text: what does phototoxic mean? referring to grapefruit essential oil:  safety information: phototoxic.  do not use if the area of application will be exposed to sunlight for 24 hours due to its phototoxicity  obviously i can see that it will have a reaction with the sunlight, but what exactly will happen?  what kind of toxins are produced? a phototoxic substance is a chemical compound which becomes toxic only when exposed to light.  phototoxicity is a phenomenon known in live-cell live cell fluorescence microscopy, where illuminating a fluorescent molecule (the fluorescently active site is called a fluorophore) causes the selective death of the cells expressing it. while not completely understood, it seems to be clear that the main cause for phototoxicity is the formation of oxygen radicals due to non-radiative energy transfer.  typically in fluorescence, photons of a certain wavelength excite electrons of the illuminated fluorophore to higher energy states. when these excited electrons return to a lower energy state, they emit a photon with a lower energy level thus causing the emission of light of a longer wavelength. this principle of fluorescence is also known as stokes shift.  unfortunately for microscopists, in many cases some of the energy is not used for this radiative energy transfer but is transferred to oxygen causing the formation of oxygen radicals. these radicals are highly toxic to living cells, sometimes killing cells in seconds.  phototoxicity in live cells depends strongly on the kind of fluorescent molecule used. the isolation and characterization of fluorescent proteins such as green fluorescent protein (gfp) has provided biologists with fluorochromes which show a much weaker phototoxic effect compared to most smaller chemically synthesized fluorescent molecules such as fitc or rhodamine. still, the energy level of excitation light as well as the duration of illumination must be minimized to ensure long-term survival of living cells during fluorescent imaging.    # may cause a skin reaction such as burning, irritation, or pigment change upon exposure to ultra-violet light (sunlight, tanning lights). www.simplers.com/herbal/definition.htm  # sun exposure causes skin pigmentation www.aworldofaromatherapy.com/aromatherapy-glossary.htm  # increases the sensitivity of the skin to sunlight and ultraviolet radiation to the point of toxicity www.lovingscents.com/glossary.htm