The devil is in the DEETails

An article published today in the journal Parasites and Vectors reviews the evidence on whether DEET is unsafe. In this guest post Dr James Logan, one of the authors and Senior Lecturer in Medical Entomology at the London School of Hygiene & Tropical Medicine, as well as Director of arctic, tells us about how we assess the dangers of chemicals like DEET, and how their risks should be balanced against their benefits.

Aedes_aegypti_biting_humanAs a medical entomologist I get a lot of people, either members of the public or health professionals, asking me about which repellents I recommend for travellers. DEET is always top of the list, particularly when travelling to tropical countries (although there are other repellents, including PMD, Picaradin and IR3535, which are effective but require more frequent reapplication).

DEET is undoubtedly the most effective active ingredient on the market. But some people seem somewhat surprised by this, as they either think that DEET doesn’t work very well, or that it is not particularly safe to use.

However, there is a mountain of scientific literature providing evidence that DEET is highly effective when used at the right concentrations (20-50% in tropical regions). DEET has been around been around since the 1950’s, available in concentrations up to 100% commercially, so we have an extremely good historic record of DEET usage. The truth is, DEET is one of the best-studied repellents in existence.

Some people have concerns over the effects of DEET on our health and this has led to many misleading claims about its safety. However, DEET, like all chemicals for human use, has been studied extensively in terms of its toxicity. Unfortunately though, there are discrepancies on how this assessment has been done. Normally the assessment is a staged process which first involves a risk assessment based on toxicity data from testing on animals, and data from case reports in humans and observational studies.

During the risk assessment it should be considered whether the adverse effects seen in animals are actually relevant to humans, what dose is required to observe the adverse effect and what exposures are actually likely by the end users. At this point in the review process a decision is usually made to determine whether there is enough concern over the data to warrant a ‘full risk assessment’ and the assignment of a ‘safe exposure limit’.

If the data do show cause for concern, a full risk assessment is developed and a ‘correction factor’ is applied during this process, which reduces the safe exposure limit by 100 times to err on the side of caution. But if a chemical shows low levels of toxicity, with a low risk to  users during the initial phase of assessment, a full risk assessment is not required; otherwise a chemical may be given an exposure limit much lower than is necessary due to the correction factor.

In the case of DEET, the US Environmental Protection Agency (USEPA) considered its effects observed in rats so modest and the dosage needed to show adverse effects so high, that it did not form sufficient grounds for having a full risk assessment. The USEPA did not, therefore, define a minimum dose of DEET for daily use. In contrast, however, an EU directive, using the same animal data, decided to do a full risk assessment which suggested that the exposure limit should be 15% applied only once per day when the correction factor was used.

The different outcome between these two results highlights the problem with the current assessment process and this assignment is not necessarily appropriate for DEET. When making decisions about the allowable concentrations of DEET it is extremely important to take into consideration the extensive use of DEET and lack of reports of adverse effects. Any assessment should also be balanced by the benefits of using DEET in avoiding diseases such as malaria and dengue fever.

If DEET products were restricted to 15% or less, as modelled in the 2010 EU directive based on toxicity estimates from animal studies alone, it is likely to result in people not being protected adequately whilst travelling and therefore put them at much greater risk of disease. There is plenty of evidence to show that low levels of DEET are not very effective, so it is important that travellers have access to repellents with greater levels of DEET that will provide the best protection.

An estimated 200 million applications of DEET to the skin are made every year, and since 1957 there have only been 14 cases of encephalopathy associated with DEET. Even if you take into account under-reporting, if DEET caused serious health problems, we would know about it by now.

Our paper is published on the same day that we launch Bug off – the very first Insect Repellent Awareness Day which aims to educate the public about the importance of using repellents such as DEET and to dispel the myths of eating vitamins and food supplements to repel mosquitoes.

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