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The Anatomy of a Smudge

Photonics Spectra
Mar 2008
Michael A. Greenwood

Industrial and household contaminants abound, but how these toxic substances get from their source onto human hands is less than certain.

Of course, it usually requires touch, but does a moist hand pick up contaminants better than a dry one? Is smudging a surface more hazardous than directly pressing on it?

To measure the quantity and extent of a substance transferred to a human hand, a team of researchers used fluorescence tagging. This, in turn, allowed them to gauge what parameters are most important in the transfer.


Researchers studied how fluorescent materials, meant to simulate toxins, were transferred to the hands of test subjects. A hand exhibits fluorescence after being pressed on a riboflavin-treated surface (a). The same hand exhibits natural fluorescence before being pressed on the riboflavin-treated surface (b). The researchers subtracted the hand’s natural fluorescence to show the fluorescence generated only from the riboflavin (c). Courtesy of Elaine A. Cohen Hubal.

The team consisted of Elaine A. Cohen Hubal of the National Center for Computational Toxicology and of researchers from the National Exposure Research Laboratory, both of which are branches of the US Environmental Protection Agency and are located in Research Triangle Park, N.C., and from Battelle Memorial Institute in Columbus, Ohio. As they have done in past studies, the investigators used riboflavin as a tracer to gauge how material is transferred. In the most recent experiments, they also used Uvitex OB. The substances were selected because of their fluorescence, nontoxicity and physicochemical properties that are similar to some pesticides.

As reported in the Feb. 1 issue of Environmental Science & Technology, the team applied the tracers to various surfaces and conducted trials in which test subjects touched the treated area in specific ways.

Transfer measurements were made by placing a subject’s hand palm side up inside a box, ∼40 cm below a Canon CCD camera. An image intensifier in-line with the camera lens provided the required increase in sensitivity to the fluorescence. Lamps positioned above the hand provided a wide band of blue (~450 nm) excitation energy. An interference filter blocked lamp energy above 500 nm to avoid reflectance interference. The remaining 600-nm fluorescence image mapped the point-to-point riboflavin transfer.

The team found that tracer and surface types, contact motion and skin condition were all factors in a transfer. They also found that transfers were more pronounced when a laminate (instead of carpet) was touched and when a surface was smudged instead of directly pressed. Additionally, moist skin picked up more tracer than dry skin. Lastly, the researchers identified a correlation between the number of times a surface was touched and the amount of material that was transferred.

Contact: Elaine A. Cohen Hubal, National Center for Computational Toxicology, US Environmental Protection Agency; e-mail:

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