Concentration, sweat, mosquito pressure and how much you apply all change how long a repellent protects you. Here is what government guidance and field studies actually show.
Concentration raises protection up to a ceiling, then stops
People often assume that a stronger repellent lasts longer. Up to a point that is true, and then it is not. The UK Health Security Agency, in the bite prevention chapter of its 2026 malaria prevention guidelines for travellers, reports that UKMEAG recommends a 50% DEET-based insect repellent if one is available. The same guidance states there is no further increase in duration of protection beyond a concentration of 50% DEET. So the useful question is not simply which number is highest on a label. It is whether the formulation, the concentration, your activity and the number of mosquitoes present allow the product to work as tested. Above the ceiling, extra strength adds little.
How long protection lasts depends on sweat, activity and mosquito pressure
Duration is not a fixed property of a bottle. The UK Health Security Agency explains that sweat-off time varies with activity, comparing sedentary activity with vigorous exercise, and that the interval between applications depends on this and on the DEET formulation and concentration. The National Travel Health Network and Centre, in its insect and tick bite avoidance factsheet last updated on 13 August 2026, adds that duration also depends on the number of mosquitoes present, sweat run off, swimming, the type of insect and individual attractants. One practical rule appears in both sources: reapply when mosquitoes begin to take an interest. That guidance turns your own observation into the useful signal, rather than a clock alone.
Most people apply far less repellent than was tested
A common reason repellents seem to fail is the amount used. The UK Health Security Agency states that travellers tend to apply almost half of the amount at which a product has been tested. The National Travel Health Network and Centre makes the same point, saying travellers apply around half the tested amount. Because of this, the stated longevity on manufacturer packaging may not be achieved, and more frequent application will be required. This matters most in places with many biting insects and during long evenings outdoors. The fix is simple and unglamorous: cover exposed skin evenly and completely, then reapply when bites start to happen. Skipping patches of skin, or using a thin film, effectively shortens your protection.
Picaridin and DEET compared in a two-year field study
Van Roey and colleagues, publishing in PLOS Neglected Tropical Diseases in December 2014, ran a field study in Cambodia over two years. Human landing collections were performed on people treated with picaridin 10% lotion, picaridin 20% spray, DEET 20% or an ethanol negative control. Based on 4996 mosquitoes collected on negative control persons, the overall five hour protection rate was 97.4%, with a 95% confidence interval of 97.1 to 97.8%, and it did not decrease over time. Picaridin 20% performed equally well as DEET 20% and better than picaridin 10%. The authors conclude that the 20% spray performs equally well as the 20% DEET solution, both preventing more than 98% of bites in that study area.
Species matter: aedines and anophelines are harder to repel
The same Cambodian study found that repellent performance varied by mosquito species. The repellents performed better against Mansonia and Culex species than against aedines and anophelines. Performance was lower against Aedes albopictus than against Aedes aegypti, and lower against Anopheles barbirostris than against several other vector species. Aedes aegypti and Anopheles minimus were the most sensitive, and Anopheles barbirostris the least. The authors also report that parity rates were higher in vectors collected on repellent treated persons than on controls. They conclude that heterogeneity in sensitivity between species could affect efficacy in public health programmes. The authors also concluded that picaridin is an appropriate product for evaluating the epidemiological impact of large scale repellent use.
PMD and DEET: why laboratory tests can look worse than real life
Colucci and Müller, in a 2018 Scientific Reports paper archived in PubMed Central, compared 15% PMD and 15% DEET in ethanolic solution. They applied 1 mL per 600 square centimetres of skin, using the same participants in laboratory arm-in-cage tests against three WHO recommended mosquito species, and in field trials at two Swiss sites using hourly human landing catches across a six-hour window starting one hour after application. In the arm-in-cage test, complete protection times for DEET were 30 minutes against Aedes aegypti and 2 hours against Anopheles stephensi and Culex quinquefasciatus. PMD gave 0.5 hours against Aedes aegypti and 0.5 to 1 hour against the other two species, so DEET slightly outperformed PMD. In the field, the median complete protection time was at least 6 hours for both repellents.
What the evidence means for choosing and reapplying
The authors of that comparison attribute the gap between laboratory and field results to mosquito landing pressure, which was magnitudes higher in the laboratory than in the field. Protection times were clearly associated with the intensity of landing rates, and the authors conclude that the arm-in-cage test simulates extreme conditions and may underestimate real efficacy where landing pressure is low to moderate. For icaridin, also called picaridin, UKMEAG advises a 20% preparation, with repellent properties and duration comparable to DEET when both are used at 20%. In some studies IR3535 has shown a shorter duration of protection than DEET at the same concentration, and it needs more frequent application, as does eucalyptus citriodora oil, known as PMD (UK Health Security Agency).
Sources
- Bite prevention: UK Health Security Agency
- Field Evaluation of Picaridin Repellents Reveals Differences in Repellent Sensitivity between Southeast Asian Vectors of Malaria and Arboviruses: PLOS Neglected Tropical Diseases
- Evaluation of standard field and laboratory methods to compare protection times of the topical repellents PMD and DEET: Scientific Reports
- Insect and tick bite avoidance: National Travel Health Network and Centre (NaTHNaC)