HOW COME?: UV rays bleach hair by breaking up molecules

What do faded curtains, bleached-out kitchen cabinets, and beach-blond hair have in common? All come courtesy of ultraviolet radiation, an invisible part of the sunlight we bask in on a summer day.

Light from the sun comes in all frequencies; some we see, some we don't. Visible light is medium-frequency, but other lower and higher frequencies are invisible to human eyes. So we can't see lower-frequency infrared rays and radio waves. And we don't have eyes capable of seeing higher-frequency ultraviolet light or X-rays.

But it's the ultraviolet part of sunlight that makes our skin darken in summer. How? First, dead skin cells on the surface absorb some of the UV. Then, under the surface, pigment-producing cells called melanocytes start making extra melanin pigment, which also soaks up ultraviolet light. The more UV rays we encounter, the more melanin piles up, making our skin darker and darker. It's our body's attempt to protect us from the DNA damage that can be caused by too much radiation.

But skin and hair are very different. Skin is an organ, layers of living cells fed by tiny blood vessels. Unlike skin, hair isn't living tissue. Hair is made of keratin, the protein that also forms fingernails and toenails (which, like hair, we can painlessly trim). Animal horns are also sheathed in keratin. And like a horn, hair gets its color from the melanin made in the skin at its root.

Hairs that lose some of their original load of pigment look faded. Stripped of all melanin, hair becomes colorless, appearing white. Once in the hair shaft, melanin can't be replenished from the base. Which is why hair lightened by the hydrogen peroxide in hair dye can't return to its natural color on its own.

But you don't need dye to (subtly) lighten your hair; hair exposed to summer sunlight will lighten, whether you like it or not. Over time, ultraviolet radiation, combined with the oxygen in the air, causes a chemical reaction in hair that breaks up the melanin molecules deposited by the scalp. So hair gradually gets a bit lighter - blonder if you have blond or light brown hair, redder if your hair is dark. The same bleaching process happens to curtain fabric (UV-A light penetrates glass) and to the wood furniture and cabinets in your home - and to animal bones in the desert.

So what does lemon juice add? What makes lemon juice so tart is its citric acid. When lemon juice coats the hair shaft, citric acid speeds up the breakdown of melanin in the sun, making your hair lighter sooner. Scientists call the process "acid-catalyzed oxidation." While saturating your hair with diluted lemon juice may help it lighten, citric acid can also damage the cuticle, leaving hair a strawlike mess. And, of course, sitting in the sun too long can heighten the risk of skin cancer. (So besides sunscreen, bring a hat to the beach!)