Mosquito Repellents: Which to Choose and How to Apply It

An effective repellent contains one of four active ingredients: DEET, icaridin, IR3535 or citriodiol (also called PMD). If the packaging doesn’t mention any of them, it isn’t doing much. And in Spain it also has to carry a visible health authorisation number on the label: repellents are biocidal products and have to be assessed by the Spanish medicines agency (AEMPS) or by the health ministry’s public health directorate.

Everything else — the brand, the smell, whether “natural” is printed in large letters — matters far less than those two things: which active ingredient it contains, at what concentration, and whether you are applying it properly.

The four active ingredients that work

Active ingredient	Also appears as	Notes on use
DEET	N,N-diethyl-m-toluamide, diethyltoluamide	The most studied. Effectiveness stops rising appreciably above roughly 50%
Icaridin	Picaridin, KBR 3023	The usual alternative to DEET, less smelly and less aggressive towards plastics and synthetic fabrics
IR3535	Ethyl butylacetylaminopropionate	Common in formulations aimed at family use
Citriodiol	PMD, para-menthane-3,8-diol	Derived from lemon eucalyptus. Not to be confused with the unformulated essential oil, which offers no equivalent guarantee

How long protection lasts doesn’t depend on the active ingredient alone. It varies with concentration, air temperature, how much you sweat, whether you swim, and whether you are wearing sunscreen. As a general rule, the higher the concentration the more hours of protection; products with under 10% active ingredient usually cover only an hour or two. Sustained-release or microencapsulated formulations can last longer even at a lower concentration.

What doesn’t work

There is a fair amount of myth here and it’s worth being blunt, because every euro spent on a useless product is an hour of unprotected skin:

• Repellent wristbands. None is authorised in the Spanish biocide registers. They aren’t on the list because they haven’t demonstrated what they promise.
• Devices emitting sound or ultrasound.
• Taking vitamin B or garlic. There is no evidence that either changes how attractive you are to mosquitoes.
• Unformulated essential oils sold as repellent. “Pure” lemon eucalyptus oil is not the same as formulated, assessed citriodiol or PMD: it hasn’t been through the same safety and effectiveness checks.
• Products combining sunscreen and repellent in one bottle. The problem isn’t that they don’t work, it’s that sunscreen needs reapplying every two hours and repellent doesn’t: follow the instructions for one and you break the instructions for the other.

Before buying, check that the authorisation number appears on the packaging. You can verify it in the AEMPS biocide search or in the health ministry’s official register of non-agricultural pesticides. The usual formats look like 553 PLAG, 23-50-12043 or ES-0015532-0000.

How to apply it properly

Almost every repellent failure is an application failure:

1. Read the label first. It gives the minimum age, how often to reapply, and which insects the product works against. A repellent effective against mosquitoes does not necessarily protect against ticks.
2. Test a small patch of skin the first time, to rule out a reaction.
3. Apply only to exposed skin, evenly. You don’t need it on clothing, and not under clothing either.
4. Never spray directly on the face. Put it on your hands and spread it, avoiding eyes and lips. Go sparingly around the ears.
5. Don’t put it on wounds, cuts, irritated or sunburnt skin, or in deep folds such as armpits or the groin.
6. Just enough. A thin layer is sufficient. Saturating the skin doesn’t extend protection; if you are still being bitten, add a little more, but don’t exceed the frequency on the label.
7. Use sprays outdoors, so you don’t inhale the product, and away from food.
8. Wash your hands after applying it.
9. Reapply as the label says, and particularly after swimming or sweating heavily.
10. When you’re done for the day, wash the treated areas with soap and water. There is no need to keep repellent on your skin all night.

Repellent and sunscreen: sun first

This is where most people get it wrong. The right order is sunscreen first, repellent second. The Spanish health ministry recommends leaving at least 30 minutes between them; other health authorities frame it simply as waiting for the sunscreen to absorb.

One practical detail worth knowing: the available data indicate that applying a DEET repellent over sunscreen reduces the sun protection factor by around a third. That isn’t a reason to give up either one, but it is a reason to reapply sunscreen more often than you would if you weren’t wearing repellent on top. On a beach day in Salou that usually means every couple of hours regardless of what the bottle claims.

Children, pregnancy and breastfeeding

Spanish guidance here is cautious and worth following to the letter:

• Under 2 months: no repellents at all. Physical barriers only: a mosquito net over the cot and pushchair, clothing covering arms and legs.
• Under 3 years: if you think a repellent is needed, check first with your paediatrician or at the pharmacy that the specific product covers that age. An adult always does the applying.
• Children generally: don’t let them handle the bottle. The adult puts it on their own hands and spreads it over the child’s exposed skin, avoiding the child’s hands (which end up in mouths and eyes). Back home, wash the treated skin with soap and water.
• Pregnancy and breastfeeding: check with a healthcare professional before using repellents.

Repellent is one layer, not the only one

No repellent replaces the other measures, and combining them pays off more than increasing the concentration:

• Covering clothing: long sleeves, long trousers, socks and closed shoes, preferably in light colours.
• Mosquito nets on windows, doors and, where needed, over the bed.
• Air conditioning where possible: it makes life harder for mosquitoes.
• Remove standing water around the house — plant pot saucers, buckets, drains, fountains, pet water bowls — because mosquitoes need very little water to breed. Change pet water every two or three days.
• Clothing and equipment can also be treated with permethrin, which is never applied to skin, only to fabric.

And don’t confuse repellent with insecticide: insecticide kills the mosquito and never goes on skin; repellent drives it away and doesn’t kill it.

When to see a doctor

A normal mosquito bite itches, swells a little and settles within a few days. Get advice if any of these happen:

• Difficulty breathing, swelling of the lips, tongue or throat, severe dizziness or widespread hives after a bite or after applying a repellent: this is an emergency, seek immediate help.
• The bite area becomes hot, increasingly red, painful or oozing, or a fever appears: it may have become infected.
• A very large local reaction still growing after 48 hours or covering a whole limb.
• Fever, joint pain, rash or severe headache in the days or weeks after travelling to an area with mosquito-borne disease. Tell whoever sees you about the trip and the dates.
• Rash, itching or swelling caused by the repellent itself: wash it off with mild soap and water, stop using it, and take the packaging with you to the appointment.
• Doubts about the right product for a small child, in pregnancy, while breastfeeding, or if you have a chronic condition.

This information is general in nature and does not replace assessment by a healthcare professional. Sources consulted: Spanish Ministry of Health — Protective measures and use of repellents (in Spanish), AEMPS — Biocides and personal care (in Spanish), Junta de Andalucía — Information note: correct use of mosquito repellents (2025, in Spanish) and CDC Yellow Book — Mosquitoes, Ticks, and Other Arthropods.