Sept. 6, 2005 — Human hands glow, but fingernails release the most light, according to a recent study that found all parts of the hand emit detectable levels of light.

The findings support prior research that suggested most living things, including plants, release light. Since disease and illness appear to affect the strength and pattern of the glow, the discovery might lead to less-invasive ways of diagnosing patients.

Mitsuo Hiramatsu, a scientist at the Central Research Laboratory at Hamamatsu Photonics in Japan, who led the research, told Discovery News that the hands are not the only parts of the body that shine light by releasing photons, or tiny, energized increments of light.

"Not only the hands, but also the forehead and bottoms of our feet emit photons," Hiramatsu said, and added that in terms of hands "the presence of photons means that our hands are producing light all of the time."

The light is invisible to the naked eye, so Hiramatsu and his team used a powerful photon counter to "see"it.

The detector found that fingernails release 60 photons, fingers release 40 and the palms are the dimmest of all, with 20 photons measured.

The findings are published in the current Journal of Photochemistry and Photobiology B: Biology.

Hiramatsu is not certain why fingernails light up more than the other parts of the hand, but he said, "It may be because of the optical window property of fingernails," meaning that the fingernail works somewhat like a prism to scatter light.

To find out what might be creating the light in the first place, he and colleague Kimitsugu Nakamura had test subjects hold plastic bottles full of hot or cold water before their hand photons were measured. The researchers also pumped nitrogen or oxygen gas into the dark box where the individuals placed their hands as they were being analyzed.

Warm temperatures increased the release of photons, as did the introduction of oxygen. Rubbing mineral oil over the hands also heightened light levels.

Based on those results, the scientists theorize the light "is a kind of chemiluminescence," a luminescence based on chemical reactions, such as those that make fireflies glow. The researchers believe 40 percent of the light results from the chemical reaction that constantly occurs as our hand skin reacts with oxygen.