The reservoir is real, but who it helps is specific. For people sensitised to dust mites, about one in five adults, the more allergen in the bedroom dust, the worse the asthma and the more frequent the symptom days ¹. Lower the household allergen load and hard outcomes move: in mite-sensitised children, severe asthma attacks dropped from 29% to 17% over a year ².

Doing one thing alone does nothing. Covers alone, sprays alone, vacuuming alone don't push the load under the trigger threshold; the combined routine in the sensitised does ³. The curtain is one piece of it.

The dust also carries chemistry. Indoor dust is the main route by which phthalates and flame retardants from vinyl slats and synthetic fabric enter the body ⁴. Small for adults; more for a toddler with hands in the mouth ⁵.

The interval depends on what is hanging, not the calendar.

The effect lands fastest if you react to dust.

• A month in: the morning throat-clear and the reach for the antihistamine fade out.
• Across a year: sleep chipped by congestion gets easier, and the daytime haze rhinitis costs you softens ⁹, ¹⁰.
• With allergic asthma, the bad week that would have come this winter may not ².
• If you don't react to dust: a cleaner room, no puff when the curtain opens, clearer light through the slats. Modest, and honest.

The curtain isn't the main mite reservoir; the mattress, pillow, and duvet are, and that's where the strongest trial evidence sits ². If the curtain matters to you, the bedding matters more.