Insect Repellents

While on the whole I think most people probably like summer, it is the time of year when those pesky insects, and in particular mosquitoes, come out to bother us. Locally, it is expected to be a particularly bad year for mosquitoes due to ideal breeding conditions. In addition to being a general annoyance and leaving itchy mosquito bites, mosquitoes and other insects can transmit diseases such as West Nile Virus (or, if you happen to be in a more tropical location, malaria).

To keep mosquitoes and other insects away, we commonly use insect repellents. There are other approaches, ranging from staying indoors (but who wants to do that in the summer) to wearing protective clothing (again not ideal) to using various candles, torches, electrical devices, etc. that hopefully keep them away, but if you’re not going to be staying in one spot I suspect most people would choose an insect repellent. So how do you decide which of the many products available is the best?

Probably the most common insect repellents are based on DEET (N,N-Diethyl-meta-toluamide). The main reason DEET is commonly used is because it is reasonably effective at repelling insects (scientists used to believe it blocked their olfactory receptors, but current thinking is that it actually repels insects due to its smell). It is available in a range of concentrations; the higher concentrations are not more effective, but their protection lasts longer.

Is DEET safe though? US EPA (the United States Environmental Protection Agency) recently summarized the results of acute, subchronic and chronic toxicity studies on DEET. They found that DEET does not appear to be carcinogenic or mutagenic, even at very high doses. Other toxic effects such as neurotoxicity and kidney effects were observed only at very high doses (much higher than those associated with use as an insect repellent) in animal studies. However, there is some evidence that it may be associated with seizures in rare cases, including 4 that led to deaths; while it is difficult to conclusively link a specific seizure to DEET exposure, overall the weight of evidence led US EPA to conclude that data are insufficient to support a direct link between DEET exposure and seizures, and even if there is a link only about 1 in 100 million users could experience seizures. Overall, using DEET appears to be much safer than being exposed to mosquitoes, ticks, and other disease-carrying insects.

How about alternatives to DEET?

A recent study found a compound called Isolongifolenone, derived from pine oil, to be as effective as DEET at repelling mosquitoes and ticks. However, after a fairly cursory search I didn’t find any good toxicity studies or reviews, and some indication that carcinogens are used in its manufacture, so a lot more study is required before I’d endorse its use.

Lemon eucalyptus oil (p-menthane-3,8-diol) is a commonly used “natural” repellent. It isn’t as effective as DEET, but is considered the most effective natural repellent. Other vegetable oils may also be used. Their toxicity has not been evaluated to the same extent as DEET, but they are not expected to be highly toxic and historical use hasn’t shown any evidence of adverse effects. However, they are not recommended for children under 3. Soybean oil is another vegetable oil repellent, which is not as effective, but can be used for younger children.

Citronella products are also used. Health Canada concluded that, while there was no evidence of imminent health risks, safety data were lacking. They are not recommended for use on infants or toddlers, are not as effective as DEET, and the duration of protection is much shorter.

Many other substances have been used in insect repellents, but are generally much less effective than DEET or lemon eucalyptus oil.

Overall, DEET-based repellents are still considered the best option, particularly if you are potentially exposed to a lot of mosquitoes and ticks. However, the concentration should be limited to reduce the possibility of toxicity. Health Canada recommends the following doses:

  • Up to 30% for adults and children over 12, applied every 6 hours.
  • Up to 10% for children 2 to 12, applied every 3 hours, to a maximum of 3 times daily.
  • Up to 10% for children 6 months to 2 years, a maximum of once per day.
  • Health Canada does not recommend DEET-based repellents for infants under 6 months

Lemon eucalyptus oil products appear to be a viable alternative as well, for adults and children over 3.

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