Professor Clare Mahon of Durham University recently led a study investigating an odd but common household problem: why white clothing stained with sunscreen sometimes turns red after being washed with bleach. What began as a simple curiosity became a detailed chemical investigation that uncovered a reaction far more complex than expected.
Smith, L. G., di Leva, D., Shaw, L. A., Hughes, E., Kenwright, A. M., Beeby, A., Wilson, M. R., & Mahon, C. S. (2025). An unexpected chlorination of an organic sunscreen. Chemical Communications, 61(51), 9314–9317. https://doi.org/10.1039/D5CC01593F
Mahon and her team examined how commercial sunscreen formulations respond when exposed to bleach. Out of the eleven products they tested, seven produced a vivid red color. Each of those seven contained the same UV-filtering ingredient, diethylaminohydroxybenzoyl hexyl benzoate, a molecule widely used in sunscreens for its ability to absorb UVA light. Previous research had shown that this ingredient can react with chlorine, but earlier explanations did not account for the intense red color it produces on fabric.
To understand what was happening at the molecular level, the researchers conducted nuclear magnetic resonance experiments and collaborated with specialists in molecular modeling. Their analysis revealed an unusual transformation known as ipso-dichlorination. In this reaction, two chlorine atoms attach to the same position on one of the molecule’s ring structures. This altered structure absorbs much of the visible spectrum except for longer red wavelengths, which is why the stained clothing appears bright red.
To test their hypothesis further, the researchers developed a modified version of the sunscreen ingredient that resists the ipso-dichlorination process. When this version was exposed to bleach, it showed almost no change in its light-absorbing behavior. This result suggests that reformulating sunscreens to prevent bleach-related staining may be possible, although additional studies would be needed before it could be adopted commercially.
The work is part of the broader ANTENNA project, a collaboration involving Durham University, Procter and Gamble, and Imperial College London. The project focuses on the chemistry behind detergents and cleaning processes, and the findings from this study may help guide both sunscreen and laundry product formulation in the future.
Beyond stained clothing, the findings have wider implications. Sunscreen ingredients such as the one investigated here can undergo similar chemical changes in swimming pool water or other chlorinated environments, sometimes forming by-products that behave differently than the original molecule. Understanding these reactions helps researchers assess both consumer safety and environmental impact.
This research demonstrates how everyday experiences, like a stubborn stain, can reveal complex chemical behavior worth investigating. The study highlights the importance of examining how common products interact and how small molecular changes can influence real-world outcomes.
Adrian graduated with a Masters Degree (1st Class Honours) in Chemical Engineering from Chester University along with Harris. His master’s research aimed to develop a standardadised clean water oxygenation transfer procedure to test bubble diffusers that are currently used in the wastewater industry commercial market. He has also undergone placments in both US and China primarely focused within the R&D department and is an associate member of the Institute of Chemical Engineers (IChemE).
