AdvicePublished: Jul. 06, 2026, 6:00 a.m.Dr. Roach: The permethrin in factory-applied gear lasts longer than self-applied permethrin, but it’ll gradually become less effective, especially when it gets exposed to sunlight and oxygen. Getty Images Getty ImagesDEAR DR. ROACH: My son is going to be spending several weeks on a lake. I was looking at a permethrin-impregnated sleeping bag liner, and I’m wondering if this type of treated sleeping bag is good to prevent bites from various insects. Is it worth the money? -- J.M.M.ANSWER: Permethrin-impregnated clothing is highly effective. Studies have shown an 80% to 90% reduction in bites when it’s used. (Permethrin can’t be used on skin as it’s inactivated by skin oils.) Sleeping bag liners and bed nets aren’t as well-studied, but they’re still r