An IPX rating is a fresh-water result. IPX4 measures splashing water; IPX7 measures a 30-minute dunk at one meter, per IEC 60529. Both tests use clean water that runs off, evaporates, and leaves nothing behind. Sweat, salt spray, and trail mud don't behave that way. Salt dries onto battery contacts as chloride that keeps pulling moisture out of the air. Skin oil softens the rubber that does the sealing. Grit lodges in a gasket groove and holds the seal a hair off its seat, so water wicks past a seal that was never actually broken. The habit that protects the hardware is unglamorous: after a sweaty or salty trip, wipe the headlamp with a damp cloth, dry it completely, and only then close the battery door for storage.
What the number on the box actually tested
Set the lamp down on the counter after a coastal hike and look at it before you touch it. The band is stiff and pale where it crossed your forehead. There's a thin white line crusted along the battery door seam, and with the door open you can see a few dark grains sitting in the gasket groove. The switch still clicks. The beam still looks normal.
That gap between "still works" and "still sealed" is where the rating quietly stops being useful, and it took me an embarrassingly long time to see it. Now I check three places in a fixed order. The band first, because it holds the most sweat. The battery door seam second, because that's the only route from outside to the contacts. The little rubber flap over the charge port third, because on most lamps it's the softest and least supported piece of rubber on the whole unit.
The splash test is a spray rig. The immersion test is a tank. Both use fresh water, both are short, and neither one is followed by two months of a lamp sitting in a humid drawer with dried salt on it.
Salt doesn't need an opening
Salt water reaches places fresh water doesn't bother with, because it arrives as a film. A sweaty hand, a wet headband, a plume of spray on a windy beach: any of those can push a thin layer of salt water into the battery compartment seam without the lamp ever going under. The water leaves. The salt stays.
Salt is hygroscopic, meaning it pulls moisture out of ordinary air and keeps the surface around it damp long after the trip is over. That's why the corrosion shows up at home, in the drawer, in a lamp you were sure was dry. A salt film is also conductive enough to matter. A faint path between a cell terminal and a spring contact won't short anything out, but it can bleed a set of batteries down over a week and it gives corrosion a place to start. White or pale green fuzz on a spring is that process halfway done.
The usual symptom isn't dramatic. It's a lamp that was fine last week and is dead in the drawer, or one that needs a shake before it turns on.
Sweat is water with more in it
Rain and sweat are not the same test even though both are wet.
Sweat carries salt, urea, lactic acid, and the oils off your skin, and it goes somewhere rain rarely reaches: straight into the foam and fabric of the headband, where it sits and concentrates as it dries. Repeated exposure does two slow things. The chloride attacks plated contacts the way sea spray does. The oils soak into the surface of the gasket and soften it. A rubber seal that has gone soft takes a set, which means it stops springing back against the door, and a seal that doesn't push back is a seal that eventually lets water by.
This is why an IPX4 lamp rated for rain can still come apart after a summer of daily use. Not because it failed the splash test, but because nobody tests the gasket after sixty cycles of salt and oil. A rain-rated lamp that only ever sees rain is genuinely fine for years. The same lamp under a sweaty forehead every day is a different piece of equipment, and it needs the wipe-down in a way the rain lamp never will.
Grit doesn't break the seal, it props it open
Mud is the one people misread. Water doesn't have to tear a gasket to get in. One grain of sand or a fleck of dried clay sitting in the channel where the gasket seats will hold the door a fraction of a millimeter off, and the seal does the rest of the damage to itself. Water moves through that gap by capillary action, slowly and often invisibly, until the contacts are already corroding.
So a headlamp that passed IPX7 when clean can absolutely leak muddy water. The rating wasn't a lie. It described a different lamp than the one you're holding, with a different gasket surface, on a different day.
The ten minutes that decides next season
Most of the work happens once, when you get home. What I do, in order:
- Brush the gasket groove dry before anything gets wet. A soft brush only. Wetting grit before you lift it out pushes mud deeper into the channel.
- Wipe the body, the headband, and the door seam with a cloth wrung out until it's barely damp. Fresh water, and the goal is to lift salt off surfaces rather than to soak the lamp.
- Let it dry fully with the door open and the gasket exposed to moving air. If the groove still feels cool and damp against your fingertip, it isn't dry.
- Close the door only after it's dry. A closed door keeps grit out during storage, but a closed door on a wet lamp holds moisture in, right against the contacts.
That sequence is the part people get backwards. Storing a headlamp open invites dust into the very channel you're trying to protect. Storing it closed while it's still wet seals a humid pocket around the battery compartment for weeks.
One thing not to do: don't reach for alcohol, contact cleaner, or any solvent on the gasket. Those pull the plasticizers out of rubber, and rubber that has lost them goes hard and cracks. The seal you were trying to protect becomes the seal that fails. A damp cloth is the whole toolkit. If a gasket is visibly cracked, flattened, or stretched, it's a replacement part, not a cleaning problem. Cranking the battery door tighter to stop a leak just deforms it further.
Look at the contacts while the door is open. If there's fuzz or a chalky film, lift it off with a dry cloth or a soft brush. Don't scrape at the plating, because once it's gone the corrosion comes back faster every time. On a rechargeable lamp, check that the charge port flap lies flat; if it doesn't, that's the weakest point on the light, and it's worth confirming it's seated before a wet trip.
What cleaning can't fix, and when none of it matters
Not every seal is serviceable. Some lamps use bonded seams or molded gaskets you can't get at, and there the care is the same wipe-and-dry with no further option. That's a limitation of the design, not a flaw in the routine.
The routine is also conditional. A lamp that lives in a jacket pocket and only comes out on cold, dry winter nights with gloves on barely meets sweat, salt, or mud. It can go a long time between cleanings. A lamp that goes to the coast or sits on a sweaty forehead in August is the one that needs this after every trip, and it's the one where skipping it turns a three-year light into a one-season light.
