The buildup under your toenails is mostly keratin, the same tough protein your nails are made of. As your nail grows, dead skin cells and keratin naturally accumulate in the space between the nail plate and the nail bed. This is normal to a degree, but excessive buildup often signals something else going on, from fungal infections to skin conditions like psoriasis.
What the Buildup Is Made Of
Your toenails are roughly 80% hard keratin and 20% soft keratin, bound together with high amounts of sulfur. The nail bed underneath constantly sheds skin cells, and some of those cells get trapped beneath the nail as it grows forward. Mix in sweat, sock fibers, dirt, and dead skin from walking barefoot, and you get the whitish or grayish paste that collects under the free edge of the nail.
A thin layer of this debris is completely normal. You can usually remove it with regular washing and gentle cleaning. But when the buildup becomes thick, discolored, or hard to scrape away, it’s worth looking more closely at the cause.
Fungal Infections: The Most Common Culprit
Toenail fungus (onychomycosis) affects about 5.5% of people worldwide, and it’s the leading cause of significant keratin buildup under the nail. The condition becomes more common with age. Dermatophytes, a group of fungi that feed on keratin, are responsible for most cases, though yeast and mold can also be involved.
Fungal infections typically start at the tip or sides of the nail and work inward. As the fungus colonizes the nail bed, it triggers your body to produce excess keratin in response, creating a thick, chalky or crumbly mass underneath the nail. This process gradually lifts the nail plate away from the bed, a separation called onycholysis.
The visual signs are distinctive. Fungal buildup tends to appear yellowish brown, and the nail itself may thicken, become brittle, or develop a rough texture. In one form of the infection, well-defined white patches appear on the nail surface that eventually merge and make the nail soft and crumbly. A bacterial co-infection can turn the debris green or black. If the infection goes untreated long enough, the entire nail can become thick, misshapen, and largely replaced by keratotic debris.
Psoriasis and Other Skin Conditions
Psoriasis is the skin condition most likely to affect the nails. It produces buildup that can look similar to a fungal infection, which makes it easy to confuse the two. Nail psoriasis causes the skin cells under the nail to turn over too quickly, creating a silvery or whitish scale that packs into the space beneath the nail plate.
A few features help distinguish psoriasis from fungus. Psoriatic nails often develop small pits, tiny depressions in the nail surface caused by flawed keratin production in the nail root. More than 10 pits on a single nail, or more than 60 across all nails, generally points to psoriasis. You might also notice salmon-colored or oily-looking spots on the nail bed, along with tiny splinter-like lines of dried blood. Another key difference: psoriasis tends to wax and wane over time, with periods where nails look better followed by flare-ups. Fungal infections, by contrast, steadily worsen without treatment.
Why It Sometimes Smells
That sharp, unpleasant odor when you clean under a toenail comes from microorganisms breaking down trapped keratin and dead skin. Bacteria thrive in the warm, moist environment inside shoes and feed on the organic material collecting beneath the nail. When a fungal infection is also present, the combination of fungal metabolites and bacterial byproducts intensifies the smell considerably. The odor alone doesn’t necessarily mean you have an infection, but persistent, strong-smelling debris paired with discoloration is a reliable signal that something beyond normal buildup is happening.
How Shoes and Moisture Contribute
Ill-fitting shoes are a primary driver of both keratin buildup and toenail fungus. When toes are pressed against the front or sides of a shoe, the repeated pressure damages the nail bed and triggers excess keratin production as a protective response. This is why runners and hikers commonly develop thickened nails with significant subungual debris, even without a fungal infection.
Moisture compounds the problem. Fungi and bacteria flourish in damp environments, so wearing the same sweaty socks all day or keeping feet in non-breathable shoes creates ideal conditions for infection. Changing socks daily and removing them promptly after exercise reduces this risk. Shoes with enough room in the toe box prevent the mechanical trauma that kickstarts the buildup cycle in the first place.
Safely Cleaning Under Your Toenails
For normal buildup, soaking your feet in warm water for 10 to 15 minutes softens the debris and makes it easier to remove. A thin, flat tool like an orangewood stick works well for gently pushing material out from under the nail edge. Avoid using sharp metal instruments aggressively, as puncturing the nail bed creates an entry point for bacteria and fungi.
If the nail is significantly thickened, professional debridement from a podiatrist is safer and more effective. Podiatrists routinely use specialized drilling tools to thin down thickened nails, relieve pressure, and clear packed debris. This is especially important if you have diabetes, poor circulation, or difficulty reaching your feet, since even a small wound in these situations can lead to serious infection.
Treating Fungal Buildup
Fungal toenail infections are notoriously stubborn. Topical nail lacquers are available both over the counter and by prescription, but their complete cure rates are modest. The most effective prescription topical clears the infection fully in roughly 15% to 26% of cases, while older formulations manage complete cures in only about 6% to 9% of users. The reason: the thick nail plate acts as a physical barrier that prevents medication from reaching the fungus underneath.
Oral antifungal medications are significantly more effective because they reach the nail bed through the bloodstream, bypassing the nail barrier entirely. Treatment typically lasts several months, and because toenails grow slowly, it can take six months to a year before you see a fully clear nail growing in. Your doctor will help determine whether topical or oral treatment makes more sense based on the severity of the infection and your overall health.
Signs That Need Professional Attention
Some changes beneath the toenail go beyond cosmetic concern. Fluid or pus draining from under the nail, increasing pain or swelling, red streaks extending from the toe, or a feeling of heat and throbbing all point to an active infection that needs treatment. A dark spot under the nail that appeared without any injury and doesn’t grow out with the nail should be evaluated promptly, as it can occasionally indicate something more serious than trapped blood. Excessive redness around the nail fold or a fever alongside nail changes also warrants a visit.