Eczema, also known as atopic dermatitis, is caused by a combination of genetic predisposition, skin barrier defects, and an overactive immune response, not by any single factor. It affects roughly 10% of the global population aged 16 and older, with rates varying from around 5% in Germany and Kenya to over 15% in South Korea and China. Understanding the interplay of causes helps explain why eczema runs in families, flares unpredictably, and responds to such different treatments.
A Faulty Skin Barrier Starts the Problem
Healthy skin works like a brick wall: tough protein cells act as the bricks, and a mortar of natural fats (called ceramides) seals the gaps between them. In eczema, both parts of that wall are compromised.
The protein side of the equation centers on a gene called FLG, which provides instructions for building a key structural protein in the outermost layer of skin. About 40 different mutations in this gene have been identified in people with eczema. These mutations produce a stunted, unusable version of the protein, leaving the skin’s protective barrier full of microscopic holes. Between 20% and 30% of people with eczema carry an FLG mutation, compared to just 8% to 10% of the general population.
The fat side matters just as much. Research comparing skin samples from people with and without eczema has found a marked reduction in ceramide levels in eczema-affected skin. One particular ceramide fraction, ceramide 1, shows the steepest decline. What’s striking is that even skin that looks completely normal on a person with eczema still has significantly depleted ceramide levels. This means the barrier defect is body-wide, not limited to patches where rashes appear. That’s why moisturizing all your skin, not just the rough spots, is a cornerstone of eczema management.
The Immune System Overreacts
A leaky skin barrier lets in allergens, bacteria, and irritants that wouldn’t normally reach the deeper layers of skin. When they do, the immune system responds, but in eczema, that response is disproportionate. The outer skin cells release alarm signals that activate a specific branch of the immune system geared toward fighting parasites and allergens rather than bacteria or viruses. This branch pumps out inflammatory signaling molecules that cause redness, swelling, and intense itching.
Two of those signaling molecules are especially important. One drives the immune system to keep producing allergy-type antibodies (IgE), which is why many people with eczema also test positive for environmental allergies. The other acts directly on the skin, disrupting the barrier further and triggering mucus production in the airways, which helps explain why eczema so often travels with asthma and hay fever. This trio of conditions, sometimes called the “atopic march,” tends to appear in sequence during childhood: eczema first, then food allergies, then respiratory allergies.
The cruel irony is that the inflammation itself damages the skin barrier even more, which lets in more irritants, which triggers more inflammation. Breaking this cycle is the primary goal of treatment.
Bacteria on the Skin Make It Worse
A specific bacterium, Staphylococcus aureus, plays an outsized role in eczema flares. It’s present on the skin of a large proportion of eczema patients, colonizing both affected and unaffected areas at rates far higher than in people without the condition.
This bacterium isn’t just a passive bystander. It releases several toxins that actively worsen the disease. One is a metalloprotease that breaks down the skin’s natural antimicrobial defenses. Another is a serine protease that directly damages the skin barrier and interacts with nerve receptors to trigger itching. A third group of toxins, called exfoliative toxins, further erode the barrier’s structural integrity. The result is a skin surface that’s both more inflamed and less capable of defending itself.
The gut may matter too. Studies comparing the intestinal bacteria of people with and without eczema consistently find that those with eczema have reduced populations of beneficial bacteria, particularly Bifidobacterium and Lactobacillus. Lower levels of these bacteria correlate with more frequent flare-ups and greater disease severity, though whether restoring them reliably improves symptoms remains an active area of investigation.
Genetics Beyond the Skin Barrier
FLG mutations are the best-studied genetic link, but they account for only a portion of eczema cases. Many people with eczema have no identifiable FLG mutation at all, and some people who carry the mutation never develop the condition. This points to a broader genetic landscape.
Eczema is strongly heritable. If one parent has atopic dermatitis, asthma, or hay fever, a child’s risk rises substantially. If both parents are affected, the risk climbs higher still. The genetic contribution likely involves dozens of genes influencing immune regulation, skin lipid production, and inflammatory signaling. No single gene “causes” eczema the way a single gene causes conditions like cystic fibrosis. Instead, multiple genetic variants each nudge risk upward, and environmental factors determine whether those risks become a visible disease.
Environmental Triggers That Spark Flares
Genetics loads the gun, but the environment pulls the trigger. Common triggers include:
- Indoor air quality: Visible mold, damp stains, perceived dry air, window condensation, and abnormal smells in the home are all associated with higher rates of eczema symptoms. Cooking with coal or wood and burning incense also increase risk.
- Chemical exposures: Freshly painted walls and new furniture (which off-gas volatile compounds) during pregnancy or early life are linked to higher childhood eczema rates.
- Tobacco smoke: Exposure to environmental tobacco smoke around the time of birth is a consistent risk factor.
- Climate: Dry air strips moisture from skin, while excessive heat triggers sweating that irritates already-compromised skin. Over a third of young children with eczema experience itching when they sweat.
Protective factors are simpler. Living in a rural location and regularly ventilating your home by opening windows both appear to lower risk, likely by reducing indoor pollutant concentrations and diversifying microbial exposures.
The Hygiene Hypothesis: Complicated Evidence
You may have heard that modern cleanliness causes eczema by depriving the immune system of the microbial training it needs in early life. This idea, known as the hygiene hypothesis, has some supporting evidence but also significant holes.
The strongest finding is the sibling effect. A study of over 600 children found that having one older sibling cut eczema risk by about 40%, and having two or more older siblings cut it roughly in half. The assumption was that older siblings bring home infections that “educate” a younger child’s immune system. But when researchers measured actual markers of infection, including antibodies to common childhood viruses, frequency of colds, and history of chickenpox, none of them explained the protective effect of older siblings. Children who caught more infections were not less likely to develop eczema.
This suggests the sibling effect is real, but its mechanism is more nuanced than simply “more germs equals less eczema.” It may involve broader microbial diversity in the household, shared skin bacteria between siblings, or other factors that haven’t been fully identified.
How These Causes Interact
What makes eczema so persistent is that its causes reinforce each other. A genetic barrier defect lets irritants and bacteria penetrate the skin. The immune system overreacts, producing inflammation that further damages the barrier. Staphylococcus aureus exploits the weakened barrier, releasing toxins that increase itching and inflammation. Scratching causes physical damage, which opens new entry points for bacteria and allergens. Environmental triggers like dry air or chemical irritants add fuel at every stage.
This self-reinforcing loop is why eczema management typically targets multiple causes at once: restoring the skin barrier with ceramide-containing moisturizers, calming the immune response with anti-inflammatory treatments, and reducing environmental triggers. No single intervention addresses the full picture, because no single cause drives the disease alone.