Eczema doesn’t have a single cause. It develops from a combination of genetic predisposition, immune system overactivity, and environmental triggers that together compromise your skin’s ability to protect itself. At least 171 million people worldwide live with atopic dermatitis, the most common form, and researchers now understand the condition as a chain reaction: a weakened skin barrier lets irritants in, the immune system overreacts, and inflammation takes hold.
A Faulty Skin Barrier Starts the Problem
Healthy skin works like a brick wall. Skin cells are the bricks, and a mixture of fats fills the gaps like mortar, keeping moisture in and irritants out. In people with eczema, that mortar is defective. Studies consistently show that the outermost layer of skin in eczema patients contains significantly lower levels of key fats, particularly a group called ceramides. When ceramides drop, the ratio between them and other skin fats shifts, and the barrier becomes leaky. Water escapes more easily (which is why eczema skin feels dry and tight), and allergens, bacteria, and pollutants slip through into deeper layers where they trigger inflammation.
One specific ceramide type that protects skin flexibility is markedly reduced in eczema patients. Another type correlates directly with how much water the skin loses: the lower its levels, the worse the barrier performs. This isn’t just a consequence of the rash. Barrier defects exist even in skin that looks completely normal in people with eczema, which is why moisturizing unaffected areas matters.
Genetics Load the Gun
The strongest known genetic risk factor for eczema involves a protein called filaggrin. This protein acts like scaffolding inside skin cells, helping them flatten into the tough, tightly packed outer layer that forms your skin’s shield. It also breaks down into natural moisturizing compounds that keep skin hydrated. When the gene responsible for filaggrin carries a mutation, the protein is either absent or drastically reduced.
In a large population-based study, 18.4% of eczema patients carried at least one copy of a filaggrin gene mutation, compared with 12.9% of people without eczema. About 4.2% of eczema patients carried two copies of the mutation, meaning their skin produced little to no functional filaggrin. Those individuals can trace their eczema directly to this deficiency. But the math also tells you something important: the vast majority of people with eczema have normal filaggrin genes. Genetics contribute, but they’re not the whole story.
If one parent has eczema, asthma, or hay fever, a child’s risk roughly doubles. If both parents are affected, the risk climbs higher still. But family history influences eczema through dozens of genes affecting skin structure, immune signaling, and inflammation, not just filaggrin alone.
The Immune System Overreacts
In eczema, the immune system tilts toward a specific type of inflammatory response. When irritants or allergens penetrate the weakened skin barrier, immune cells release signaling molecules that recruit more inflammatory cells to the area. Two of the most important signals are produced by a class of immune cells that specialize in fighting parasites and responding to allergens. In eczema, these cells activate when they shouldn’t, flooding the skin with inflammatory compounds that cause redness, swelling, and intense itching.
These same signals also tell other immune cells to produce antibodies associated with allergic reactions, which is why many people with eczema also develop hay fever or asthma over time. This progression from eczema in infancy to respiratory allergies later in childhood is common enough that doctors have a name for it: the atopic march. The inflammatory signals don’t just cause symptoms. They also further damage the skin barrier, suppressing the production of the very fats and proteins the skin needs to repair itself. This creates a vicious cycle where barrier damage triggers inflammation, and inflammation worsens barrier damage.
Bacteria Fuel the Cycle
Up to 70 to 90% of people with eczema carry Staphylococcus aureus on their affected skin, compared to roughly 20 to 30% of healthy people who carry it only in the nose. This bacterium thrives on eczema skin because the weakened barrier, altered fat composition, and reduced natural antimicrobial defenses create an ideal environment.
S. aureus doesn’t just passively sit on the skin. Different strains produce toxins and enzymes that further break down the skin barrier, provoke additional immune responses, and can trigger flares. This is why eczema patches sometimes become suddenly worse, with increased oozing, crusting, or spreading. The bacterial overgrowth also crowds out beneficial microbes that would normally keep the skin ecosystem balanced, compounding the problem.
Environmental Triggers That Provoke Flares
While genetics and immune dysfunction create the underlying vulnerability, environmental exposures often determine when and how severely eczema flares. Air pollution is one trigger with growing evidence behind it. A German birth cohort study found that early-life exposure to fine particulate matter (PM2.5) was associated with a 69% increased risk of doctor-diagnosed eczema. Another study found that a modest increase in daily PM2.5 concentration was linked to 40% higher odds of symptom flares. Nitrogen dioxide and sulfur dioxide exposure show similar patterns, with eczema-related medical visits rising 3 to 6% for every incremental increase in pollutant levels.
Prenatal exposure matters too. Exposure to nitrogen dioxide, carbon monoxide, and coarser particulate matter during the first trimester of pregnancy has been linked to a higher risk of eczema developing before a baby reaches six months of age.
Beyond pollution, common everyday triggers include:
- Fragrances and preservatives in soaps, lotions, and laundry detergents
- Metals, especially nickel, found in jewelry, belt buckles, and phone cases
- Wool and synthetic fabrics that trap heat and create friction
- Sweat, particularly in skin folds
- Temperature extremes and low humidity, which accelerate moisture loss
- Certain hair dyes containing paraphenylenediamine, permanent dye’s most common allergen
- Topical antibiotics containing neomycin, a frequent cause of allergic skin reactions
Emotional stress can also provoke flares, likely because stress hormones amplify the same inflammatory pathways already overactive in eczema skin.
The Hygiene Hypothesis
One compelling theory for why eczema rates have climbed in industrialized countries over the past several decades involves microbial exposure during early life. First proposed in 1989, the idea is straightforward: growing up in environments with diverse microbes helps train the immune system to distinguish genuine threats from harmless substances like pollen or pet dander. Without that training, the immune system is more likely to overreact.
Studies of children raised on farms in Central Europe found they encountered a wider range of microbes and had lower rates of eczema and other allergic conditions than children in the same region who didn’t grow up on farms. Research has also shown that having older siblings, who bring home germs from school and daycare, correlates with reduced eczema risk. One study found that more older siblings meant different gut bacteria colonization patterns in infancy and a lower chance of developing eczema. Reduced gut microbial diversity in the first weeks of life has been linked to a higher risk of allergic disease years later.
This doesn’t mean dirt prevents eczema or that hygiene is bad. It means the modern tendency toward highly sanitized environments, fewer infections in early childhood, smaller family sizes, and widespread antibiotic use may deprive the developing immune system of the microbial signals it needs to calibrate properly.
Contact Dermatitis: A Different Mechanism
Not all eczema is atopic. Contact dermatitis, which looks similar, develops when your skin reacts to a specific substance it touches. There are two types. Irritant contact dermatitis happens when a chemical directly damages skin cells, no immune memory required. Frequent handwashing, solvents, and harsh cleaning products are common culprits. Allergic contact dermatitis involves a true immune response where your body becomes sensitized to a substance after repeated exposure, then reacts every time it encounters that substance again.
The most common metal triggers are nickel, chrome, and mercury. Fragrance ingredients and preservatives in personal care products rank among the top non-metal causes. Permanent hair dyes, nail polish, eye shadow, and even some sunscreens can provoke reactions. If your eczema appears only in specific locations (earlobes, wrists, around the eyes, or on the hands), a contact allergen is worth investigating through patch testing with a dermatologist.