A couple of weeks ago, I went out for an evening walk in my neighborhood and made it about 15 yards before being mobbed by a pack of unruly mosquitoes. The Black Hills sits in a semi-arid steppe climate, which normally keeps the local mosquito population under control. Growing up as a kid here, I rarely bother with bug spray. However, this evening was an exception. I strategically withdrew to the house, grabbed a bottle of repellent, sprayed down, and confidently strode back out.

I made it approximately 100 yards before being assaulted by the pests again. The repellent was 15% DEET, but it must have been an ancient one, because it should have lasted longer than two city blocks. By the time I got back inside, I had a smattering of red bites to remind me that consumer DEET does not last forever — both on your skin and (apparently) on the shelf.

How Long DEET Actually Lasts

DEET (N,N-diethyl-meta-toluamide) is still the gold-standard mosquito repellent and has been since the U.S. Army developed it in the 1940s. The duration of protection scales with concentration, but not linearly. As a rough rule: a product around 10% DEET buys you roughly two hours of protection, and around 20% buys closer to four to six. Duration keeps increasing with concentration but not indefinitely — efficacy tends to plateau around 50%, and higher percentages do not extend protection by much; they just feel oilier.

Two things will quietly cut into this. The first is the obvious one: sweat, swimming, and skin friction wear the layer off faster than the label assumes. The second is less talked about: DEET formulations degrade over time and especially when stored in a hot car or garage. Even a four- or five-year-old bottle of 15% DEET may not behave like the label promises — the active ingredient is reasonably stable, but the formulation isn't always. If yours predates a recent presidential administration, it is probably worth replacing.

West Nile

Standing in the kitchen scratching my arms, I started thinking about the different diseases these mosquitoes might be vectors for, especially those with dermatologic manifestations. In the upper Great Plains, the relevant answer most years is West Nile virus, a flavivirus first identified in West Nile district of Uganda in 1937 and now established across temperate North America. The primary U.S. vector is the Culex mosquito, with birds as the main reservoir and amplifying hosts; humans are dead-end hosts. Cases tend to cluster in late summer and fall.

Many West Nile infections are clinically silent — the great majority of people who acquire it never know it. Among those who do develop symptoms, the typical picture is a mild flu-like illness: fever, myalgias, headache, sometimes anorexia, with an incubation period typically 2 to 6 days, and ranging up to 14. From a dermatologic standpoint, the noteworthy feature is the rash: roughly half of symptomatic patients develop a viral exanthem of erythematous macules and papules (little red dots) on the upper trunk that resemble measles or roseola. Most mild illness resolves within a week, though a prolonged fatigue is common afterward.

The small fraction of patients who go on to develop neuroinvasive disease — encephalitis, meningitis, acute paralysis — are the reason West Nile is taken seriously. Neuroinvasive disease can include confusion, neck stiffness, cranial nerve palsies, and generalized weakness, and carries a mortality rate around 10%. There is no specific antiviral therapy; care is supportive. Older adults and the immunocompromised are at higher risk for severe disease.

What the AAD Recommends

The American Academy of Dermatology's patient guidance on bug bite prevention is straightforward and worth a read in full. The short version:

  • Use an EPA-registered repellent. DEET at 20–30% on exposed skin is the most-cited dermatology recommendation. Picaridin (20%), IR3535, and oil of lemon eucalyptus (OLE) are other alternatives.
  • Treat the clothing, not just the skin. Permethrin-treated clothing adds a meaningful second layer of protection and lasts through several washes.
  • Apply sunscreen first, then repellent. And avoid the combination sunscreen + repellent products — the two reapplication schedules are different, and you end up either under-protected from UV or over-applying DEET.
  • Skip the spray on infants under two months. Use mosquito netting on strollers and carriers instead. Also avoid oil of lemon eucalyptus and its refined active ingredient PMD in children under three.

Full guidance is at the AAD: How to prevent and treat bug bites.

Back to the Walk

I did not, fortunately, end up with West Nile — just a collection of itchy red bites. My personal go-to combination for mosquito bites is oral levocetirizine, the over-the-counter R-enantiomer of cetirizine (Zyrtec) that tends to cause less drowsiness than regular cetirizine. Importantly, I combine this with a thin layer of topical fluocinonide ointment (a prescription-strength steroid) on the largest, angriest bite marks. The bites were largely settled within two to three days. As a fun aside, if you do find yourself at the doctor's office for bug bites, there is actually a specific medical diagnosis code for "arthropod assault."

Don't be victimized by an arthropod assault: bug spray quietly ages out in a drawer and with hot temperatures. Guess I will follow my own advice and update the DEET supply…

Disclaimer: This article is general dermatologic education and not medical advice. Persistent or severe bite reactions, or any concerning symptoms after a bite, deserve evaluation by your physician.

References

American Academy of Dermatology. How to prevent and treat bug bites. Accessed June 2026.

Centers for Disease Control and Prevention. West Nile virus — clinical signs and symptoms. Accessed June 2026.

Centers for Disease Control and Prevention. Mosquitoes, ticks, and other arthropods — CDC Yellow Book. Accessed June 2026.