Do the deodorant toilet bowl blocks used in public restrooms cause cancer?

“Chemical compounds in household products like mothballs and air fresheners can cause cancer by blocking the normal process of cell suicide,” reported University of Colorado researchers this week.

The chemical compounds at issue are naphthalene, which is used in mothballs, and para-dichlorobenze (PDCB), which is used in deodorant toilet bowl blocks and other air fresheners.

The study spawned worrisome headlines from the United Press International (“Mystery of carcinogenic mothballs solved”) and “[Colorado University] sniffs out cancer link in mothballs”).

“This study shows why mothballs and some air freshener products may be harmful to humans,” said study author Ding Xue. “Understanding how carcinogenic compounds can trigger tumor growth is important for federal regulatory agencies that deal with human exposure to hazardous chemicals,” Xue added.

Since consumers use more than one million pounds of naphthalene and PDCB annually, should this new study cause worry?

First, the researchers did not study whether the chemicals actually caused cancer in humans. Instead, they studied the effects of the chemicals on nematodes – worms, that is. When the worms were exposed to the chemicals, cells that normally would have died, instead survived, according to the researchers.

Before addressing the more general proposition that chemical-caused biological effects in worms might be reasonable predictors of chemical effects on humans, consider the existing studies involving actual human exposure to naphthalene and PDCB. We can only consider such studies very briefly, however, since there are none that link either chemical to cancer in humans.

As the Environmental Protection Agency says on its web site, “Available data are inadequate to establish a causal association between exposure to naphthalene and cancer in humans” and “No information is available on the carcinogenic effects of [PDCB] in humans.”

And it’s not like naphthalene and PDCB are new substances that no one knows anything about. Naphthalene was produced in the early 19th century and was in widespread use as an insect repellent (moth balls) by 1900. The use of PDCB as an insecticide/moth repellent dates back to 1912.

By 1934, 21 million gallons of naphthalene and PDCB were used annually in the U.S. alone. Virtually everyone is exposed to naphthalene – not only from moth balls, but also from the burning of coal and gasoline – and PDCB, typically from toilet deodorant blocks.

Moreover, despite the large scale production of naphthalene and PDCB, no studies report higher cancer rates among workers who would be expected to have relatively high exposures to the chemicals.

That brings us to the University of Colorado’s worms.

Neither naphthalene nor PDCB reportedly caused cancer in the worms – it’s not even clear that worms can get cancer in the first place. The chemicals reportedly merely delayed cell death, which may be linked with cancer, but not necessarily. Not only is the relevance of this particular biological event to cancer development unclear, but relevance of worm biology to humans is also questionable -- worms and humans, after all, belong to different animal groups or phyla. So why use worms in the first place if their relevance to humans is dubious?

“[Testing chemicals] on lab rats can take two years to complete,” Xue said in a media release. “But we can do the same kind of [tests with worms] in two weeks,” Xue added. Xue’s goal, therefore, is to be able to test chemicals on worms rather than animals because the process is shorter and less expensive.

Xue’s idea is not entirely illogical, but it does increase the uncertainty in extrapolating research results -- at least lab rats are in the same animal phylum as humans. But Xue may want to reconsider the whole notion of laboratory testing of chemicals for their potential to cause cancer.

The idea that typical human exposures to chemicals increase cancer risk and that testing chemicals on lab rats is a good way to find out whether particular chemicals pose cancer risks is a now largely discredited idea leftover from 1970s-era hysteria over chemicals. Since the 1970s, millions of lab rats have been poisoned with thousands of chemicals to produce uncountable cancer scares and incalculable regulatory and consumer costs.

But when you compare the results of those high-dose lab rat experiments with much lower-dose, real-life human exposures to chemicals, it’s become clear over the years that the lab rat experiments are of little-to-no relevance to humans.

Based on Xue’s naphthalene/PDCB study, it doesn’t appear that lab worms represent an improvement over lab rats. So next time you’re in a public restroom and you see one of those toilet deodorant blocks, you can flush away any cancer anxiety that you may have.

Steven Milloy publishes and He is a junk science expert, an advocate of free enterprise and an adjunct scholar at the Competitive Enterprise Institute.

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