or nicotine, are used as pesticides.
Status: MISLEADING, IRRELEVANT.
- SLS is an irritant, as anybody who got foam in his eyes can testify. While filming
The Pink Panther, Robert Wagner was temporarily blinded by an industrial strength foaming
agent used in the bathtub scene; I suspect that SLS was involved. Several articles listed
in the toxnet site confirm that rinsing with water will take care of the irritation, with no
lasting side effects. Other common chemicals, such as capsaicin or table salt, are even
more powerful as irritants. Peeling onions will get sulfuric acid
in your eyes. Status: MISLEADING, IRRELEVANT.
- The tobacco companies denied for decades that cigarette smoke causes cancer, but in this case we are
led to believe that a simple phone call settles the matter. Note that there are several other foaming agents available, so there is no reason to stick to a known carcinogenic.
If all else fails, you don't actually need foam to make a good shampoo; customers associate
lots of bubbles with a cleaner effect, but the two things are almost unrelated. Status:
Part 4: Birth of a hoax
This is entirely my opinion, but... who created the hoax? The original author might have
thought that scaring lots of people was funny (remember, SLS is really ubiquitous), but
I suspect that some companies used it as free advertising to push their products.
They just had to mix a shampoo formula that doesn't contain SLS, put it in the list of
"good, natural personal care products" and let the meme machines do the rest. Probably they
also raised the price of those shampoo bottles: they are the good guys fighting against
cancer mongers, after all...
Different points of view are welcome. Different points of view supported by reputable sources are especially welcome.