Science, Journalism, and Bug Spray

Watch out! The BBC report that -

Deet bug repellent 'toxic worry'

While The Telegraph are even more concerned -

Insect repellent Deet is bad for your nerves, claim scientists

This is in reference to a new paper about the widely-used insect repellent DEET. The BBC, as usual, performed slightly better than the Telegraph here. They included quotes from two experts making it clear that the research in question was preliminary and in no way proves that DEET is dangerous to humans. But they still ran the headline implying that DEET could be "toxic", which is the only thing most people will remember about the article. As you'll see below, this is quite misleading.

DEET is an insect repellant, generally used to prevent mosquito bites. You spray it on your skin, clothes, mosquito nets, etc. If you've ever been to a tropical country, you'll probably remember it. It has a distinctive smell, it stings the eyes and throat, and, most distressingly, it dissolves plastics. My watch fell off in Thailand because DEET ate through the strap.

That aside, DEET is believed to be safe, so long as you spray it instead of drinking it. Hundreds of millions of people have used it for decades. And it works, which means it saves lives. Mosquitoes spread diseases like malaria, yellow fever, Dengue, and plenty more. They can all kill you. This is why any health professional will advise you to use mosquito-repellants, preferably DEET-based ones, when visiting risk areas.

So it would be massive news if DEET was found to be dangerous. But it hasn't. What's been found is that, in animals and in test-tubes, DEET is a cholinesterase inhibitor. Cholinesterase is an enzyme which breaks down acetylcholine (ACh), a neurotransmitter. If you inhibit cholinesterase, ACh levels rapidly increase. This can cause problems because ACh is the transmitter that your nerves use to communicate with your muscles. As ACh builds up, your muscles don't stop contracting, and you suffer paralysis, until you can't breathe. This is how "nerve gas" works.

But we know DEET isn't a strong cholinesterase inhibitor, when used normally, because people don't get cholinergic effects after using it. The toxicity of cholinesterase inhibitors is acute. You get paralyzed, and suffer other symptoms like uncontrollable salivation, crying, vomiting, and incontinence. You'd know if this happened to you.

Cholinesterase inhibitors are not, as various media reports have said about DEET, "neurotoxic" , they do not cause "neural damage". They act on the nerves, but they do not damage the nerves. In fact people with Alzheimer's take them (in low doses!), as do people with the nerve disease myasthenia gravis.

So the fact that DEET can act as a cholinesterase inhibitor in the lab changes nothing. It's still safe, at least until evidence comes along that it actually causes harm in people who use it. You can't show that something is harmful by doing an experiment showing how it could be harmful in theory.

To be fair, there is one cause for concern in the paper - in the experiments, DEET interacted with other cholinesterase inhibitors, leading to an amplified effect. That suggests that DEET could become toxic in combination with cholinesterase inhibitor insecticides, but again, the risk is theoretical.

The media should never have reported on this paper. The science itself is perfectly good, but the results are completely irrelevant to the average person who might want to use DEET. They are of interest only to biologists. If people decide not to use DEET on the basis of these reports, they are putting themselves in danger. Others have noted that journalists almost always report on laboratory experiments like these as if they were directly relevant to human health. They're not.

Appendix: In one of the articles, an expert says that "I also would guess that the actual concentration [of DEET] in the body is much lower than they had to use in the study to see an effect in the mouse tissues." But we don't have to guess, we can work it out. DEET had detectable effects in mammalian tissues at a concentration of 0.5 millimole. A millimole is a unit of concentration; 1 millimole is 0.19 grams DEET per liter of water. (Molar weight of DEET = 191g/mole). The human body is 60% water by weight. A person weighs, say, 75 kg, which means roughly 50 liters of water. That means that to achieve the level of DEET used in this study, you would need to absorb into your body about 50 x 0.19 = 9.5 grams of DEET (assuming it was evenly distributed in your body).

That's a huge amount. But maybe it's not completely impossible, bearing in mind that DEET might be absorbed through the skin? Is there any data on DEET levels in humans? Yes. This paper reports on the development of a way of measuring DEET in human blood. This method could detect DEET at levels from 1 ng/mL to 100 ng/mL. I assume that the upper limit was chosen because no-one ever gets more DEET than that. 100 ng / mL = 100 micrograms / L = 0.52 micromolar = 0.0005 millimolar. That's 1000-fold too low, and that's the upper limit.

This was just a back-of-the-envelope calculation so please feel free to critique it, but, I find it reassuring.

[BPSDB]

Corbel, V., Stankiewicz, M., Pennetier, C., Fournier, D., Stojan, J., Girard, E., Dimitrov, M., Molgo, J., Hougard, J., & Lapied, B. (2009). Evidence for inhibition of cholinesterases in insect and mammalian nervous systems by the insect repellent deet BMC Biology, 7 (1) DOI: 10.1186/1741-7007-7-47