Oxidative stress is damage that happens when highly reactive oxygen molecules interact with environmental factors like sunlight or heat to create a chemical reaction called oxidation. Oxidation can show up as brassiness when hard, unfi ltered tap water deposits metals and minerals into the porous, melanin-depleted structure of blonde hair.
When UV light hits those metals, it triggers oxidative reactions that generate free radicals, which form yellow-orange oxidation (brassines) inside the hair shaft. Because blonde hair lacks melanin’s natural UV protection, this “metal-plus-sunlight” chemistry keeps repeating with every exposure to light and heat so even after toning, the hair color continues to warm and yellow until the metals causing the reaction are removed.
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