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IR3535: a gentle, effective repellent

Prevention Updated Jul 2026 5 min read
Mosquito-repellent

The short answer

IR3535 is an EPA-registered insect repellent that works against mosquitoes. It is applied to exposed skin and is a common active ingredient in family-oriented products, including some that combine a repellent with sunscreen. Its chemical name is ethyl butylacetylaminopropionate, and it is a synthetic molecule derived from the amino acid beta-alanine, which is why the EPA classifies it as a biopesticide (EPA Skin-Applied Repellent Ingredients; EPA biopesticide fact sheet). Compared with the strongest options, it has a mild feel and a favorable safety record, though it can irritate the eyes if it gets into them (NPIC Insect Repellents Fact Sheet).

The EPA lists IR3535 as one of its registered skin-applied repellent active ingredients, alongside DEET, picaridin, and oil of lemon eucalyptus, with roughly 45 products registered (EPA Skin-Applied Repellent Ingredients).

How it works and effectiveness

IR3535 discourages biting insects from landing and feeding on treated skin. The EPA notes it can be used against mosquitoes, deer ticks, body lice, and biting flies (EPA biopesticide fact sheet).

How long it protects depends heavily on concentration. Products with less than 10 percent of an active ingredient may offer protection for only short periods, roughly 1 to 2 hours (NPIC Insect Repellents Fact Sheet). Higher concentrations last much longer. In a field study published in the Journal of Medical Entomology, complete protection times for 10 percent and 20 percent IR3535 formulations ranged from 7.1 to 10.3 hours against mosquitoes, and from 9.1 to 12.2 hours against blacklegged ticks (Carroll 2008). The study also noted that many test subjects withdrew before the product failed, so real-world protection may run even longer than those measured figures (Carroll 2008).

Effectiveness can vary by mosquito species, so protection time on a label is best read as a guide rather than a guarantee for every situation.

Safety

IR3535 has a long track record. It was developed by Merck KGaA in Germany and has been used as a repellent in Europe for over 20 years without significant adverse effects reported (EPA biopesticide fact sheet). The EPA registered the first product containing IR3535 in 1999 and, based on the available toxicity data, made a determination of reasonable certainty of no harm for the general population, including infants and children (EPA biopesticide fact sheet).

NPIC rates IR3535 as very low in oral, inhalation, and dermal toxicity for short-term exposure (NPIC Insect Repellents Fact Sheet). The main caution is the eyes. IR3535 is an eye irritant, so it should be kept away from the eyes and applied carefully, especially on children who may rub their faces (NPIC Insect Repellents Fact Sheet). Diluted IR3535 did not cause skin irritation in studies, though undiluted material can (NPIC Insect Repellents Fact Sheet).

If the product does get into an eye, rinsing promptly with water helps reduce irritation (NPIC Insect Repellents Fact Sheet).

How to apply

  • Apply only to exposed skin and over clothing, following the product label. The label is the authoritative instruction for concentration, age guidance, and reapplication.
  • Do not spray directly onto the face. Spray onto your hands first, then wipe onto the face, keeping well clear of the eyes and mouth.
  • Take extra care with children. Apply to your own hands first, then to the child, and avoid their hands, which often reach their eyes.
  • Reapply according to the label, and sooner if you have been sweating heavily or in water. In hot, humid conditions repellents may need reapplying more often.
  • When you are back indoors, washing treated skin with soap and water is a reasonable routine step.

How it compares to DEET and picaridin

All three are EPA-registered skin-applied repellents (EPA Skin-Applied Repellent Ingredients). DEET is the longest-studied and, at higher concentrations, tends to deliver the longest protection, which is why it is often the default for high-exposure settings. Picaridin sits close behind and is popular for feeling lighter on the skin.

IR3535 is generally chosen for its gentle feel and its long, favorable safety history rather than for maximum staying power. At low concentrations its protection can be shorter than a comparable DEET product, but at 10 to 20 percent it provides many hours of coverage in field testing (Carroll 2008; NPIC Insect Repellents Fact Sheet). One practical note: fewer studies have looked at combining IR3535 with sunscreen, so if you use a combination product, follow its specific label rather than assuming the two ingredients behave independently (NPIC Insect Repellents Fact Sheet).

For most everyday outdoor time, a 10 to 20 percent IR3535 product is a reasonable, mild option. For long exposure in areas with heavy biting pressure or disease risk, a higher-strength DEET or picaridin product may be the more dependable choice.

Sources

  1. EPA, Skin-Applied Repellent Ingredients. Lists IR3535 as a registered active ingredient with roughly 45 products, alongside DEET, picaridin, and oil of lemon eucalyptus. https://www.epa.gov/insect-repellents/skin-applied-repellent-ingredients (accessed 20 July 2026).
  2. EPA, Biopesticide fact sheet, aminopropionic acid, ethyl ester (IR3535). Notes use against mosquitoes, deer ticks, body lice, and biting flies; over 20 years of use in Europe; reasonable certainty of no harm including infants and children. https://www3.epa.gov/pesticides/chem_search/reg_actions/registration/fs_PC-113509_01-Jan-00.pdf (accessed 20 July 2026).
  3. National Pesticide Information Center (NPIC), Insect Repellents Fact Sheet. Very low short-term toxicity ratings; eye irritant; skin irritation from undiluted material; protection under 10 percent may last only 1 to 2 hours; limited data on sunscreen combinations; CDC recommendation. https://npic.orst.edu/factsheets/repellents.html (accessed 20 July 2026).
  4. Carroll, S. P. (2008). Prolonged Efficacy of IR3535 Repellents Against Mosquitoes and Blacklegged Ticks in North America. Journal of Medical Entomology, 45(4), 706. Complete protection times of 7.1 to 10.3 hours against mosquitoes and 9.1 to 12.2 hours against blacklegged ticks for 10 and 20 percent formulations. https://academic.oup.com/jme/article/45/4/706/883450 (accessed 20 July 2026).
  5. EPA, Pesticide Product Label, BUG REPELL IR3535 20%. Example of a registered 20 percent IR3535 product. https://www3.epa.gov/pesticides/chem_search/ppls/070759-00004-20250305.pdf (accessed 20 July 2026).

Related

  • [Placeholder: DEET repellent guide]
  • [Placeholder: Picaridin repellent guide]
  • [Placeholder: How to choose a mosquito repellent]
  • [Placeholder: Applying repellent to children safely]

This page is educational and is not medical or pest-control advice. Always read and follow the product label, and consult a qualified professional for guidance specific to your situation.

Last reviewed 20 July 2026.