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Why cold water only: blood is protein-based. Heat causes the protein to coagulate and bond to the fabric fibres — the same way an egg white sets in a pan. Once that happens, the stain is extremely difficult to remove. This applies to the wash and the dryer: never machine-dry a garment until the stain is completely gone.
Period blood on sheets: treat it exactly the same way — it's the same protein stain. The two things that matter are cold water and acting before it dries. A cold-water rinse and soak with a little dish soap or an enzyme detergent handles most of it. For dried stains, an overnight soak in cold water with oxygen bleach is the most effective approach, and hydrogen peroxide works well on white bedding. Keeping a bottle of hydrogen peroxide by the laundry is genuinely the easiest fix.
What to avoid: hot water, the dryer before the stain is gone, chlorine bleach on anything but white cotton (it can react with blood proteins and set a yellow mark), and vigorous scrubbing, which spreads the stain and damages the weave.
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