Summary of Recommendations:
For best protection, use a combination of permethrin impregnated clothing and 1)DEET of 10-30% if mosquitos are the primary concern, or 2)IR3535, Picaridin, or PMD if ticks, biting flies or midges are of concern.
Children – No DEET for children under two months. No Picaridin for children under two years. No PMD for children under 3 years. Maximum DEET concentration of 30% for all children.
Pregnancy – No specific recommendations to avoid any insect repellents during pregnancy. IR3535 and Permethrin are FDA Pregnancy Category B, and can be used safely in pregnancy. All other repellents are FDA Pregnancy Category N (not classified). Many sources recommend avoiding PMD use in pregnancy, but no developmental toxicities have been reported.
Summary of efficacy and toxicity:
|Mosquitoes||Ticks||Flies & Midges||Toxicity|
|Picaridin||Medium to High||Medium||High||Minimal|
|IR3535||Medium to High||Medium||High||Moderate – ingestion|
|Citronella||Low to Medium*||Low*||Low*||Minimal|
|Permethrin||High||High||High||Moderate – ingestion|
|*Limited by short duration of efficacy|
The majority of arthropod borne disease is not preventable by vaccine. The exceptions to this are yellow fever, Japanese encephalitis and some regional tick-borne viruses in Eastern Europe.1 This means the most common mosquito borne infection in the US, West Nile Virus (which can cause serious meningoencephalitis), does not have a vaccine.1 Mosquitoes can transmit both viruses and parasites, but only the females bite (males feed on flower nectar).1,2 They are attracted by visual stimuli (dark clothing more so) and volatile substances (including repellents) from afar, and by heat, carbon dioxide released from skin, lactic acid and acetone (in breath) closer up.1,2 The amount of CO2 released primarily determines a person’s attractiveness, meaning males, adults and larger persons have more propensity for being bit.2 Ticks are more versatile, transmitting bacteria, viruses and parasites. They may be asymptomatically co-infected with multiple pathogens, and can pass those pathogens onto their progeny.1
Insect repellents are organic or chemical agents that discourage insect contact and biting by making the atmosphere within four centimeters of the skin potently noxious. The FDA regulates repellents and approves the topical versions for use and safety in pregnancy.1 An ideal repellent works on a broad range of arthropods, does not damage skin or clothing, can be applied with sunscreen, is not oily or overly odorous, and is difficult to remove by sweating or wiping.1,2 As it wears off, the mosquitos will make contact first, and only bite when more has worn off, giving you a window to reapply.2 I will split the repellents into two categories: synthetic chemicals and plant-based oils.
Probably the most well-known repellent , DEET was first used by the military to replace the toxic chemical dialkyl phthalate, and marketed to the public starting in 1956.1 DEET causes damage to rayon, spandex and leather clothing (cotton, wool and nylon are okay).1,2 Some recent resistance to DEET has been noted in the Anopheles mosquito.2
Concentration: Can range from 5-100%, but most products are 30-40% or less.1,2 There is a plateau in insect repellent effects as you exceed 50% concentration, and 50-75% DEET has been shown to cause erythema with vesiculobullous skin necrosis and residual scarring.1 Against Anopheles mosquitoes, concentrations of 4-15% provided >95% protection for 5-11 hours, and 19-25% concentrations provided 100% protection for 3-10 hours. Over 25% concentration showed no better protection.2 Against Culex mosquitoes, DEET generally provided >90% protection through 7 hours.2 DEET does not work as a great repellent for ticks, as only the higher concentrations show decent efficacy.1
Toxicity: Toxicity has been recorded mainly in children related to instances of over-application and ingestion. Symptoms were mainly neurlogic (lethargy, headache, confusion, ataxia, tremors, seizures, acute encephalopathy), but cardiovascular3 (orthostatic hypotension, bradycardia) and allergic and cutaneous manifestations (urticarial, hemorrhagic vesicubullous erosions, mainly with higher concentrations) have been noted.
Recommendations in Pregnancy: DEET is considered safe in pregnancy. Although it does cross the placenta, no developmental toxicity has been recorded. It has not been classified by the FDA.
Recommendations in Children: Do not use in children under two months old. Use concentrations less than 30% for all children.1 Use just enough to cover the skin, heavier application does not equal better protection. Do not apply under sunscreen; in animal models when applied under sunscreen, there was increased absorption of DEET.4 Do not use combined sunscreen and repellent as sunscreen needs to be reapplied more often.5
First introduced in Europe, Picaridin has been available in the US since 2005 and offers advantages such as no odor, non-sticky/greasy feel, and no damage to clothes or plastics.1 It is effective against mosquitoes, flies, chiggers and ticks and comes as a lotion, spray or wipe.1
Concentrations: Picaridin comes in 7-20% concentrations. Like DEET, it has a longer duration of effectiveness against mosquito carriers of arbovirus and filarial parasites than the anopheline transmitter of malaria (6-8 hours vs. 1-6 hours). At 20% concentration, picaridin appears to work better than DEET against ticks, providing at least 1-5 hours of complete protection (some studies say 12 hours) and working across a broader range of tick species.1,2
Toxicity: Low to no toxicity.
Recommendations in Pregnancy: Picaridin is considered safe in pregnancy. Although it does cross the placenta, no developmental toxicity has been recorded. It has not been classified by the FDA.
Recommendations in Children: No toxicity has been recorded, but the manufacturers recommend not to use it in children under two years old.1
IR3535 (ethyl butylacetylaminoproprionate; 3-N-butyl-N-acetyl aminoproprionic acid)
Initially marketed as a skin emollient and moisturizer, hunters began to use IR3535 after noticing its effective protection against biting midges (“no-see-ums”). It is superior to DEET in its protection against biting midges, blackflies (carriers of onchocerciasis) and sandflies (carriers of leishmaniasis).1 Similar to DEET, IR3535 can damage or dissolve plastics.
Concentrations: IR3535 comes in concentrations of 10-30% in various products. At low concentrations it is comparative to DEET and PMD in repelling mosquitoes, while at 20% it better than DEET, offering 10-14 hours of near-complete protection.1,2
Toxicity: Can be irritating to the eyes with direct contact. No skin irritation or other toxicity has been reported in humans.
Recommendations for Pregnancy: Classified in FDA Pregnancy Category B. Safe for use in pregnancy.
Recommendations for Children: No adverse effects reported in children.
Oil of Lemon Eucalyptus (p-menthan-3,8-diol (PMD))
PMD skirts the edge of the synthetic chemical and plant oil categories. While it is derived from the natural oil of the lemon eucalyptus leaf, PMD is the synthetic version of its repellent component, and the pure extract is not registered with the EPA.1,2 It is not volatile in nature, so it tends to last longer than other plant compounds.2
Concentrations: PMD comes in a spray formulation and is found in concentrations of 10-40% . It is equal in efficacy to DEET as a mosquito repellent, providing 2-8 hours of complete protection.1,2 However, it is reportedly better than DEET for tick protection, reducing attachment and feeding by 77%, providing complete protection for 2-3hrs and protecting against a broader range of tick species.1,2 It also has shown some effectiveness in protecting against some species of biting midges.1
Recommendations for Pregnancy: Since the CDC recommends avoiding use in small children, many sources recommend avoiding its use in pregnancy. While no reports of developmental toxicity exist, essential oils and/or their metabolites may still cross the placenta. With better alternatives easily available, PMD should be avoided in pregnancy.
Recommendations for Children: Should not be used in children under three years old due to skin irritation issues.5
Plant Based Oils
Concentrations: Citronella comes in concentrations from 0.5-20%.1 It is not effective against flies, fleas, biting midges or ticks, and should only be used when mosquitos are the primary or only concern.1 Due to its volatility it has a shorter duration of action, and will provide 100% protection against mosquitos for two hours, dropping to 60% protection by four hours.6 If mixed with 5% vanillin (a fixative), the duration of 100% protection can be extended to six hours.6 As a general rule, essential oils are volatile and short acting, but their duration can be increased by the use of fixatives (liquid paraffin, vanillin, salicyluric acid, mustard, coconut oil).6
Toxicity: Little to no toxicity. Skin irritation is minor concern. No adverse effects reported over the last 65 years of use.
Recommendations in Pregnancy and Children: No specific recommendations but appears safe in sensitive populations. No developmental toxicity reported. No adverse events in children reported.
Permethrin is a combination repellent and insecticide derived from crushed flowers of Chrysanthemum cinerarifolium. It works by blocking sodium channels, followed by acetylcholinesterase inhibition leading to a fatal paralysis.1 It was first used in agriculture, then introduced to the military in 1990. It is applied only to clothing or fabrics (doesn’t damage fabrics, plastic or rubber), and does not show additional benefit if applied directly to the skin.1 It does not interfere with the function of moisture wicking fabrics. It can be applied to clothing, bed nets, tents, sleeping bags, etc. and provides great protection against mosquitoes, flies, biting midges, chiggers, fleas, sandflies and ticks.1 While the manufacturer recommends keeping treated fabrics stored in closed, dark places, stating the product breaks down in four days if left in open oxygenation and sunlight, at minimum the fabrics should be retreated after 5-70 washings.1 It provides better tick protection than DEET and picaridin, but is typical used in combination with a topical insect repellent.
Toxicity: Only seen in massive ingestions or inhalations. Symptoms include ataxia, hyperactivity, hyperthermia, seizures and paralysis.1
Recommendations in Pregnancy: FDA Pregnancy Category B (no adverse effects in animal studies). Considered safe in pregnancy. No developmental toxicity reported.
Plant oil based candles
While there is no solid evidence supporting or refuting their effectiveness , one study looked at a variety of these candles and found compounds that displayed effective repellent properties against several species of mosquitoes at low concentrations: catnip (Lamiaceae), hairy basil (Lamiaceae), citronella, vetiver and clove (myrtaceae). Hairy basil at a 10% concentration was shown to be a potential natural insecticide, as it caused a 90% mortality of the nearby mosquitoes in the study.2,7 An East Africa, Lamiaceae branches are hung in the windows as a natural insect repellent, and a study found that their effect could last between 2-8 hours.2
While this has been a traditional remedy, a double-blind, placebo controlled trial disproved its efficacy. 8
Insecticides are chemical or organic agents that kill insects, usually with a neurotoxin.1 The Environmental Protection Agency regulates their use, and any approved insecticide should have an EPA registration number (usually found on the back label). This number means that it is approved for both human safety and effectiveness. If a product is unregistered and does not have this number, it means that although it is approved for human safety, it has not shown sufficient efficacy (mostly this is related to short half-lives or short duration of protection).
Carbamates and Organophosphates are approved for outdoor use only. Synthetic pyrethroids are the only insecticides approved for indoor use. Mosquito coils are a compact paste or powder made with pyrethroids and other volatile chemicals (such as formaldehyde). Although they are effective to discourage mosquito biting, they smoke and smolder, releasing particulates and volatile chemicals which have an unknown risk for lung disease or carcinogenesis with repeated exposure.1
Pesticide EPA Fact Sheets
1. Diaz JH. Chemical and Plant-Based Insect Repellents: Efficacy, Safety, and Toxicity. Wilderness Environ Med. 2016;27(1):153-163. doi:10.1016/j.wem.2015.11.007.
2. Lupi E, Hatz C, Schlagenhauf P. The efficacy of repellents against Aedes, Anopheles, Culex and Ixodes spp. – a literature review. Travel Med Infect Dis. 2013;11(6):374-411. doi:10.1016/j.tmaid.2013.10.005.
3. Clem JR, Havemann DF, Raebel MA. Insect repellent (N,N-diethyl-m-toluamide) cardiovascular toxicity in an adult. Ann Pharmacother. 1993;27(3):289-293.
4. Ross EA, Savage KA, Utley LJ, Tebbett IR. Insect repellent [correction of repellant] interactions: sunscreens enhance DEET (N,N-diethyl-m-toluamide) absorption. Drug Metab Dispos. 2004;32(8):783-785.
5. Hexsel CL, Bangert SD, Hebert AA, Lim HW. Current sunscreen issues: 2007 Food and Drug Administration sunscreen labelling recommendations and combination sunscreen/insect repellent products. J Am Acad Dermatol. 2008;59(2):316-323. doi:10.1016/j.jaad.2008.03.038.
6. Nerio LS, Olivero-Verbel J, Stashenko E. Repellent activity of essential oils: a review. Bioresour Technol. 2010;101(1):372-378. doi:10.1016/j.biortech.2009.07.048.
7. Tisgratog R, Sanguanpong U, Grieco JP, Ngoen-Kluan R, Chareonviriyaphap T. Plants traditionally used as mosquito repellents and the implication for their use in vector control. Acta Trop. 2016;157:136-144. doi:10.1016/j.actatropica.2016.01.024.
8. Rajan T V, Hein M, Porte P, Wikel S. A double-blinded, placebo-controlled trial of garlic as a mosquito repellant: a preliminary study. Med Vet Entomol. 2005;19(1):84-89. doi:10.1111/j.0269-283X.2005.00544.x.