An environmental allergen is an airborne allergen such as dust, insect spores, or pet hair — the inhaled triggers of allergy, as distinct from food allergens that are eaten. House-dust-mite droppings, grass and tree pollens, mould spores, and animal dander provoke the classic responses: sneezing, streaming eyes, wheezing, and eczema flares. The food connection is closer than it looks — the same immune system, the same IgE antibodies, and the cross-reactivities that link birch pollen to apple allergy or dust mites to shellfish sensitivity. Food factories must control environmental allergens too: flour dust sensitises bakers’ lungs, and airborne milk or egg powder can contaminate “free-from” lines. Managing them means ventilation, filtration, cleaning, and monitoring — the indoor environment treated as carefully as the ingredient list.