The skin is the largest organ of the human body1 and performs essential functions, such as regulating body temperature, defending against external aggressors, and preventing water loss.2 It also has immunological, endocrine, and metabolic roles that are crucial for maintaining homeostasis.3 Among its layers, the stratum corneum, the outermost layer of the epidermis, serves as the primary barrier,4 providing protection while maintaining selective permeability.5 This barrier function protects the skin from toxins, ultraviolet radiation, and microorganisms (Fig. 1) and is supported by the commensal microbiota, physical structure, and immune defenses.6 However, certain dermatological conditions, such as psoriasis,7 atopic dermatitis,7 and obstructive sleep apnea syndrome,8 as well as external agents such as water,9 tattoos,10 and sunscreen,11 can compromise this protective function.

Fig. 1.

Skin barrier function. The stratum corneum is essential for proper skin barrier function because it helps prevent penetration by external irritants and microorganisms, while reducing transepidermal water loss and maintaining skin hydration.

Insect repellents are widely used topical products, particularly in tropical regions and during summer, to protect against mosquito bites and vector-borne diseases. These products can be categorized as synthetic compounds, such as N,N-diethyl-meta-toluamide (DEET), introduced in 1956,12 and plant-derived formulations, such as citronella essential oil from Cymbopogon nardus. Both are effective insect repellents, but their effects on skin barrier function remain underexplored. DEET is known for its high efficacy but has been associated with skin reactions, such as erythema and dermatitis, in some individuals.13 Citronella has historically been used with fewer adverse effects,14 although it generally has a milder efficacy profile.15

Individuals with preexisting skin conditions, such as atopic dermatitis or psoriasis, may be more susceptible to alterations in skin homeostasis,16 potentially exacerbating symptoms or compromising barrier integrity. In addition, certain populations, such as children, older adults, and individuals with sensitive skin, may require alternative formulations with minimal impact on skin health.17 Despite the widespread use of insect repellents, it remains unclear whether plant-based or synthetic formulations are more suitable for different population groups, particularly in terms of their effects on skin barrier function. Therefore, it is essential to establish a scientific basis for their impact, starting with healthy individuals before extending research to more vulnerable populations.

This study aimed to evaluate the impact of DEET-based and citronella-based repellents on skin barrier function by analyzing skin homeostasis parameters, including transepidermal water loss (TEWL), temperature, erythema, sebum production, pH, stratum corneum hydration (SCH), melanin levels, elasticity, and hardness. TEWL, a key parameter for assessing skin barrier integrity, was selected as the primary variable. The secondary objective was to compare differences in the effects of the two repellents on skin barrier function.

This study is, to our knowledge, the first to evaluate the effects of plant-based and synthetic insect repellents on skin barrier function. The parameters evaluated showed that both repellents affected skin barrier function. Between the 2 repellents, the synthetic repellent produced more pronounced changes, although these changes were not necessarily associated with harmful effects.

Temperature is an important parameter when assessing skin barrier function, as shown by Denda et al.18 In the present study, skin temperature was significantly higher after the application of both repellents, especially after application of the synthetic repellent. In contrast, Kenefick et al. found no differences in temperature, thermoregulation, or thermal sensation after application of a DEET-based insect repellent.19 Regarding erythema, values were significantly higher only after application of the plant-based repellent compared with control. A similar finding was reported by de Andrade et al., who observed increased erythema after citronella oil application.20 An increase in erythema may indicate altered skin homeostasis,21 suggesting that citronella oil may act as a potential skin irritant.

Melanin levels were significantly higher after application of both repellents compared with control, with the greatest increase observed after DEET application. This increase may indicate a skin stress response to the repellents,22 potentially leading to hyperpigmentation or uneven dark spots. However, the formulations themselves may also have contributed to the observed changes. For example, the chromatic properties of the synthetic repellent, which contained lavender and geraniol oils, could have affected measurements taken shortly after application. The lower variability observed in the citronella group may reflect differences in how each formulation interacts with individual skin types or is absorbed by the skin.

Skin pH was significantly lower after application of the plant-based repellent than after control or synthetic repellent application. Citronella is an essential oil that contains compounds such as citronellal, geraniol, and citronellol, which are slightly acidic.23 When applied to the skin, these compounds can lower surface pH because the skin acid mantle is naturally acidic, with an approximate pH of 4.5–5.5.

The application of both plant-based and synthetic insect repellents led to a decrease in TEWL. Similarly, de Andrade et al. found decreased TEWL after citronella application.20 This effect may be due to compounds present in the repellents that help counteract potential skin barrier damage. Many insect repellents, especially those containing oily or emollient ingredients, may form a thin layer on the skin surface, acting as a barrier that retains moisture and prevents water from escaping through the epidermis. This protective layer may reduce TEWL by minimizing water evaporation from the outermost skin layer. In some cases, repellents may contain substances that help maintain stratum corneum integrity and thereby enhance barrier function. For instance, natural oils in citronella may have moisturizing properties, whereas DEET may create a film that reduces TEWL.

In fact, DEET application resulted in a significant increase in SCH and R0 elasticity, probably because of these occlusive properties. Both parameters are fundamental for assessing skin barrier function.24,25 In this study, the repellents did not appear to be harmful; rather, they increased hydration and decreased water loss, thereby improving certain skin barrier parameters. Sebum, another important parameter for assessing barrier function, was higher after application of both repellents. Sebum production is generally beneficial because it generates a mechanical layer that protects against external noxae; however, disproportionate sebum production could favor colonization by Cutibacterium acnes or Malassezia yeasts.25 In this study, sebum increased considerably after DEET application, doubling preapplication values.

The protection time provided by citronella oil is shorter than that provided by DEET.26 Studies have shown that DEET-based repellents can provide protection for 6–8h, whereas citronella-based formulations offer a much shorter duration, often approximately 1.9h or less.27 Consequently, citronella requires more frequent reapplication to maintain effectiveness, which may increase exposure to its components and potential interactions with the skin barrier.28 In contrast, although DEET induced more pronounced changes in skin barrier parameters, its prolonged efficacy reduces the need for frequent reapplication, thereby minimizing prolonged exposure to potential irritants and occlusive agents present in the formulation. This may explain why, despite causing initial changes in skin barrier parameters such as TEWL, these alterations do not necessarily translate into clinically significant adverse effects.

Finally, our findings highlight the significant influence of exposome factors on changes in skin barrier function, regardless of the applied condition: water, citronella-based repellent, or DEET-based repellent. Although specific alterations varied depending on the substance, certain exposome factors, particularly sun exposure and alcohol consumption, consistently emerged as the most influential across all conditions.29,30 Under the control condition, sun exposure was the primary factor affecting skin temperature, erythema, elasticity, and hardness, followed by smoking, age, and BMI.31 This suggests that baseline skin characteristics, shaped by chronic environmental and lifestyle exposures, significantly influence skin reactivity even in the absence of active ingredients.

After application of the DEET-based repellent, sun exposure and alcohol consumption were again the dominant factors influencing temperature, TEWL,32 erythema,33 and elasticity.34 These findings suggest that preexisting skin barrier alterations associated with these factors may increase susceptibility to further disruption after DEET exposure. In contrast, the response to citronella-based repellent was influenced not only by sun exposure and alcohol consumption but also by skin phototype, BMI, and age. Individuals with higher skin phototypes showed greater erythema and a decrease in pH, whereas those with higher BMI showed an increase in elasticity. These results indicate that certain natural skin characteristics may modulate responses to botanical repellents differently than responses to synthetic repellents.35 Overall, our findings underscore that skin barrier function is affected not only by topical applications but also by exposome factors. This highlights the importance of considering individual exposome profiles when evaluating dermatological responses to external agents and reinforces the need for personalized skin care and protection strategies.

One strength of this study is its novelty, because the effects of insect repellents on skin barrier function have not been extensively studied. In addition, the study focused on 2 types of insect repellents commonly used by the general population, making the findings clinically relevant and applicable.

This study also has limitations. The tested formulations included ingredients with potential emollient or moisturizing properties, such as natural oils in the citronella repellent and lavender and geraniol oils in the DEET repellent. These components may have influenced the observed changes in skin barrier function. Future studies should distinguish the effects of individual compounds from those of the complete formulation. Increasing the sample size and conducting studies across different regions would further strengthen the findings and enhance their generalizability.

This study provides important insights into the effects of plant-based and synthetic insect repellents on skin barrier function, focusing on key parameters such as TEWL, SCH, elasticity, and sebum production. The findings show that both repellents significantly altered several skin parameters, although the overall effects remained within the normal range for skin barrier function. Citronella significantly decreased TEWL and pH while increasing SCH and sebum levels. Similarly, DEET reduced TEWL and increased SCH, elasticity, and skin temperature. When the effects of both repellents on skin barrier function were compared, DEET produced greater changes in variables such as temperature, TEWL, and elasticity. However, DEET provides a longer duration of protection, which reduces the need for product reapplication.

These findings show that insect repellents can alter skin barrier function and underscore the need for careful use of these products, particularly in individuals with sensitive skin or preexisting skin conditions. Although the observed changes remained within the normal range for skin barrier function, caution is warranted when these products are used in vulnerable populations. Therefore, while insect repellents play a crucial role in preventing insect bites and insect-borne diseases, particularly in tropical regions, they should be used with care, especially by individuals with compromised skin barrier integrity.

Informed consent was obtained from all participants involved in the study.

This work was supported by the Universidad de Granada under project PPJIB-2024-39, as part of the “Precompetitive Research Projects for Young Researchers – Modality B” program.

None declared.