by Chris Goodman, Pharmacist         (pdf version)

Which repellents work?  Which are safe? 
According to Chapel Hill’s dermatologist, Dr. Mark Fradin, “DEET has a remarkable safety profile after 40 years of use and nearly 8 billion human applications.” He maintains that DEET has been subjected to more scientific and toxicologic scrutiny than any other repellent substance. DEET was developed by the U.S. Army in 1946 and was registered for use by the general public in 1957. In 1998, the Environmental Protection Agency (EPA) completed a comprehensive re-assessment of DEET, and concluded that as long as consumers follow label directions and take proper precautions, insect repellents containing DEET do not present a health concern. In addition to DEET, the EPA has now registered several other active ingredients for use as insect repellents on skin, meaning the EPA has reviewed and approved these ingredients for efficacy and safety when used according to product labeling. These include: picaridin, oil of citronella, and oil of lemon eucalyptus.

July 2002 in The New England Journal of Medicine, Dr. Fradin published
an article comparing 16 insect repellents available nationally at that time. His conclusions were that the non-DEET repellents tested did not provide protection
for durations similar to those of DEET-based repellents and cannot be relied on to provide prolonged protection in environments where mosquito-borne diseases are a substantial threat. DEET-based repellents remain the gold standard of protection under circumstances in which it is crucial to be protected against mosquito bites that may transmit disease.

Fradin does admit that DEET is not a perfect repellent. It may be washed
off by perspiration or rain and its efficacy decreases dramatically with rising
outdoor temperatures. DEET can damage plastics, dissolve watch crystals, frames of glasses, and certain synthetic fabrics such as rayon and spandex. DEET does not damage cotton, wool, or nylon.

It’s not that 50% DEET is more effective than lower concentrations, but the
duration of protection is longer. Greater than 50% however, appears to have no
added benefit. A product with 23.8% DEET provided an average of 5 hours of
protection; 20% almost 4 hours; 6.65% almost 2 hours. (FYI: Skin-So-Soft 10
minutes!) In other comparisons, 50% DEET products in ethanol provided 6-8 hours protection, whereas 33% DEET in a controlled release base (Ultrathon™) gave protection for 6-12 hours.

Nearly 100% protection can be achieved when DEET repellents are used in combination with permethrin-treated clothing. Permethrin is registered by EPA for use on clothing, shoes, bed nets, and camping gear. CDC recommends reapplying permethrin after every 5 washings.

Picaridin has been used in Europe and elsewhere for years. Initially in the
U.S., only 7% and 15% solutions were available. Now, Sawyer has a 20% picaridin spray. In a study comparing various concentrations of picaridin and DEET, the 9.3% picaridin formulation provided >95% protection for only 2 hours. A 19.2% picaridin and DEET 33% in a controlled release base (Ultrathon™) product provided >95% protection for 8 hours. Other studies have found picaridin to be less irritating than DEET. It is nearly odorless and has no adverse affect on plastics, synthetics, plastic coatings and sealants. DEET was more likely to be described as sticky or oily feeling when compared to picaridin.

Oil of lemon eucalyptus is a plant based repellent. In two recent publications, when tested against mosquitoes in the U.S. it provided protection similar to repellents with low concentrations of DEET. It has not been tested against mosquitoes that spread malaria and other diseases which occur internationally. Oil of citronella is another plant derived repellent. In Fradin’s study, citronella 10% had a range of protection of 7 to 60 minutes.

When deciding which repellent to buy, consider the amount of time that you will need protection. If you need short term protection (several hours), picaridin 7-15%, low concentrations of DEET, or oil of lemon eucalyptus are adequate. Now that picaridin is available in a 20% concentration, it does offer an alternative to products with 30-50% DEET that have been the standard recommended repellent for international travelers. However, the CDC does state that the non-DEET repellents have not been as thoroughly tested as DEET.

Other alternatives? Well…the ONLY place on earth to totally avoid the pesky mosquito is Antarctica!

 How to Use Repellents Safely:

• Read and follow all label directions and precautions.
• Do not apply to cuts, wounds, or irritated skin.
• Do not apply to hands of children (if child tends to put hands in mouth), or near eyes and mouth.
• Do not allow young children to apply the product themselves.
• The CDC recommends up to 50% DEET for adults and children > 2 months old for protection in malaria risk areas.
• Oil of lemon eucalyptus labeling specifies that it should not be used on children <3 years old.
• Use just enough repellent to cover exposed skin.
• Do not use repellents under clothing.
• Do not use sunscreens that contain DEET, because sunscreen needs to be applied often and liberally and DEET is applied sparingly.
• When using sunscreen and repellents together, apply the sunscreen first and then repellent.
• Keep in mind that DEET can reduce a sunscreen’s SPF by 33% when used together.
• Avoid over-application of DEET products. Consider using a controlled release product, which has been shown to protect from 6-12 hours. Reapply if you start to be bitten.
• After returning indoors, wash DEET-treated skin with soap and water. (Cutter says that picaridin does not need to be washed off after returning indoors.)
• Use of DEET products may cause skin reactions in rare cases. Trying a different product or concentration might be helpful.
• Recommendations for DEET (and picaridin) use in pregnant or lactating women are the same as for other adults according to the CDC.
For aerosol and pump sprays:
• Do not spray in enclosed areas.
• To apply to face, spray on hands first and then rub on face. Do not spray directly onto face.

Campus Health OTC Pharmacy stocks:

Controlled release DEET products with 30-33% DEET (Sawyer Ultra 30™ lotion & Ultrathon™ Cream)
Ben’s 30 Wilderness™ Pump Spray (30% DEET)
20% Picaridin (Sawyer Insect Repellent™ spray)
Permethrin 0.5% spray
For short term protection (NOT for international travelers): Cutter Advanced™ Pump Spray (7% picaridin)

References: 1. Fradin MS, Day JF. Comparative Efficacy of Insect Repellents against Mosquito Bites. NEJM 2002;347 (1):13-18. 2. US EPA Reregistration of the Insect Repellent DEET http://www.epa.gov/opp00001/citizens/deet.htm (April 28, 1998). 3. Fradin MS. Mosquitoes and Mosquito Repellents: A Clinician’s Guide. Ann Intern Med 1998;128:931-940. 4. Scheinfeld, N. Picaridin: a new insect repellent. J Drugs in Dermatology Jan-Feb 2004. 5. The Science Behind Picaridin. www.picaridin.com/science.htm 6. CDC Health Information for International Travel, 2008. 7. Petersen LR, Marfin AA. West Nile Virus: A Primer for the Clinician. Ann Intern Med 2002;137:173-179. 8. Frances SP,et al. Effectiveness of repellent formulations containing deet against mosquitoes in NE Thailand. J Am Mosq Control Assoc 1996 Jan;12(2Pt1):331-3. 9. www.upto dateonline.com accessed 6/20/07. 

Updated May 2008