Fearsome Yellow

yellowNext time you think of welcoming someone home by tying a yellow ribbon around an old oak tree, you might want to think again. According to a widely circulating report the yellow dye could leave a toxic residue on your hands. What are we talking about? PCBs. Actually one specific PCB, namely PCB-11. Polychlorinated biphenyls have become an environmental pariah, accused of being endocrine disruptors and carcinogens. Quite a comedown for chemicals that were once revered as ideal heat transfer fluids and insulating materials in electrical equipment. They were phased out in the 70s when researchers discovered that these compounds persisted in the environment and were toxic to animals. Aside from PCBs’ ability to cause a type of acne known as chloracne, no significant adverse effects have been noted in humans. In two classic cases, one in Japan and one in Taiwan, a number of people became ill after consuming rice bran oil that had become contaminated with PCBs, but it turned out that the problem was toxins that had formed when the PCBs were heated to a high temperature. Polychlorinated biphenyls are no longer produced but some can form as a byproduct of certain chemical reactions. This is where the yellow dye comes into the picture.

In many cases, although certainly not always, yellow pigments are made by mixing blue pigments with green ones. A classic blue pigment is “phthalocyanin blue” which was discovered accidentally by a chemist working at a plant that was producing phthalimide, a chemical used to make certain plastics. He was troubled by blue contamination of the product that was eventually traced to a by-product formed when the phthalimide reacted with trace amounts of iron leaching out from the metal reactor. Research then showed that substituting copper for iron resulted in a more stable pigment. And if this blue pigment, also known as Monastral blue, were reacted with chlorine, it was converted to a green color, appropriately named “phthalocyanin green.”

This is where the issue of PCBs arises. When the blue pigment is reacted with chlorine, PCB-11 forms as a contaminant and is carried through to the yellow dye that is made by mixing the blue and green phthalocyanins. This dye is used in many fabrics, paper products and paints that we come into contact with. Hence the warnings. Has anyone ever shown that people exposed to yellow clothing have higher levels of PCBs in their blood? No. And given the trace amounts of PCBs present in the yellow dye, nobody is ever likely to show anything like that. But just mentioning PCBs and yellow clothing in the same sentence is enough to make some people shed these garments. Coincidentally, yellow is the color of fear. In this case, irrational fear. Headlines such as “Your favourite yellow sweatshirt could be making you sick” amount to needless fear mongering. Makes me sick.

Joe Schwarcz

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