Even though this question has actually been floating around the web for over 10 years, no one has really given a definitive answer to the topic. On Yahoo Answers alone there were over 56 threads within the last 5 years asking a similar form of the question. The American Optometric Association issued this warning, "Food coloring, while safe for consumption, is not necessarily sterile. Using it on contact lenses puts the individual at risk for an eye infection. And remember that certain dyes and tints, depending on what they are derived from, may cause a reaction like red or irritated eyes in some people." And yet, many view this warning as vague and more hypothetical, reasoning that since they don't mention specific instances of bad side effects, there probably aren't any. None of these specific risks have been tested obviously, so it is hard to speculate what the food coloring could do to your eye, but nowhere online does it mention anything about pH or possible chemical burns from the dye. After watching many youtube videos of people attempting the feat, nearly all of them had trouble keeping their eyes open or not crying/shouting/screaming about how bad it burns or stings (much to the amusement of those filming the videos.) One person who blogged about the experience even admitted that hours later after rinsing his eyes they still throbbed. So what's causing the burning? A few first year optometry students decided to take a closer look.
It turns out it is extremely hard to find any values for what the actual pH (how acidic or basic something is) of different food coloring dyes, so we measured them ourselves. We tested two different brands of food coloring, a brand name and a generic so to speak; we used McCormick and Market Pantry (Target brand). In order to accurately measure the pH, we couldn't use color-changing strips since the color of the dye would likely interfere with the color the strip would change, so we used a digital waterproof pH meter. Below are the pH measurements we collected:
For the Market Pantry brand food coloring dyes, the pH measurements were all very close and rather acidic, averaging a value of approximately 3.14. The McCormick brand, however, were slightly less acidic, where two of the colors (red and blue) averaged a pH of 4.73 and the other two colors (yellow and green) were nearer neutral with an average of 6.85 pH. The differences in pH values obtained in the two different brands could be contributed to the different preservative agents used between the two and the amount of preservative used. Just to give you an idea of how acidic the Market Pantry brand dyes are, a pH value of 3 is about how acidic vinegar is. Our results of different pH's for different colors and brands actually make sense when thinking about how it seemed like the color of dye made a difference in how much it stung when dropped in the eye. When reading through many responses to the subject of whether it is safe to put food dye in your eye, a lot of people reasoned that it was probably safe since it is edible and doesn't irritate the stomach. There are 2 problems with this train of thought: 1) The stomach is highly acidic itself and has a pH of 2, so it is more able to tolerate acidic substances than your eye and 2) when you consume food coloring, typically you are not drinking it straight from the bottle, you put very few drops in whatever food product you are making, and the pH of those few drops are very unlikely to have much of an effect on the overall pH of that particular foodstuff (which is very different from placing many drops of the dye directly and undiluted into your eye.) To summarize at this point, there are some food colorings that are acidic enough to cause a chemical burn if you put them in your eye:
(It's probably not as cool as it was in Fight Club when it's happening in your eye.)
