Of the products tested, those containing DEET provided the longest lasting protection. The complete protection times of DEET based repellents correlated positively with the concentration of DEET in the
repellent. The formulation containing 23.8% DEET protected for an average of 301.5 minutes.

There was a statistically significant difference in complete protection time between each DEET based repellent and the product with the next higher concentration of DEET. The IR3535-based repellent protected against mosquito bites for an average of 22.9 minutes.The citronella based repellent we tested protected for 20 minutes or less.

There was no significant difference in protection time between the slow-release formulation containing 12% citronella and the formulation containing 5% citronella or the two formulations containing 10% citronella. The repellent containing only 0.05% citronella provided less protection than the Skin-So- Soft mineral-oil-based moisturizer.

Repellent impregnated wristbands, containing either 9.5% DEET or 25% citronella (by weight), protected the wearer on average, for only 12 to 18 seconds. In our study, 11 of the 12 non-DEET products had complete protection times of less than 23 minutes.

All other botanical repellents we tested provided protection for an average duration of less than 20 minutes. Repellent-impregnated wristbands offered no protection.

Conclusions. Currently available non-DEET repellents do 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.

INSECT-TRANSMITTED disease remains a major source of illness and death worldwide. Mosquitoes alone transmit disease to more than 700 million persons annually. Malaria kills 3 million persons each year, including 1 child every 30 seconds. In many circumstances, applying repellent to the skin may be the only feasible way to protect against insect bites. Given that a single bite from an infected arthropod can result in transmission of disease, it is important to know which repellent products can be relied on to provide predictable and prolonged protection from insect bites. Commercially available insect repellents can be divided into two categories ­ synthetic chemicals and plant derived essential oils. The best known chemical insect repellent is N,N- diethyl-m-toluamide (DEET). Many consumers, reluctant to apply DEET to their skin, deliberately seek out other repellent products. We compared the efficacy of readily available alternatives to DEET based repellents in a controlled laboratory environment.

Skin-So-Soft Bath Oil, which consumers commonly claim has a repellent effect on insects, provided only a mean of 9.6 minutes of protection against aedes bites in our study. This extremely limited repellent effect has previously been documented in other studies. Thousands of plants have been tested as potential botanical sources of insect repellent. Most plant-based insect repellents currently on the market contain essential oils from one or more of the following plants: citronella, cedar, eucalyptus, peppermint, lemongrass and geranium.

All botanical repellents that we tested in our initial studies, regardless of their active ingredients and formulations, gave very short lived protection, ranging from a mean of about 3 to 20 minutes. Most alternatives to topically applied repellents have proved to be ineffective. No ingested compound, including garlic & thiamine (vitamin B1), has been found to be capable of repelling biting arthropods. Small, wearable devices that emit sounds that are purported to be abhorrent to biting mosquitoes have also been proved to be ineffective. In our study, wristbands impregnated with either DEET or citronella similarly provided no protection from bites, consistent with the known inability of repellents to protect beyond 4 cm from the site of application.

Multiple factors play a part in determining how effective any repellent will be. These factors include the species of the biting organisms and the density of organisms in the immediate surroundings: the user's age, sex, level of activity, and biochemical attractiveness to biting arthropods; and the ambient temperature, humidity and wind speed. As a result, a given repellent will not protect all users equally. Examination of the ranges of complete protection times shows variation in the ability of each repellent to protect different subjects. Our study shows that only products containing DEET offer long-lasting protection after a single application.

DISCUSSION

Protection against arthropod bites is best achieved by avoiding infested habitats, wearing protective clothing and applying insect repellent. The insect repellents that are currently available to consumers are either synthetic chemicals or are derived from plants. The most widely marketed chemical based insect repellent is DEET, which has been used worldwide since 1957. DEET is a broad spectrum repellent that is effective against many species of mosquitoes, biting flies, fleas and ticks. The protection provided by DEET is proportional to the logarithm of the dose: higher concentrations of DEET provide longer-lasting protection.

Most commercially available repellent formulations contain 40% DEET or less, and the higher concentrations are most appropriate to use under circumstances in which the biting pressures are intense, the risk of arthropod transmitted disease is great, or environmental conditions promote the rapid loss of repellent from the surface of the skin. In our study, a formulation containing 23.8 percent DEET provided an average of five hours of complete protection against A. aegypti bites after a single application. Depending on the formulation and concentration tested, DEET based repellents have been shown in other studies to provide complete protection against arthropod bites for as long as 12 hours, even under harsh climatic conditions.

Certain plant derived repellents may provide short lived efficacy. Frequent reapplication of these repellents would partially compensate for their short duration of action. However, when one is travelling to an area with prevalent mosquito-borne disease that could be transmitted through a single bite, the use of non-DEET repellents would seem ill advised.

Given our findings, we cannot recommend the use of any currently available non-DEET repellent to provide complete protection from arthropod bites for any sustained outdoor activity.

Although this study shows that DEET based products can be depended on for long-lasting protection, they are not perfect repellents. DEET may be washed off by perspiration or rain, and its efficacy decreases with rising outdoor temperatures.

DEET Safety, despite the substantial attention paid by the lay press every year to the safety of DEET, this repellent has been subjected to more scientific and toxicologic scrutiny than any other repellent substance. The extensive accumulated toxicologic data on DEET have been reviewed elsewhere. DEET has a remarkable safety profile after 40 years of use and nearly 8 billion human applications. Fewer than 50 cases of serious health effects have been documented in the medical literature since 1960, and three quarters of them resolved without sequelae. Many of these cases involved long-term, heavy, frequent or whole-body application of DEET. No correlation has been found between the concentration of DEET used and the risk of toxic effects. As part of the Re-registration Eligibility Decision on DEET, released in 1998, the U.S. Environmental Protection Agency reviewed the accumulated data on the toxicity of DEET and concluded that "normal use of DEET does not present a health concern to the general population". When applied with common sense, DEET based repellents can be expected to provide a safe as well as a long-lasting repellent effect.

Until a better repellent becomes available, DEET based repellents remain the gold standard of protection under circumstances in which it is crucial to be protected against arthropod bites that might transmit disease.