Do mosquito repellents reduce the effectiveness of sunscreens?

However, while insect repellents and sunscreens have become summer essentials, a recent study questions their compatibility. To evaluate their interactions, human skin biopsies were used ex vivo, allowing reproduction of conditions close to reality. Three products were tested: a repellent containing 35% IR3535, an SPF 50 sunscreen formulated with avobenzone, octyl triazone and anisotriazine as UV filters, and a hybrid product combining these UV filters with plant extracts, including lemon eucalyptus essential oil.

Products were applied alone (50 μL) or in combination (25 μL each), beginning with sunscreen. The reverse application order (repellent before sunscreen) was not tested; performing this test in parallel could have yielded different results. After 30 minutes’ incubation, the explants were exposed to either intense UVB radiation (300 mJ/cm²) for three minutes or natural sunlight with a UV index of eight for two hours (144 mJ/cm²). Two controls were included: a skin explant exposed to sunlight without protection (positive control) and one unexposed (negative control). Skin structure was then examined using haematoxylin and eosin staining. Researchers assessed several skin parameters, including the morphology and density of cell nuclei and their distribution in the epidermis.

Using computational analysis, researchers extracted a high contrast mean value (HCM), a quantitative indicator of halo formation around cell nuclei in histological images. These halos correspond to areas of cytoplasmic disorganisation and suggest a Cellular apoptosis potentially induced by UV. The HCM values measured 24 to 48 hours after UVB exposure are presented in histograms A and B below.

It appears that applying both a sunscreen containing avobenzone, octyl triazone and anisotriazine and a mosquito repellent containing IR3535 markedly reduces sun protection.

When sunscreen is applied alone, HCM remains comparable to that of unexposed explants, confirming its expected efficacy. In contrast, applying repellent after sunscreen results in a significant decrease in protection, with values similar to those observed without photoprotection. These results suggest that the repellent alters the efficacy of UV filters, or neutralizes them completely. It is notable that the hybrid product, designed to offer dual protection against mosquitoes and UV radiation, provides only minimal sun protection in this test, which calls its efficacy into question.

We can also question another point: HCM values at 48 hours are sometimes lower than those at 24 hours, which is counterintuitive. This decrease could indicate product degradation over time, partial loss of active compounds or partial cellular recovery in the absence of re-exposure. The citronella essential oil in this formulation may have altered the distribution or stability of the UV filters, thus reducing their protective capacity. It is also possible that the combination of these ingredients changes the product’s refractive index, thus reducing UV ray absorption.

A study published in 1997 had already reached a similar finding: the simultaneous application of a DEET-containing repellent and a sunscreen formulated with ethylhexyl methoxycinnamate, oxybenzone and ethylhexyl salicylate had reduced the sun protection factor by 28% in 14 volunteers. These results, echoing recent observations, suggest that the interaction between certain repellents and UV filters could compromise sunscreen efficacy.

No explanation has been provided on how mosquito repellents and sun protection products might interact. Possible mechanisms include UV filter degradation via reaction with repellent compounds, mechanical alteration, or dilution. The experiments tested a limited number of products: one repellent and one sunscreen under laboratory conditions and another pairing in 1997. However, there are 27 UV filters regulated in Europe, including both chemical and mineral filters, and not all interact with mosquito repellents.

Scientists tested the efficacy of sunscreen, with or without mosquito repellent, and of the hybrid product during two-hour sun exposure (histogram C). The results are less conclusive. Unlike UVB-only exposure, sunscreen alone no longer provides adequate protection. The HCM value measured under this condition is comparable to that with sunscreen and repellent or with the hybrid product, suggesting reduced efficacy. This result indicates that even without repellent, sunscreen protection declines during prolonged exposure to natural sunlight. One hypothesis is that the UV filters degraded during exposure. These data do not allow conclusions about the interaction between sunscreen and mosquito repellent under "normal" exposure conditions.

Despite the relevance of the initial results, additional real-world studies with a large sample and varied formulations would help clarify interactions between repellents and sunscreens.

But how can you combine sun protection and mosquito repellent?

It does not mean that a potential incompatibility between sun protection and insect repellent requires you to choose one over the other. Until more is known about the extent of this interaction, it is advised toleave at least 20 minutes between the application of these products. In line with US Centers for Disease Control and Prevention recommendations, apply sunscreen first so filters bind to the skin, then follow with insect repellent. Remember to reapply the sunscreen, after swimming, sweating, or every two hours, to maintain optimal protection throughout exposure.