Thallium contamination of food has become a controversial topic on websites and other public media. However, after reviewing numerous research studies in this area, we have not found as much controversy about thallium and its relationship to food from a science perspective. Instead, what we have found is a substantial amount of agreement about thallium and the way it affects our food supply.
As a quick summary: we do not believe that thallium poses a special risk to persons who enjoy cruciferous vegetables, except in situations where unusual environmental contamination is known to be present, for example, in areas surrounding certain types of industrial facilities like smelters, cement factories, coal-fired power plants, sulfuric acid plants, coal ash plants, or natural gas fracking facilities. The use of wastewater from these types of facilities for crop irrigation, or the use of solid waste from these facilities for crop fertilizer can also raise food levels of thallium to unacceptable amounts. However, these problematic environmental circumstances appear to be the exception rather than the rule in the food studies that we have reviewed.
The following sections of this article will give you a better understanding of thallium and cruciferous vegetables, as well as our reasons for encouraging you to continue enjoying these unusually nutrient-rich foods.
Thallium is a naturally occurring element found throughout the earth's crust, in soil, and in seawater. Soils that are more acidic typically contain greater levels of thallium, as do soils rich in organic matter and other elements to which thallium readily binds (like sulfur, carbon, chlorine, and carbon). All studies that we have seen show natural background levels of thallium in soil.
Naturally low levels of thallium in the environment correspond to very small amounts of thallium that are naturally present in our blood and urine. Food and water are our two most common sources of thallium exposure, but thallium can also move into our body through inhalation and skin contact. Inhalation and skin contact studies usually involve exposure in a toxic workplace environment. Tobacco smoking can also be a source of inhalation exposure to thallium, and research studies show smokers to have consistently higher levels of thallium in their bodies than non-smokers.
Plants can naturally uptake thallium from the soil through their root systems. Although naturally occurring amounts of thallium in soil can vary widely, average amounts of thallium in U.S. soils based on estimates by the U.S. Environmental Protection Agency (EPA) fall into the range of 100 ppb—1 ppm, with "ppb" standing for "parts per billion" and "ppm" standing for "parts per million." These levels translate into actual amounts of thallium in the range of 100 micrograms to one milligram (1,000 micrograms) in every 2.2 pounds of soil. As plants grow, they can update this naturally occurring soil thallium into their cells. Most studies show greater concentrations of thallium in the leaves of plants than in their stems or roots. In addition, older and more mature leaves appear to contain more thallium than fresher and less mature leaves.
Plants are not all the same in their uptake of thallium. "Bioaccumulation" and "bioconcentration" are the two terms most commonly used to describe the uptake of substances by plants and animals. Researchers rank foods on their uptake of elements like thallium by calculating rates of uptake that are referred to as "bioaccumulation factors" (BAF) or "bioconcentration factors" (BCF). These terms are simply two different ways of referring to the same process. Cruciferous vegetables (including cabbage, kholrabi, and kale) have been consistently shown to have greater potential for thallium update than most non-cruciferous vegetables. However, we have also seen studies in which potatoes, sweet potatoes, onions, soybeans, and eggplant have disproportionately taken up high amounts of thallium. For example, in one study involving wastewater irrigation on farmland adjacent to a sulfuric acid manufacturing facility in China, the BCF values for thallium were: green cabbage 12.0; sweet potato root 11.1; soybeans 7.5; and eggplant 4.5. By comparison the BCF value for lettuce in this same study was 1.3. As described earlier, many different factors affect thallium update from the soil, including the soil pH and organic composition.
As you can see from the numbers above, kale is by no means the only vegetable that can accumulate higher amounts of thallium. In fact, in another study that we reviewed, when three cruciferous vegetables were compared for their thallium concentrations—kale, rape, and kholrabi—kale actually came out lowest in thallium uptake, with roughly half as much thallium as rape, and about one fourth as much thallium as kholrabi.
Worldwide, thallium intake by humans has been estimated at about 1-5 micrograms per day. In the United Kingdom, a 2008 study showed 2 micrograms of average intake, with upper range amounts of approximately 4 micrograms.
The oral chronic reference dose (RfD) for some of the more toxic forms of thallium (including thallium sulfate) as set by the U.S. Environmental Protection Agency (EPA) is 0.00007 milligrams per kilogram of body weight per day. This guideline translates into 5.6 micrograms of thallium intake per day for persons weighing 70 kilograms (154 pounds). This amount is considered to pose no appreciable health risk over a lifelong course of daily intake.
The EPA has also set a maximum contaminant level (MCL) for thallium in drinking water of 2 micrograms per liter and considers this level to be protective of human health.
We have seen one study from northeastern Spain in which average intake of thallium was 9.5 micrograms per day and above the U.S. EPA level. But we have also seen studies from countries in Europe where average intake fell substantially below the U.S. EPA level, including some studies where average intake of thallium fell below 1 microgram per day. As a general rule, these intake estimates match up fairly well with the naturally occurring amounts of thallium found in soil and water worldwide.
However, of real concern in all the studies that we have reviewed is the excessive amount of thallium that can become deposited in soil and water from hazardous waste. We have seen studies from an industrially contaminated site in southwestern Guizhou Province in China; a sulfuric acid manufacturing plant in western Guangdong Province in China; and a zinc smelter in Poland which consistently show excessively high amounts of thallium in soil, water, and foods grown in the contaminated location (or fertilized with waste products from the location, or watered with wastewater from the location). In the majority of these cases, levels of thallium in the foods were two to ten times higher than naturally occurring amounts. In some cases, the food amounts rose substantially higher. All of the elevated amounts of food thallium in these environmental contamination studies were able to raise daily intake levels above the recommended EPA limit of about 5-6 micrograms per day (for a 154-pound person).
Of potentially large concern in the U.S. is the impact of natural gas fracking on contamination of groundwater with a variety of unwanted substances, including thallium. For example, some irrigation aquifers in California have been found to be contaminated with thallium from fracking water. The use of these contaminated water sources on vegetable crops could raise thallium levels in the harvested vegetables to unacceptable levels.
At present, we know of no way for you to definitely determine whether vegetables that you eat are grown in non-contaminated soils or produced without the use of contaminated fertilizers or irrigation water. It is certainly possible for you to purchase kale, other cruciferous vegetables, or vegetables in general that were grown in soil that has naturally higher levels of thallium than the U.S. soil average. However, we have not seen research to suggest that this possibility is a widespread concern for any type of vegetable grown in the U.S., including cruciferous vegetables. For this reason, we feel confident in recommending full enjoyment of all vegetables—including all cruciferous vegetables like kale—when produced under natural growing conditions.
Of greater concern to us is the issue of environmental contamination. While we recommend full enjoyment of vegetables grown under natural conditions, we also recommend avoidance of vegetables grown under highly contaminated conditions. So is there some foolproof way for you to determine whether your vegetables were grown in soil that was free from excessive thallium contamination, or without the help of thallium-contaminated fertilizers or thallium-polluted irrigation water?
While the ultimate answer to this question is "no," we believe that you can still lower your risk of thallium-contaminated vegetables to an acceptable level by purchasing certified organic vegetables or their equivalent. At least in one respect, you are guaranteed to lower your risk since sewage sludge cannot be used in the production of certified organic foods, and this type of fertilizer typically contains unwanted amounts of thallium. In addition, organic certifiers are required to follow the EPA guidelines for water contamination as set forth in the federal Clean Water Act (CWA), and these guidelines include a maximum contaminant level for thallium. In some respects, however, the organic standards for irrigation water are far less reassuring than the sewage sludge standards since they do not directly prohibit use of any particular irrigation water type. For example, we know of no federal organic regulation which would prohibit the use of thallium-contaminated fracking water in production of certified organic vegetables. However, an organic certifying agency that was concerned about irrigation water quality might still investigate and report its findings to a local and/or state government and ask that steps be taken to help remedy the problem (for example, temporary closure of a contaminated irrigation aquifer by the state government).
In short, we consider purchase of certified organic vegetables to be a worthwhile step in lowering your risk. And if you live in an area where there is ongoing environmental contamination from the types of industrial practices described in this article, we recommend a call to your state organic association for suggestions about additional vegetable options in your area.