Chili peppers cause problems for eczema-prone skin through direct chemical irritation, not through an immune-system allergy. Capsaicin — the compound that makes peppers hot — activates pain and heat receptors in the skin and mucous membranes. This is a controlled chemical irritant effect. On eczema-affected skin, this irritation can trigger the itch-scratch cycle: the burning sensation leads to scratching, which damages the skin barrier, which worsens eczema.

The hotter the pepper (higher Scoville rating), the more capsaicin and the more irritation. A particularly common trap is handling chillies and then touching the face or any area of eczema — even trace amounts on the hands can cause an immediate flare. Cooking reduces but does not eliminate capsaicin.