Hair that shifts toward red or reddish-brown without any dye usually comes down to one of a few causes: sun exposure breaking down your darker pigments, mineral buildup from your water supply, nutritional changes, or simply your natural pigment balance becoming more visible over time. The good news is that most of these causes are identifiable and reversible.
How Hair Color Works at the Pigment Level
Your hair gets its color from two types of melanin. Eumelanin produces black and dark brown tones, while pheomelanin produces reddish-brown and yellow tones. Every strand of hair contains both. Black hair is roughly 99% eumelanin and 1% pheomelanin. Dark brown and blonde hair run about 95% and 5%. Truly red hair sits around 67% eumelanin and 33% pheomelanin.
The key point: pheomelanin is always there, even in the darkest hair. It’s just masked by the heavier eumelanin. Anything that selectively reduces eumelanin, whether that’s sunlight, chemicals, or changes inside your body, will let that underlying reddish pigment show through. That’s why “turning red” is so common compared to, say, spontaneously turning jet black.
Sun Exposure Degrades Dark Pigment First
UV radiation is the most common reason hair develops a reddish or warm cast, especially during summer months. Sunlight generates oxygen-based free radicals inside the hair shaft that chemically break apart melanin molecules. Eumelanin is broken down through a process called ring opening, where the molecular structure of the pigment literally splits apart. Pheomelanin goes through a similar degradation process, but here’s what matters: pheomelanin is far more sensitive to UV light overall, yet eumelanin degrades at a rate and in a way that reveals the underlying warm tones before pheomelanin itself fades.
UV damage also breaks down cystine, a sulfur-containing amino acid that helps hold the hair fiber together. As cystine degrades, the hair shaft becomes more porous, allowing water and dissolved oxygen to penetrate further. This creates a cycle where the initial sun damage makes the hair more vulnerable to additional oxidation, which continues shifting the color even after you’ve come inside. If you spend a lot of time outdoors and your hair has gradually shifted warmer, this is likely the primary cause.
Minerals in Your Water
Over 85% of U.S. homes have hard water, meaning the tap water is high in dissolved minerals like calcium, magnesium, iron, and copper. These minerals deposit onto and into the hair shaft every time you shower. Iron is the biggest culprit for reddish or orange tones. Copper tends to create brassy warmth in lighter hair and can even produce greenish tints in very blonde or bleached hair.
If your hair color shift coincided with a move to a new home, a change in your water source, or seems to get worse over time without any other explanation, mineral buildup is worth investigating. The discoloration from minerals sits on and within the cuticle layer of the hair, so it won’t wash out with regular shampoo. It accumulates gradually, which is why many people don’t connect it to their water right away.
How to Remove Mineral Buildup
Chelating shampoos are specifically designed for this. Unlike regular clarifying shampoos that remove product buildup and oil, chelating formulas contain ingredients that chemically bind to metal ions (iron, copper, calcium, magnesium) and pull them off the hair when rinsed. Think of the chelating agent as a molecular claw that grabs onto the mineral deposits so water can carry them away. Using a chelating shampoo once a week or every two weeks can gradually strip away the discoloration. If minerals are your problem, you should notice a difference within a few washes.
Chlorine and Pool Water
Swimming in chlorinated pools can shift your hair color noticeably. Research measuring precise color changes found that chlorinated water at typical pool pH made dark brown hair measurably redder and more yellow, even without any UV exposure. When UV light and chlorine act together, the effect intensifies because chlorine works as an oxidizing agent that accelerates the same pigment breakdown that sunlight causes on its own.
If you swim regularly, this effect compounds over the season. The chlorine also damages the hair cuticle in ways similar to UV exposure, making the fiber more porous and more susceptible to further color shifts from sun, minerals, or additional chlorine exposure.
Nutritional Factors
Your body needs specific nutrients to produce melanin, and shortfalls can reduce pigment production enough to visibly change your hair color. The enzyme responsible for building melanin (tyrosinase) requires copper to function. When copper levels drop, the enzyme becomes less active, and the hair that grows during that period contains less total melanin. Since eumelanin production appears more sensitive to this disruption, the result can be hair that looks warmer or more reddish rather than simply lighter.
Severe protein deficiency produces a dramatic version of this effect. In clinical cases, children with significant malnutrition develop visibly lighter or reddish bands in their hair corresponding to periods of inadequate nutrition. This is sometimes called the “flag sign,” alternating bands of normal and lighter color that record nutritional ups and downs like a timeline. You don’t need to be severely malnourished for milder versions of this to occur. Periods of strict dieting, very low protein intake, or absorption issues can reduce the nutrients available for pigment production. The total melanin content of hair drops measurably during periods of nutritional stress.
Iron deficiency is also worth considering, not because of a direct pigment mechanism, but because low iron affects many cellular processes, and the rapidly dividing cells in hair follicles are among the first to feel it.
Aging and Gradual Pigment Changes
As you get older, the cells that produce melanin in each hair follicle gradually slow down. Before hair goes fully gray or white, it often passes through intermediate shades. For people with dark brown or black hair, this transitional phase can look distinctly reddish or auburn because the fading eumelanin reveals the pheomelanin that was always present underneath. This is essentially the same principle as sun damage, but driven by biology rather than external exposure.
This shift can start earlier than you might expect. Some people notice warmer tones in their late twenties or early thirties, well before any obvious gray hairs appear. If you’re noticing red tones primarily in new growth near your roots rather than at the ends, age-related pigment changes are a likely explanation.
How to Address Unwanted Redness
The right fix depends on what’s causing the shift. For sun-driven changes, UV-protective hair products (sprays or leave-in conditioners with UV filters) and wearing a hat on high-exposure days will slow ongoing damage. The reddish tones already present in sun-damaged hair are permanent in those strands, since the eumelanin has been chemically destroyed, but new growth from the roots will come in at your natural shade.
For mineral-related redness, chelating shampoos are the first step. A shower filter designed to reduce iron and copper can prevent reaccumulation. For chlorine damage, rinsing hair with clean water before and after swimming reduces how much chlorine the hair absorbs, and a swim-specific shampoo after pool sessions helps remove residual oxidants.
If you want to cosmetically neutralize red tones, color theory provides a straightforward approach. Green sits opposite red on the color wheel, so products containing green or blue-green pigments counteract reddish warmth. These come as color-depositing shampoos, conditioners, or professional toner additives. Green-based products target true red tones, while blue-green formulas work better on orange-red or brassy warmth. These are temporary pigment deposits that wash out over several shampoos, so they require regular use.
For nutritional causes, addressing the underlying deficiency resolves the issue over time, though only in new growth. Hair that has already grown out won’t change color from improved nutrition. Given that hair grows roughly half an inch per month, it can take many months before the corrected color fully replaces the affected strands.