Technical DEET is composed of more than 95 percent m-DET isomers. Ortho (o-DET) and para (p-DET) isomers are slightly more and less toxic than m-DET, respectively (Ambrose and Yost, 1965). The chemical identity of DEET is shown in Table 5.1, and Table 5.2 summarizes its physical and chemical properties.
Chemical Identity of DEET
|Chemical class||Aromatic amide (N,N-dialkylarylamides); repellent|
|Trade names||DEET, OFF, Delphene, MGK diethyltoluamide, Detamine, Metadelphene, Chemform, Chiggar-Wash, Muskol, Cutter, Repel, Old Time Woodsman|
|CAS Registry number||134-62-3|
Physical and Chemical Properties of DEET
|Color/form||Colorless to off-white, light-yellow, amber liquid|
|Water solubility at 25°C||Practically insoluble|
|Partition coefficient (Kow)||100|
|Soil sorption coefficient (Koc)||300|
|Vapor pressure at 20°C||5.6 x 10-3 mm Hg|
|EPA toxicity classification||Class III|
|NIOSH IDLH value||NA|
|EPA IRIS RfD||NA|
|EPA IRIS RfC||NA|
|NA = not available.|
DEET insect repellent is part of a complete repellent system used by U.S. military personnel that has shown excellent efficacy in preventing arthropod-borne disease (Young and Evans, 1998). Until 1989, the standard-issue insect repellent of the U.S. military consisted of 75 percent DEET in an alcohol base. By 1984, the 3M Company (St. Paul, Minnesota) had developed a slow-release, polymer-based product containing 33 percent DEET, which is now the repellent provided to all U.S. military personnel. This product is available to the general public from 3M as Ultrathon. Three DEET products were shipped to the Gulf area; these products are detailed in Table 5.3.
Formulations of DEET Available During ODS/DS
|Unit Size||Application Areas|
|6840-01-284-3982||3M insect/arthropod repellent||Cream||
||2-oz tube||Skin and clothing|
|6840-00-753-4963||Insect repellent, clothing and personal application||Liquid||
||2-oz bottle||Skin and clothing|
|6840-00-142-8965||Cutter insect repellent stick||Stick||
||1-oz stick||Skin and clothing|
|Source: Provided by OSAGWI.|
Carbon dioxide and lactic acid are among the most important cues mosquitoes use to locate a host. It is believed that DEET repels mosquitoes by inhibiting lactic acid receptors on their antennae (Davis and Sokolove, 1976).
DEET can enter the body through several exposure pathways, including dermal and ocular exposures, inhalation, and ingestion. Some consider DEET an ideal permeant of skin (Stinecipher and Shah, 1997), and it has been reported to accelerate the dermal penetration of pharmaceuticals (Windheuser et al., 1982), raising the concern that DEET may also increase dermal penetration of pesticides, since they are often used together (Moody et al., 1987). Several studies in animals and humans have shown that, following absorption, DEET is completely metabolized prior to elimination in the urine (Schmidt et al., 1959; Smith et al., 1963a; Selim et al., 1995).
After DEET is applied to the skin, it is partially absorbed, but some also evaporates or is rubbed off by clothing, the latter accounting for the majority of loss (Smith et al., 1963b). Following absorption, DEET does not appear to accumulate in the superficial layers of the skin. In a definitive study, DEET was absorbed across the forearms of human volunteers within two hours of application, but the rate of elimination via excreta was more rapid than the rate of absorption (Selim et al., 1995). This is consistent with expected absorption patterns of low-molecular-weight, lipophilic chemicals (Scheuplein, 1967) such as DEET.
DEET has been shown to affect the cardiovascular and nervous systems. The mechanism of cardiovascular toxicity has been investigated in dose-response experiments with intraperitoneal injections of DEET in rats, studies of hypodynamic responses of dogs following intraperitoneal DEET injections, and studies of the effect of atropine in blocking DEET-induced hypotension and bradycardia (Leach et al., 1988). These experiments showed a significant effect of hypotension. In the dog study, there was a significant reduction in cardiac output but no change in stroke volume and total peripheral resistance, suggesting that the observed hypotension was a result of DEET-induced bradycardia.
Episodes of severe DEET toxicity in mammals are usually related to a direct action on the nervous system. Experimental animals that received large doses of DEET have manifested coma and death. Animal studies have suggested that DEET is not a selective neurotoxin (Osimitz and Grothaus, 1995; Schoenig et al., 1996).
Reported cases of severe DEET toxicity in humans have involved mainly encephalopathies in children. The vast majority of these cases occurred in female children exposed to topical DEET, so it was hypothesized that another mechanism of DEET toxicity that may occur with smaller systemic doses is perturbation of ammonia metabolism (Heick et al., 1988), resulting in hyperammonia. In this case, DEET would be especially toxic to individuals with genetic or acquired defects in ammonia metabolism, such as female carriers of ornithine carbamoyl transferase (OCT) deficiency (this condition is usually fatal in neonatal males). Heick et al. (1988) injected normal mice with DEET and observed acutely increased ammonia levels. While this result, as reported in case-study observations, suggests hyperammonia as a primary mechanism of acute DEET toxicity, the authors also point out several cases that suggest hypersensitivity reactions (Miller, 1982; Roland et al., 1985). Furthermore, cases of DEET-associated seizures in boys (MMWR 1989; Lipscomb et al., 1992) and men (MMWR, 1989; Veltri et al., 1994) may discredit the hypothesis that OCT deficiency is the predisposing factor for DEET CNS toxicity (Lipscomb et al., 1992), or may at least suggest that there is yet another responsible mechanism.
Exposure to DEET as Reviewed in the Scientific Literature
It is beyond the scope of this literature review to speculate about the magnitude of exposures to pesticides by individuals during ODS/DS. However, Robbins and Cherniack (1986) provide some exposure information that may prove useful. It should first be noted that limited information is available for estimating exposure from what these authors refer to as "conventional consumer use practices." Table 5.4 presents predictions based on limited mid-range data points for DEET exposure (USEPA, 1980). Robbins and Cherniack (1986) rightfully point out that considerable error may be associated with some of the estimates, which were made during the mosquito season and reflect 60 applications per year for military personnel and four days of use per week for Everglades biologists--the latter intended to represent high-dose use. It should also be noted that military personnel were using the old 75 percent DEET repellent. Although this formulation was available in limited quantities during ODS/DS, a 33 percent DEET extended-duration formulation was the primary DEET repellent used there. Robbins and Cherniack also included data from the preliminary report of a NIOSH Health Hazard Evaluation, based on survey data of Everglades Park Service employees. These data are not presented in Table 5.4 because of their preliminary nature and the fact that they included a range of DEET concentrations (15 percent to 75 percent) applied for a seven-month period (as opposed to six months in the EPA study). Nonetheless, this evaluation calculated an estimated exposure of >2 kg of DEET over seven months, which can be approximated as 9.5 g/day. The EPA provides some additional estimates of exposure to DEET, calculated assuming one application per day and standard body weights: 12.10 and 9.68 mg DEET/kg/day (USEPA, 1998a).
Skin Permeation and Absorption of DEET
Uncertainty about the degree of percutaneous absorption of DEET in humans complicates an objective assessment of effects. Generally, the amount of DEET that permeates the skin is closely related to the repellent formulation. Using commercially available products, Stinecipher and Shah (1997) found that the cumulative amount of DEET that permeated human skin in vitro ranged from approximately 6 percent to 100 percent, depending upon the repellent tested. Earlier research suggested that approximately 9 percent to 56 percent of applied DEET permeates the skin, although only approximately 15 percent is systematically absorbed (Robbins and Cherniack, 1986). However, in vitro studies involving infinite-dose applications of DEET to human skin have agreed closely: Stinecipher and Shah calculated the steady-state flux of DEET at from 21 to 63 µg/cm2/hr (Stinecipher and Shah, 1997), while Moody et al. calculated it to be from 20 to 60 µg/cm2/hr (Moody et al., 1995).
To determine DEET absorption accurately, it is necessary to recover all applied DEET, achieving mass balance. Few studies have been successful in this approach. One study that reports good mass balance (88.7 percent to 94.3 percent of radioactivity from 14C-labeled DEET accounted for, depending upon formulation applied) is that of Selim et al. (1995). In this study, 14C-labeled DEET formulations of 100 percent and 15 percent in ethanol were applied to the forearms of two groups of six human volunteers. After eight hours, the skin was washed, and samples were taken by applying tape to the skin at one, 23, and 45 hours after rinsing. Serial blood, urine, and stool samples were also analyzed, and radioactivity was used as the marker to estimate biodistribution of DEET. Plasma radioactivity indicated absorption of DEET within two hours of application, but elimination was rapid and was complete four hours after the eight-hour exposure period. Most of the DEET was washed off the skin, and most of that which was absorbed was metabolized: Six major metabolites were observed in the urine, the primary route of excretion. These results definitively refute earlier suggestions (Bloomquist and Thorsell, 1977; Spencer et al., 1979; Snodgrass et al., 1982; Stinecipher and Shah, 1997) that the epidermis may serve as a depot for DEET, with subsequent slow release to the circulation. Based upon the percentage of applied DEET recovered in the total excreta, dermal absorption of DEET ranged from 3 percent to 8 percent (mean = 5.6 percent) of 100 percent DEET and 4 to 14 percent (mean = 8.4 percent) of the 15 percent DEET-in-ethanol formulation.
Table 5.5 compares dermal absorption in DEET reported in different studies and with different test subjects. However, these results may not accurately represent human exposure conditions, where individuals apply repeated doses of DEET to the skin without washing off previous doses.
As with many pesticides, the majority of health effects reported to have been caused by DEET are acute. In fact, there appears to be no evidence in the literature that suggests chronic low-level exposure to DEET produces effects lasting months or years after exposure. DEET has been associated with a suite of symptoms, which are summarized in Table 5.6.
Federal law requires pesticides that were first registered before November 1, 1984, to be re-registered to ensure that they meet evolving, more stringent health standards. DEET was subjected to this process in 1998. The EPA concluded that DEET is generally of low acute toxicity, and on the basis of the available toxicological data, the agency stated that normal use of DEET does not present a health concern to the general U.S. population (USEPA, 1998a). It should be noted that the EPA assumes that the general population receives "subchronic exposure" to DEET; that is, users are expected to be exposed to DEET intermittently for only days or weeks.
In Vivo and In Vitro Dermal Absorption of DEET
||Solvent of Application||
|Feldman and Maibach, 1974||Human [a]||
||16.17 ± 5.10|
|Moody and Nadeau, 1993)||Human [b]||
||27.7 ± 4.24|
|Selim et al., 1995||Human [a]||
||5.6 (range 3-8)|
|Selim et al., 1995||Human [a]||
||8.4 (range 4-14)|
|Reifenrath et al., 1981||Hairless dog [a]||
||12.8 ± 4.6|
|Reifenrath et al., 1980||Hairless dog [a]||
||9.4 ± 3.6|
|Moody and Nadeau, 1993||Pig [b]||
||15.3 ± 0.82|
|Reifenrath et al., 1984||Weanling pig [a]||
|Moody and Nadeau, 1993||Guinea pig [a]||
||30.0 ± 5.96|
|Moody and Nadeau, 1993||Guinea pig [b]||
||10.9 ± 1.40|
|Moody and Nadeau, 1993||Rat [a]||
||41.0 ± 10.51|
|Moody and Nadeau, 1993||Rat [b]||
||21.4 ± 2.17|
|Moody and Nadeau, 1993||Mouse [b]||
||36.2 ± 27.5|
|Source: Stinecipher and Shah (1997), with additions and re-ordering in the present report.
[a] In vivo studies.
[b] In vitro studies.
[c] Approximation; the authors reported applying approximately 15 mg of 98.8 percent DEET to a 4 x 6-cm area.
[d] Approximation; authors reported applying approximately 12 mg of 15 percent DEET formulation in ethanol to a 4 x 6-cm area.
Reported Signs and Symptoms of DEET Toxicity
|Affected Area||Sign or Symptom|
|Source: Clem et al. (1993).|
In studies using laboratory animals, DEET generally has been found to be of low acute toxicity. It is slightly toxic by the eye, dermal, and oral routes and has been placed in the EPAs Toxicity Category III (the second lowest of four categories) because of these effects (USEPA, 1998a).
Generally, neurotoxic symptoms dominate at near-lethal doses of DEET in rats and other animals. The rat oral LD50 is 2 to 4 g DEET/kg (Ambrose and Yost, 1965).  Rats given a single oral dose of 500 mg DEET/kg displayed increases in thermal response time and possible decreased rearing activity (Schoenig et al., 1993).
Most reports of severe DEET adversity in humans describe neurologic symptoms, and most of the severe adverse reactions occur in children (Veltri et al., 1994; Osimitz and Murphy, 1997; Fradin, 1998). Reports of DEET adversity have described manic psychosis, cardiovascular events, anaphylaxis, and several cases of contact urticaria and irritant contact dermatitis. These are included in Table 5.7, which summarizes reports of health effects on humans attributed to DEET exposure. It is possible that examples of DEET sensitivity may be missed, especially in children, as cases may easily be misdiagnosed as a viral encephalitis (Zadikoff, 1979). Deaths in adults have occurred following large doses, and blood levels of DEET in fatal systemic poisonings have ranged from 168 mg/L to 240 mg/L (Tenenbein, 1987). In contrast, a blood concentration of 3 mg/L was measured after routine application of a repellent to a 30-year-old male volunteer (Wu et al., 1979).
Manic psychosis occurred in a 30-year-old man who, for a period of three weeks, applied DEET daily and then sat in a light-bulb-heated box, apparently for self-medication to treat a rash. Sedation and incoherence were noted for short periods after each application session, and the man was admitted to a hospital after displaying aggressiveness and psychotic ideation. Clinical improvement (haloperidol) was complete within six days, atypical for classic endogenous mania. The authors point out the structural similarities between DEET and certain CNS-active drugs such as N,N-dimethyl acetamide and doxapram hydrochloride (Snyder et al., 1986).
A cardiovascular event occurred in a 61-year-old woman who applied a DEET-containing repellent (unknown concentration) "liberally" to all exposed skin prior to gardening. She suffered bradycardia and hypotension but recovered without sequelae (Clem et al., 1993). Other reports of cardiovascular DEET toxicity have included two cases of apparent suicide via ingestion of DEET containing repellents (Tenenbein, 1987; Veltri et al., 1994). The case of anaphylaxis listed in Table 5.7 was a woman with brief exposure to DEET; her symptoms returned when she was re-exposed to DEET in an emergency room, indicating a possible hypersensitivity (Miller, 1982).
Reported Health Effects in Humans Following the Topical Application of DEET
|Reference||Affected Area||Sex/Age (yr); Possible Predisposition||DEET Concentration (%)||Pattern of Use (dermal unless otherwise noted)||Symptoms||Outcome|
|Heick et al., 1980||CNS||F/6; OCT heterozygote||15||>10 occasions||Headaches, ataxia, disorientation, cerebral edema||Death|
|de Garbino and Laborde, 1983||CNS||F/1.5||20||Frequent||"Acute encephalopathy"||Death|
|Zadikoff, 1979||CNS||F/5||10||Nightly for 3 mo||Headaches, ataxia, seizures, agitation, opisthotonos, generalized edema||Death|
|Tenenbein, 1987||CNS||F/1||47.5||Ingested 25 mL||Seizures, opisthotonos||Recovery|
|Hall et al., 1975||CNS||F/7.5||10||Application and ingestion||Opisthotonos||Recovery|
|Zadikoff, 1979||CNS||F/1.5||10||Ingestion of unknown but probably small amount||Opisthotonos, ataxia||Recovery|
|Gryboski et al., 1961||CNS||F/3||15||Daily for 2 wk||Ataxia, encephalopathy||Recovery|
|Roland et al., 1985||CNS||F/8||15 & 100||Copious for 4 days||Seizures, rash, restlessness||Recovery|
|Edwards and Johnson, 1987||CNS||F/1.5||20||3 mo||Ataxia, movement disorder, drooling, opisthotonos, opsoclonus, myoclonus||Recovery|
|Lipscomb et al., 1992||CNS||M/5||100 & 15||Brief||Seizures||Recovery|
|MMWR, 1989||CNS||4 cases: M/3-7||NA||NA||Seizures||Recovery|
|Tenenbein, 1987||CNS||F/14||95||Ingested 50 mL||Unconsciousness, seizures||Recovery|
|Veltri et al., 1994||CNS||M/17||17.9||Saturated clothing||Ataxia, possible seizure or unconsciousness||Recovery, incomplete follow-up|
|Veltri et al., 1994||CNS||M/adult; ingested phenothiazine drug same day||20.9||Sprayed entire body||Dystonia||Recovery|
|Snyder et al., 1986||CNS||M/30||70||Daily application fol-lowed by dry sauna, 3 wk||Aggressiveness, psychotic ideation, psychomotor hyperactivity, rapid and pressured speech, tangentiality, flight of ideas, grandiose delusions, auditory hallucinations||Recovery|
|Veltri et al., 1994||Cardiovascular, CNS||M/33||NA||Intentionally ingested 8 oz of DEET repellent||Cardiorespiratory arrest, hyperglycemia (day 2), seizures, intravascular coagulopathy, cerebral edema||Death|
|Tenenbein, 1987||Cardiovascular, CNS (plus bowel||F/33||95||Ingestion of up to 50 mL||Hypotension, seizure, coma, bowel infarction||Death|
|Clem et al., 1993||Cardiovascular||F/61||NA||Liberal application, frequency NA||Bradycardia, hypotension||Recovery|
|Miller, 1982||Cutaneous or allergic reaction||F/42||52||Touched companion who had just applied repellent||Anaphylaxis||Recovery|
|Maibach and Johnson, 1975; von Mayenburg and Rakoski, 1994; Wantke et al., 1996||Cutaneous or allergic reaction||3 cases: 2 M + 1 F/4-35||NA||Urticaria developed 10-30 min after application||Wheals||Recovery|
|Reuveni and Yagupsky, 1982||Cutaneous or allergic reaction||10 cases: M/18-20||33-50||Military, applied to skin and then slept||Hemorrhagic bulla and erosions, confined to antecubital fossa||Recovery in 9 of 11; scarring in 2 of 11|
Source: Adapted from Osimitz and Murphy (1997) and Fraden (1998).
CNS = central nervous system; NA = not available; OCT = ornithine carbamoyl transferase.
A particularly important study examined 9,086 human exposures to DEET-containing insect repellents that were reported to 71 Poison Control Centers (PCCs) between 1985 and 1989 (Veltri et al., 1994). In these cases, most of the adverse effects were related to the route of exposure, rather than the age or gender of the patient or the concentration of DEET in the repellent formulation. Symptoms were most likely to occur following inhalation or ocular exposures, and these accounted for 2.0 percent and 31.9 percent of the total cases handled by the PCCs, respectively. Ingestion and multiple-route exposures accounted for 49.4 percent and 12.6 percent of all cases, respectively. Of all cases, 39.8 percent of the patients had symptoms that were considered related to DEET exposure; 54 percent of the patients were asymptomatic. After the study, 74.8 percent of the patients were followed long enough to determine a definitive outcome; of these, 98.9 percent either experienced no effects or had symptoms that were transient, resolved rapidly, and usually involved the skin or mucous membranes. Of 889 patients who were evaluated in a health care facility, 81.4 percent were discharged after initial treatment and 4.9 percent were admitted for medical care. (The remainder were lost to follow-up.) The authors suggest that in most patients, symptoms, if present, were not serious and resolved quickly. Sixty-six patients experienced more pronounced or prolonged symptoms, but these resolved without apparent sequelae. Five patients (all male) were reported to have suffered serious health effects. One had a dystonic reaction. He had ingested prochlorperazine (known to cause distonia) earlier in the day, so a synergistic reaction with DEET was not excluded. Two of the patients experienced eye irritation, which was treated at home. Two other patients were treated and released from an emergency room: A 17-year-old male who saturated his clothing with repellent (17.9 percent DEET) was ataxic and possibly suffered a seizure; a 33-year-old male experienced diminished sensation and hypotension one week after using repellent.
Chronic, Reproductive, Genetic, and Carcinogenic Effects
As mentioned above, DEET underwent scrutiny during an EPA re-registration process in 1998 (USEPA, 1998a) which concluded that human exposure to DEET was usually brief, and long-term exposure was not to be expected. Based on laboratory animal studies, the EPA concluded that DEET is of low acute toxicity (Toxicity Category III). Further, DEET has been classified as an EPA Group D carcinogen (not classifiable as a human carcinogen), and mutagenicity tests for DEET (Ames assay, chromosomal aberration assay, and unscheduled DNA synthesis assay) were all negative, indicating that DEET is not mutagenic.
No reports were found of long-term effects in humans from chronic exposures to DEET (with the exception of rare reports of scarring), so there is no evidence to suggest such a scenario is of great concern in predicting the potential health effects of DEET in PGWV. As seen in Table 5.7, there have been some reports of subacute, subchronic, or possibly chronic human exposures to DEET; but with the exception of three deaths in children (at least one of whom was confirmed to have had a predisposing condition), these exposures resulted in no long-term effects. The following summarizes some of the animal studies considered by the EPA in the re-registration of DEET (USEPA, 1998a).
In a two-year chronic toxicity/carcinogenicity study in rats, 60 rats of each sex received 98.3 percent DEET in their diet. No toxicity was seen in male rats at the highest dose (100 mg/kg/day), but female rats displayed decreases in food consumption and body weight and an increase in cholesterol levels at their highest dose (400 mg/kg/day). At this same dietary concentration, 400 mg DEET/kg/day, beagle dogs in a separate one-year study displayed decreases in food consumption and body weight, an increase in the incidence of ptyalism, and a decrease in cholesterol levels. No compound-related effects on reproduction (e.g., fertility, gestation, or viability) were noted in rats given DEET in their diet at up to 5,000 ppm for two consecutive generations.
No correlation between the concentration of DEET in a repellent and the frequency or severity of effects is supported by the literature. Further, it is difficult to quantify consistently the temporal relationship between the onset of CNS symptoms and exposure to DEET, but the reaction is generally rapid, as is the resolution in most cases. There have been a relatively small number of severe adult effects related to DEET exposure. While a pattern of potentially severe neurotoxicity in children who have been exposed to DEET is emerging, the total number of reported cases is very small compared with the population exposed. This pattern has not been observed in adults. The reasons for this disparity are unknown but may be related to the fact that children have a different surface-area-to-volume ratio than adults. Generally, patients who are reported to present severe health effects related to DEET use recover without reported sequelae.
Concern about the interactive effect of DEET with other chemicals may be warranted (see Chapter Eight), but the available literature is not complete enough to allow definitive conclusions to be drawn at this point. It is difficult to extrapolate the results of animal studies to long-term human effects, and the possibility of chemical interactions compounds the uncertainty inherent in the process. This is not to say, however, that further research should not be undertaken. A prudent approach may be to, first, more accurately determine the exposures that warrant further study. If it is determined that coexposures warrant further investigation, it may be sensible to examine common routes of exposure for resulting bioavailability before investigating specific toxicological endpoints. In addition to this area for potential research, efforts to explain the broad variety of outcomes associated with DEET exposure may be warranted, especially for cases of hypersensitivity.
 There is some evidence, however, that there is little or no evaporation of DEET from the skin of rats (Schoenig et al., 1996) and humans (Selim et al., 1995).
 LD50 is the median lethal dose. This is a statistically derived single dose that can be expected to cause death in 50 percent of test animals when administered by the route indicated. It is expressed as the weight of a substance per unit weight of animal.
 As the authors point out, the data analyzed were voluntarily reported to a national database by the PCCs. These data are useful but not without limitations: "The data included in this report were not obtained from a sample that is generalizable to the population of the US. The data represent those persons who have an exposure and report that exposure to a PCC. It is unknown how those persons differ from those who do not call a PCC." However, the cases reported do provide some insight into the common routes of exposure associated with specific health outcomes and some measure of the severity of effects.