An insect repellent is a substance applied to skin, clothing, or other surfaces which discourages insects (and arthropods in general) from landing or climbing on that surface. There are also insect repellent products available based on sound production, particularly ultrasound (inaudibly high frequency sounds). These electronic devices have been shown to have no effect as a mosquito repellent by studies done by the EPA and many universities.
Insect repellents help prevent and control the outbreak of insect-borne diseases such as malaria, Lyme disease, dengue fever, bubonic plague, and West Nile fever. Pest animals commonly serving as vectors for disease include the insects flea, fly, and mosquito; and the arachnid tick.
Common insect repellents
- Birch tree bark is traditionally made into tar. Combined with another oil (i.e., fish oil) at 1/2 dilution, it is then applied to the skin for repelling mosquitos
- DEET (N,N-diethyl-m-toluamide)
- Essential oil of the lemon eucalyptus (Corymbia citriodora) and its active compound p-menthane-3,8-diol (PMD)
- Icaridin, also known as picaridin, Bayrepel, and KBR 3023
- Nepetalactone, also known as "catnip oil"
- Citronella oil
- Neem oil
- Bog Myrtle
- Dimethyl carbate
- Tricyclodecenyl allyl ether, a compound often found in synthetic perfumes.
- IR3535 (3-[N-Butyl-N-acetyl]-aminopropionic acid, ethyl ester)
- Ethylhexanediol or "6-12 repellent," discontinued in the US in 1991 due to evidence of causing developmental defects in animals
- Permethrin is different in that it is actually a contact insecticide.
- A more recent repellent being currently researched is SS220, which has been shown to provide significantly better protection than DEET.
Synthetic repellents tend to be more effective and/or longer lasting than "natural" repellents. In comparative studies, IR3535 was as effective or better than DEET in protection against mosquitoes. However, some plant-based repellents may provide effective relief as well. Essential oil repellents can be short-lived in their effectiveness, since essential oils can evaporate completely.
A test of various insect repellents by an independent consumer organization found that repellents containing DEET or picaridin are more effective than repellents with "natural" active ingredients. All the synthetics gave almost 100% repellency for the first 2 hours, where the natural repellent products were most effective for the first 30 to 60 minutes, and required reapplication to be effective over several hours.
For protection against mosquitos, the U.S. Centers for Disease Control issued a statement in May 2008 recommending equally DEET, picaridin, oil of lemon eucalyptus and IR3535 for skin. Permethrin is recommended for clothing, gear, or bed nets. In an earlier report, the CDC found oil of lemon eucalyptus to be more effective than other plant-based treatments, with a similar effectiveness to low concentrations of DEET. However, a 2006 published study found in both cage and field studies that a product containing 40% oil of lemon eucalyptus was just as effective as products containing high concentrations of DEET. Research has also found that neem oil is mosquito repellent for up to 12 hours. Citronella oil's mosquito repellency has also been verified by research, including effectiveness in repelling Aedes aegypti, but requires reapplication after 30 to 60 minutes.
- Children may be at greater risk for adverse reactions to repellents, in part, because their exposure may be greater.
- Keep repellents out of the reach of children.
- Do not allow children to apply repellents to themselves.
- Use only small amounts of repellent on children.
- Do not apply repellents to the hands of young children because this may result in accidental eye contact or ingestion.
- Try to reduce the use of repellents by dressing children in long sleeves and long pants tucked into boots or socks whenever possible. Use netting over strollers, playpens, etc.
- As with chemical exposures in general, pregnant women should take care to avoid exposures to repellents when practical, as the fetus may be vulnerable.
Regardless of which repellent product used, it is recommended to read the label before use and carefully follow directions. Usage instructions for repellents vary from country to country. Some insect repellents are not recommended for use on younger children.
In the DEET Reregistration Eligibility Decision (RED) the United States Environmental Protection Agency (EPA) reported 14 to 46 cases of potential DEET associated seizures, including 4 deaths. The EPA states: "... it does appear that some cases are likely related to DEET toxicity," but observed that with 30% of the US population using DEET, the likely seizure rate is only about one per 100 million users.
The Pesticide Information Project of Cooperative Extension Offices of Cornell University states that, "Everglades National Park employees having extensive DEET exposure were more likely to have insomnia, mood disturbances and impaired cognitive function than were lesser exposed co-workers".
The EPA states that citronella oil shows little or no toxicity and has been used as a topical insect repellent for 60 years. However, the EPA also states that citronella may irritate skin and cause dermatitis in certain individuals. Canadian regulatory authorities concern with citronella based repellents is primarily based on data-gaps in toxicology, not on incidents.
Within countries of the European Union, implementation of Regulation 98/8/EC, commonly referred to as the Biocidal Products Directive, has severely limited the number and type of insect repellents available to European consumers. Only a small number of active ingredients have been supported by manufacturers in submitting dossiers to the EU Authorities.
In general, only formulations containing DEET, icaridin (sold under the trade name Saltidin and formerly known as Bayrepel or KBR3023), IR3535 (3-[N-Butyl-N-acetyl]-aminopropionic acid, ethyl ester) and Citriodiol (p-menthane-3,8-diol) are available. Most "natural" insect repellents such as citronella, neem oil, and herbal extracts are no longer permitted for sale as insect repellents in the EU; this does not preclude them from being sold for other purposes, as long as the label does not indicate they are a biocide (insect repellent).
Insect repellents from natural sources
There are many preparations from naturally occurring sources that are repellent to certain insects. Some of these act as insecticides while others are only repellent.
- Achillea alpina (mosquitos)
- alpha-terpinene (mosquitos)
- Sweet Basil (Ocimum basilicum)
- Callicarpa americana (Beautyberry)
- Camphor (moths)
- Carvacrol (mosquitos)
- Castor oil (Ricinus communis) (mosquitos)
- Catnip oil (Nepeta species) (nepetalactone against mosquitos) (although attracts cats)
- Cedar oil (mosquitos, moths)
- Celery extract (Apium graveolens) (mosquitos) In clinical testing an extract of celery was demonstrated to be at least equally effective to 25% DEET, although the commercial availability of such an extract is not known.
- Cinnamon (leaf oil kills mosquito larvae)
- Citronella oil (repels mosquitos)
- Oil of cloves (mosquitos)
- Eucalyptus oil (70%+ eucalyptol), (cineol is a synonym), mosquitos, flies, dust mites)
- Fennel oil (Foeniculum vulgare) (mosquitos)
- Garlic (Allium sativum) (rice weevil, wheat flour beetle)
- Geranium oil (also known as Pelargonium graveolens)
- Lavender (ineffective alone, but measurable effect in certain repellent mixtures)
- Lemon eucalyptus (Corymbia citriodora) essential oil and its active ingredient p-menthane-3,8-diol (PMD)
- Lemongrass oil (Cymbopogon species) (mosquitos)
- Marigolds (Tagetes species)
- Marjoram (Spider mites Tetranychus urticae and Eutetranychus orientalis)
- Neem oil (Azadirachta indica) (Repels or kills mosquitos, their larvae and a plethora of other insects including those in agriculture)
- Oleic acid, repels bees and ants by simulating the "Smell of death" produced by their decomposing corpses. It is a 400 millions years old natural mechanisms helping to sanitise the hives or to escape predators
- Pennyroyal (Mentha pulegium) (mosquitos, fleas), but very toxic to pets.
- Peppermint (Mentha x piperita) (mosquitos)
- Pyrethrum (from Chrysanthemum species, particularly C. cinerariifolium and C. coccineum)
- Rosemary (Rosmarinus officinalis) (mosquitos)
- Spanish Flag (Lantana camara) (against Tea Mosquito Bug, Helopeltis theivora)
- Tea tree oil
- Thyme (Thymus species) (mosquitos)
- Yellow Nightshade (Solanum villosum), berry juice (against Stegomyia aegypti mosquitos)
- Andrographis paniculata extracts (mosquito)
Inactive substances – carriers
In 2002, the New England Journal of Medicine published an article that found products containing essential oils such as catnip or geranium oil, when combined with a suitable carrier oil such as soybean, have been found to be effective as natural repellents. This was based on testing done by Johns Hopkins and Cornell Universities. Other commercial products offered for household mosquito "control" include small electrical mats, mosquito repellent vapor, DEET-impregnated wrist bands, and mosquito coils containing a form of the chemical allethrin. Mosquito-repellent candles containing citronella oil are sold widely in the U.S. These have been used with mixed reports of success and failure.
Less effective methods
Some old studies suggested that the ingestion of large doses of thiamin could be effective as an oral insect repellent against mosquito bites. However, there is now conclusive evidence that thiamin has no efficacy against mosquito bites. Some claim that plants like wormwood or sagewort, lemon balm, lemon grass, lemon thyme and the mosquito plant (Pelargonium) will act against mosquitoes. However, scientists have determined that these plants are “effective” for a limited time only when the leaves are crushed and applied directly to the skin.
There are several, widespread, unproven theories about mosquito control, such as the assertion that vitamin B, in particular B1 (thiamine), garlic, ultrasonic devices or incense can be used to repel or control mosquitoes. Moreover, some manufacturers of "mosquito repelling" ultrasonic devices have been found to be fraudulent, and their devices were deemed "useless" in tests by the UK Consumer magazine Which?, and according to a review of scientific studies.
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