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Relax, the borax in slime will not kill your kids

Citizen Scientist asks a medical toxicologist how worried we should be about Health Canada's warnings over borax, found in popular arts and crafts materials.

Health Canada released a report on borax last summer. But a toxicologist tells us the situation is under control. Slime on.

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Health Canada released a report on borax last summer. But a toxicologist tells us the situation is under control. Slime on.

The Great Slime Panic of 2017. If you have school-age kids, you know what I’m talking about.

Mania for slime – a bouncy, stretchy concoction of water, craft glue, food colouring and borax – is sweeping social media.

But Health Canada recently recommended against using borax in children’s arts and crafts, citing possible “developmental and reproductive health effects.”

As a rule, I trust Health Canada’s scientific expertise completely. But something about this dictum didn’t sit right with me.

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Canadians have been using borax, a powdery white mineral, as a household detergent for more than a century. And small, harmless amounts of boron, its elemental form, naturally occur in fruits, vegetables and drinking water.

Borax can irritate skin and eyes, and you definitely shouldn’t eat it, but could playing with goo really harm your child’s future reproductive health? And why is the government suddenly concerned about it?

The second question is easier. Health Canada has launched a long-term project evaluating the safety of chemicals used for a long time, but perhaps not scrutinized enough back in the day.

For insight into the first question, I asked medical toxicologist Dr. Andrew Stolbach to walk me through Health Canada's weighty, 11,000-word safety report.  

The highlights: Long-term exposure to high doses of boron is linked to testicular abnormalities in dogs and rats. These studies are small and mostly more than 20 years old.  

Using the animal data, researchers calculated a “benchmark dose” – a low level of boron exposure we can agree is safe for people. Put another way, they’re 95 per cent sure the real safe dose is actually much higher, Stolbach said. The benchmark is 2.9 mg of boron per kilogram of body weight per day.
There are a few small studies of people exposed to high levels of boron for years – up to 25 times the benchmark. They had slightly elevated rates of fertility problems and miscarriage. But the difference was not significant.

By testing boron levels in people’s blood, Health Canada figured out that the average Canadian consumes between 1/27 and 1/290 the benchmark dose of boron.

But considering the uncertainty in the data, those numbers are high enough that we should be avoiding other sources of boron, like slime, the agency said.

Dr. Stolbach found the conclusion “puzzling.”
“I think 1/27 is pretty good, especially when we’re not talking about cancer," he said. "We’re talking about a questionable effect.”
So how much boron could your kid absorb while playing with slime? Anywhere from 0.063 mg per kilogram of weight to 1.3 mg/kg if they eat it. (I’ve asked how Health Canada settled on these numbers. They confirmed it is a mathematical model.)
“That’s such a huge range. It’s either (nearly) nothing, or half the safe amount,” Stolbach said.

Furthermore, he added, public health authorities write guidelines with the worst-case scenario in mind -- like a child who gets an unusually high amount of boron from drinking water and household sources, and also plays with slime every day, and also has medical conditions that make him more vulnerable.

Bottom line: Stolbach is fine with his five-year-old daughter playing with borax slime now and then.

“To me, it’s a very, very small risk. And it’s a theoretical risk.”

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