Eczema doesn’t have a single root cause. It results from a collision of genetic vulnerability, a weakened skin barrier, and an immune system that overreacts to triggers that wouldn’t bother most people. About 204 million people worldwide live with atopic dermatitis (the most common form of eczema), and the condition affects roughly 4% of children and 2% of adults globally. Understanding what drives it requires looking at several interlocking systems in the body.
A Faulty Skin Barrier Starts the Problem
Healthy skin works like a brick wall. Skin cells are the bricks, and a mix of natural fats fills the gaps like mortar, keeping moisture in and irritants out. In eczema, that mortar is defective. The outermost layer of skin loses water far faster than normal. In healthy children, the skin loses about 6.2 grams of water per square meter per hour. In eczema, that number climbs significantly, even in patches that look completely clear.
A major reason for this leakiness is a shortage of ceramides, the waxy fat molecules that hold the skin barrier together. People with eczema have significantly lower levels of two key ceramide types (ceramide 1 and ceramide 3) compared to people with healthy skin, while their cholesterol levels in the skin run higher. This imbalance weakens the barrier enough that allergens, bacteria, and chemical irritants can slip through into deeper skin layers, where they provoke inflammation.
The Genetic Link: Filaggrin and Beyond
The strongest genetic risk factor for eczema is a mutation in the gene that produces filaggrin, a protein essential for building and maintaining the skin’s outer layer. Roughly 27% of young eczema patients carry a filaggrin gene mutation. When filaggrin is missing or reduced, skin cells don’t mature properly, and the barrier develops microscopic gaps. Those gaps are enough to let outside molecules penetrate the skin and trigger an immune response.
But filaggrin mutations account for only about a quarter of cases, which means most people with eczema have a normal filaggrin gene. Other genetic variations affect how the immune system behaves, how skin fats are produced, and how the body responds to microbial invaders. Eczema runs in families, and having one parent with eczema, asthma, or hay fever raises a child’s risk substantially. The condition is part of what doctors call the “atopic march,” a progression where eczema in infancy can precede food allergies, asthma, and allergic rhinitis later in life.
An Immune System Stuck in Overdrive
Once the skin barrier lets irritants through, the immune system mounts a response that’s disproportionate to the actual threat. The inflammation in eczema is driven primarily by a branch of the immune system called the Th2 pathway, which normally handles parasitic infections and allergic responses. In eczema, this pathway stays chronically activated.
Two signaling molecules sit at the center of this process. These immune messengers promote the creation of more allergic immune cells, which in turn recruit inflammatory cells like eosinophils, basophils, and mast cells into the skin. Those cells release compounds that cause redness, swelling, and intense itching. The signaling molecules also directly weaken the skin barrier further by disrupting the production of ceramides and other protective proteins. This creates a vicious cycle: barrier damage lets irritants in, the immune system overreacts, and the immune reaction damages the barrier even more.
Bacteria That Exploit the Broken Barrier
The skin has its own microbiome, a community of bacteria that normally keeps the surface healthy. In eczema, that community gets taken over by Staphylococcus aureus, a bacterium that thrives on damaged skin. While only 5% to 30% of healthy people carry S. aureus on their skin, 75% to 100% of eczema patients have it on their affected patches.
The colonization rate is highest during flares: 74% of acute eczema lesions harbor S. aureus, compared to 38% of chronic patches. The density of these bacteria correlates directly with how inflamed the skin is. In acute flares, scratching physically breaks the skin and gives bacteria easy access. In chronic eczema, the bacteria exploit defects in the tight junctions between skin cells, settling in for the long term. S. aureus doesn’t just take advantage of the broken barrier; it actively worsens it by releasing toxins that provoke further immune activation and itching.
The Gut Connection
The relationship between gut health and eczema has become one of the more compelling areas of research. The concept is called the gut-skin axis: what happens in the digestive tract influences inflammation throughout the body, including the skin.
The key players are short-chain fatty acids, molecules produced when gut bacteria ferment dietary fiber. These compounds, particularly butyrate and propionate, help train the immune system to tolerate harmless substances rather than attacking them. They do this by promoting the development of regulatory immune cells that suppress allergic responses. People with eczema tend to have lower levels of these protective fatty acids in their stool, along with fewer of the bacterial species that produce them. Studies of infants have found that lower butyrate and propionate levels in the first year of life are associated with developing eczema. The severity of eczema in older patients correlates negatively with the abundance of butyrate-producing bacteria in the gut.
Environmental Triggers That Push Things Over the Edge
Genetics loads the gun, but the environment pulls the trigger. Several external factors have been linked to eczema onset and flares.
Air Pollution
Traffic-related air pollution is a measurable risk factor. Fine particulate matter (PM2.5) is associated with a twofold increase in the risk of developing eczema. Nitrogen dioxide, carbon monoxide, and other combustion byproducts also correlate with higher eczema rates in children. A German study found that simply living closer to a major road increased eczema prevalence in young children. These pollutants bind directly to the outer layer of skin, where they can provoke inflammation and weaken barrier function. The effects can be immediate: research tracking daily pollution levels against medical visits shows that eczema flares spike on the same day pollution rises.
Hard Water
The mineral content of tap water matters more than most people realize. Hard water, which is high in calcium and magnesium, damages the skin barrier through several pathways. It makes soap harder to rinse off, leaving irritating residues that dissolve protective skin fats and raise the skin’s pH. Healthy skin is mildly acidic, and the alkaline minerals in hard water push it toward a more neutral pH, which compromises barrier function and activates enzymes that break down skin proteins. The calcium in hard water may also interfere with normal signaling between skin cells. Together, these effects increase allergen penetration and bacterial colonization.
Climate and Indoor Environment
Low humidity dries the skin and accelerates water loss. Central heating in winter compounds this by dropping indoor humidity further. Indoor irritants like dust mites, pet dander, and volatile organic compounds from cleaning products and furniture can all trigger flares in sensitized skin. Even emotional stress worsens eczema by shifting immune function toward the same allergic pathways that drive the condition.
Why It Varies So Much Between People
One of the most frustrating aspects of eczema is how differently it behaves from person to person. A child with severe filaggrin deficiency and high S. aureus colonization may have persistent, widespread disease, while someone with a milder barrier defect might only flare during winter or after contact with a specific detergent. The condition also shifts with age: it tends to affect the face and scalp in infants, the creases of elbows and knees in older children, and the hands and eyelids in adults. Women are affected slightly more often than men, with a global prevalence of 2.8% compared to 2.4%.
Because eczema sits at the intersection of genetics, immune function, the skin microbiome, gut health, and environmental exposures, no two cases have exactly the same root driver. For some people, barrier repair through consistent moisturizing and avoiding harsh cleansers makes the biggest difference. For others, targeting the overactive immune response is what finally brings relief. The condition is best understood not as having one root cause, but as a system where multiple vulnerabilities stack on top of each other until the skin can no longer cope.