Shrek the friendly ogre delighted audiences in the 2010 movie hit “Shrek Forever After.” But for fast food giant McDonald’s, Shrek turned out to be a nightmare. As a cross promotional feature, the company introduced a set of glasses decorated with images of Shrek and other characters from the film. After millions of the glasses had been sold, a problem cropped up that led to a large scale recall. The yellow pigment used on the cups turned out to be cadmium sulphide, a substance toxic even in small amounts. The concern was that the pigment might rub off on children’s hands and end up being ingested if they then put their hands into their mouth.
Cadmium was discovered in 1817 by Professor Friedrich Strohmeyer in Germany while looking into a problem encountered by apothecaries who were making calamine lotion for skin care. The process involved heating “calamine,” a natural ore of zinc carbonate, to produce zinc oxide, which is the active ingredient in calamine lotion. Sometimes the lotion would end up with a yellow discolouration which Strohmeyer determined was due to a mineral contaminant that he eventually identified as a compound of cadmium.
It was the colour of cadmium compounds that led to their first commercial use. Artists loved the bright yellow of cadmium sulphide and the reds and oranges resulting from a mixture of cadmium sulphide and cadmium selenide. Vincent van Gogh used cadmium sulphide to impart the yellow colour to his flowers in his famous “Flowers in a Blue Vase.” Unfortunately, with time, cadmium sulphide oxidizes to cadmium sulphate, which is white, resulting in the original colour of the painting being slowly altered. Claude Monet's famous yellow hues were also achieved with cadmium pigments.
Cadmium paints are still used today, although they are being phased out. Indeed, Sweden has submitted a report to the European Chemical Agency claiming that artists rinsing their brushes in the sink are responsible for spreading cadmium over agricultural land via sewage sludge.
Cadmium is a cumulative toxin and the World Health Organization has suggested 70 micrograms as the maximum daily safe intake. Ingesting some cadmium is unavoidable because it shows up in crops. How does it get there? Sewage sludge and phosphate rock, both used as fertilizer, can harbour cadmium. As a result, a hamburger can contain about 30 micrograms of cadmium that can be traced to the grass or hay the cow ate, and ultimately to the soil in which the feed was grown. Coal also contains cadmium compounds that can end up in the atmosphere from where they find their way into soil via rain. Other cadmium compounds may also be released from the nickel-cadmium battery industry, although modern pollution control methods minimize such losses. Cadmium can be also be found in significant amounts as a contaminant in zinc ores and some is released into the environment when the ore is mined as well as when it is smelted into zinc.
Nobody actually carried out a study to determine how much cadmium pigment can rub off onto little hands when gripping a Shrek glass, but it could well be less than what is found in the hamburger those hands are clutching. Still, eliminating any avoidable source of cadmium is desirable, especially since there is suspicion that cadmium compounds may be carcinogenic. Cadmium can also build up in joints and the spine causing a disease that the Japanese have named “Itai-Itai,” which translates as “ouch-ouch,” due to the painful sounds made by victims as cadmium accumulates.
A classic case of environmental cadmium toxicity can be traced back to the early 1900s, although its cause was not identified until the 1960s. It was obvious that something was going on in the vicinity of the Jinzu River and its tributaries in China. People were getting sick, screaming in pain and dying prematurely. Suspicion fell on the river and the mining companies that for years and years had been disgorging their wastes into the water. The mountains upstream were rich in minerals that contained silver, lead, copper and zinc, and mines had been operating there for centuries. As demand for these metals increased in the twentieth century, more and more mining wastes found their way into the river, including increased amounts of cadmium ores.
River water was used for irrigation of rice fields, and since rice absorbs cadmium effectively, the metal accumulated in the food supply and consequently in the bodies of the population. The result was ouch-ouch disease. Although cadmium was only identified as the cause around 1965, by the late 1940s it had become obvious that the disease was linked to the water supply and mining companies began to store their wastes instead of releasing them into the river. This prevented more people from contracting cadmium poisoning, but nobody really knows how many victims the mining operations had since they began to pollute the Jinzu River back in the sixteenth century.
In 1966 in England a construction worker died and several others were sickened as a result of inhaling cadmium fumes. The men were using a welding torch to remove bolts as they were dismantling a construction tower used in the building of a bridge. It is common practice to electroplate steel bolts with cadmium, particularly those exposed to water. This is especially useful when there is contact with sea water since cadmium reacts with salt to form an impervious layer of cadmium chloride. In this case the men inhaled the cadmium vapourized by the heat of the welding torch and suffered an acute reaction.
Shrek glasses are not the only items aimed at children that have caused a concern about cadmium. With lead being non-grata, cadmium has been turning up in jewelry aimed at young girls, mostly originating in China. If pieces are accidentally swallowed, or if the jewelry comes into frequent contact with the mouth, enough cadmium may enter the circulation to cause harm. Jewelry made with cadmium should to go the way of the Shrek glasses.