With summer approaching, so too approaches the time to stock up on repellents for the seasonal onslaught of insects hungry for human blood. There are a number of different chemicals that are responsible for the repellent effects of the various sprays or creams available – and chemistry can also offer possible explanations for why some people are just that much more attractive to the humble mosquito than others.
Firstly, it’s worth considering what it is that makes humans so alluring to mosquitos and the like. 1-octen-3-ol is the compound to blame – it’s present in human sweat and breath, and acts as an insect attractant. It’s so potent in this regard that it’s used in insect traps to lure in their unsuspecting prey. Interestingly, it’s more commonly referred to as ‘mushroom alcohol’, as a particularly optical isomer of this compound is produced by mushrooms and considered largely responsible for their aroma and flavour.
In terms of repellents, there are a wide range of both natural and synthetic compounds that exhibit insect-repelling activity, but in the EU and the US, there are four main compounds that are approved for use: DEET, Icaridin, Citrioldiol, and IR3535. Each of these differ slightly in their effectiveness and characteristics, but all of them work in a similar way, by producing an odour that insects find repulsive.
DEET is by far the most commonly used compound of the aforementioned group. It was originally developed by the US army for use World War 2, and has been available to the public as an insect repellent since 1957. It is sold in a variety of concentrations – at 100%, it has an approximate efficacy of 12 hours, but in the more commonly available 20-30% solutions, this drops to a maximum of 8 hours. Although some controversy has dogged DEET, if used as directed, guidelines have stated that it represents no risk to humans. However, it has been linked with seizures in a very small proportion of people (estimated to be around 1 in 100 million users). It’s also recommended in some countries that children under the age of 12 use a lower concentration of DEET spray than adults.
Though it may be the most common insect repellent, DEET has its drawbacks; it has an unpleasant smell, greasy feel, and can also damage plastics. For this reason, a number of other synthetic alternatives are available. Icaridin, also known as picaridin, is one of these, which has the advantage of being odourless and not damaging to plastics, unlike DEET. It also seems to provide comparable protection to DEET, and is effective against an equally large range of insects. IR3535 is another catchily named alternative compound, which also provides a similar efficacy to DEET, although some studies have shown this to be slightly lower against certain species of mosquito. Though perhaps in part because its use is less widespread, it’s worth noting that, apparently, no negative effects have ever been reported for IR3535.
The only natural compound that is approved in the EU for sale as an insect repellent is citriodiol, or the essential oil of the lemon eucalyptus tree. Overall, studies seem to show that this compound provides slightly shorter protection than DEET, but as well as acting as a repellent, it can also act as a miticide and kill mites and some insects. Despite the oil’s natural origin, some commercial products are made with synthetic versions of the active compound, p-menthane-3,8-diol. This synthetic version is no different chemically from the naturally occurring version of the compound, and acts in exactly the same way.
All this talk of repellents is well and good – but why is it that some people seem to be able to repel mosquitos without them? Whilst science still can’t be sure of the precise reasons, researchers have relatively recently made a discovery which may shed some light on this question. They found that certain compounds, that were either secreted through the skin, or generated by bacteria on the surface of the skin, can actually act to render a person effectively ‘invisible’ from a certain species of mosquito’s senses. One of these compounds is called 1-methylpiperazine, though they have also identified a number of other compounds which contain small heterocyles (rings of carbon atoms also containing atoms other than carbon or hydrogen). Obviously, this could have implications for future methods of preventing insect bites, and differing levels of these compounds from person to person could be one of several factors affecting why mosquitos just prefer some people more than others.
Thanks to Amanda Edward for the idea for this graphic!
References & Further Reading
- Protection Times Data Table – EPA
- Insect Repellents – Principles, Methods & Uses – M Debboun & others
- Chemical Invisibility Cloaks Thwart Mosquitos – C&EN Magazine
- Mushroom Alcohol – Molecule of the Month
- Mosquito photo: photochem_PA, Creative Commons BY licence