Exposure to Rare Earth Elements Workers in this industry breathe in dust that contains REEs. Levels of REEs in dust become higher when they are concentrated during processing. Exposure may also occur through ingestion by hand-to-mouth contamination if a worker does not wash their hands before eating, drinking, or smoking, and it may be possible for REEs to be absorbed through the skin. Information on how REE exposure may affect the human body is limited. Most of what is known is from laboratory tests on animals, case reports of disease, and bioaccumulation studies of people living near REE mining regions in China (see the Environmental Reviews article from 2021 in the list of resources below). A 1963 study of animal mortality and rare earth nitrates and oxides published in Toxicology and Applied Pharmacology shows that individual REEs have similar levels of toxicity. The most well-known health effect associated with inhalation exposure to REE particulate is pulmonary fibrosis, or scarring of the lungs. Studies such as the one published in 2021 in Environmental Reviews have found evidence that environmental exposures to REEs—for example, those among people living near REE operations—may negatively affect other parts of the body, including the blood, brain, and liver. However, the lack of epidemiological studies of occupational exposure to REEs means that there is a knowledge gap with respect to their possible long-term health effects. Given the lack of research on the toxic effects of REE exposure, it may come as no surprise that only one of the 17 REEs, yttrium, has an occupational exposure limit. OSHA’s permissible exposure limit for yttrium is 1 mg/m3 as an eight-hour time-weighted average (TWA). This limit is based on the Threshold Limit Value (TLV) set by ACGIH that was last updated in 2001. At that time, there was insufficient data to determine whether yttrium was carcinogenic, a sensitizing agent, or whether absorption through the skin was a significant route of exposure. With only one OEL for REEs, how should OEHS professionals assess risk associated with breathing in REEs? Defaulting to OSHA’s nuisance dust TWA limits of 10 mg/m3 for inhalable dust and 3 mg/m3 for respirable dust is not appropriate and will underestimate the health risk. This is because nuisance dust limits are applicable only to non-soluble and relatively nontoxic particulate, characteristics that do not apply to REEs. In 1989, NIOSH addressed this problem when the agency conducted a health hazard evaluation at a manufacturing plant that created magnetic products from two REEs, neodymium and dysprosium. To evaluate risk to health from inhalation, NIOSH used the eight-hour TWA OEL for yttrium of 1 mg/m3 for each of the REEs. Assuming that all REEs are of similar toxicity (as seen in animal studies) and that they all cause disease in the inhalable region of the lung (referred to as the “toxic site of action”), this is an appropriate approach given the lack of established limits. In mining, a worker will not be exposed to any one REE in isolation; they will be exposed to all REEs present in the ore at the same time. It can also be assumed that the health effects of REEs are additive rather than antagonistic or synergistic. In this case, the total amount of REEs inhaled by a worker can be compared to the TWA OEL of 1 mg/m3. The following steps are recommended when conducting a baseline exposure assessment of REE particulate at an REE mining operation: Collect full-shift personal samples for REE particulate. Collect the inhalable fraction of particulate. If possible, also collect the respirable fraction. In the future, it may be discovered that some of the REEs have a toxic site of action in the respirable part of the lungs. Analyze the air samples for the individual REEs. REEs can be quantified by a laboratory. OEHS professionals who don’t have the means or the budget for laboratory analysis can estimate the REE concentration based on previous ore assays. It’s important to note that this method may underestimate or overestimate the exposure based on the stage of the mining process in which the worker was involved. Compare the inhalable fraction of total REEs to the TWA OEL of 1 mg/m3. If available, another best-practice TWA value developed by a company can be used. Be sure to verify that the assumptions made when developing the best-practice TWA value align with current research. Implement control measures and devise future sampling plans based on risk. Dust management practices such as the use of dust collection systems in the milling process—especially if indoors—will help decrease exposure. Vehicle cab filtration and seals and routine maintenance are examples of other ways to manage dust. Radiation Risks Ionizing radiation from naturally occurring radioactive materials (NORMs), such as uranium (U-238) and thorium (Th-232), can cause cancer and adverse reproductive effects. NORMs occur in soils and rocks around the world and may lead to radon exposures in homes. Uranium and thorium commonly occur in the ore bodies that are mined for REEs and may become further concentrated during processing. Chronic low-level exposure to ionizing radiation at REE operations may require additional controls to verify that workers are not at increased risk of disease. A worker may be exposed to ionizing radiation from naturally occurring uranium and thorium though four main pathways: 1. direct external gamma radiation 2. inhalation of radon (Rn-222) and thoron (Rn-220) and their radioactive progeny 3. inhalation of long-lived half-life radioactive dust 4. ingestion of radioactive materials OEHS professionals add the first three pathways together to determine overall exposure or dose. Workers may inadvertently ingest dust from hand-to-mouth contamination or simply swallow larger particles of dust that are inhaled and cleared into the mouth. Radioactive rock is not particularly digestible, and the crushed rock will mostly pass right through the body. Therefore, this kind of internal radiation exposure is not as important as radiation exposure from inhalation. But personal hygiene is always a good radiation protection practice to prevent ingestion through hand-to-mouth contamination. The most current, definitive reference on which Canada and most other countries base their radiation dose limits is the 2014 International Atomic Energy Agency (IAEA) publication “Radiation Protection and Safety of Radiation Sources: International Basic Safety Standards, General Safety Requirements Part 3.” These limits include an annual dose of 20 millisieverts (mSv) averaged over five consecutive years and an annual dose of 50 mSv in any given year. The IAEA, along with most federal radiation protection legislation, references the “as low as reasonably achievable” (ALARA) principle, which requires employers to reduce exposure levels as much as reasonably possible, even if they are already meeting the legislated dose limits. The ALARA principle is applied to hazards that may cause cancer. In theory, any exposure to a carcinogen, no matter how small, can potentially be harmful to exposed individuals. The ALARA principle considers social and economic factors in determining what is “reasonable.” North America has a long history of mining and processing uranium, so what makes exposure to ionizing radiation from NORMs a hazard unique to REE mining and processing? First, concentrations of thorium are commonly higher in REE ores compared to uranium ore. Table 1 provides the average concentrations of uranium and thorium oxides at three major REE mining operations in addition to the average concentration in 19 Canadian REE deposits. Uranium mining deposits in North America do not have high levels of thorium, and therefore exposures from the decay of Th-232 may be left out of legislation and exposure assessments. For example, separate samples must be collected to evaluate exposure from the radioactive gas that is created from the decay of Th-232 (known as thoron) and its radioactive progeny. This could underestimate risk of exposure from ionizing radiation.

The second reason that exposures from NORMs are unique to REE mining and processing is that, in North America, they do not fall under the jurisdiction of federal regulatory bodies that regulate industries that are part of the nuclear fuel cycle. In the United States, this is the Nuclear Regulatory Commission, and Canada has the Canadian Nuclear Safety Commission. REE mining and processing facilities would fall under the jurisdiction of these federal agencies if they were to sell concentrated uranium or thorium as a byproduct, thereby contributing to the nuclear fuel cycle. But this is not yet happening in North America. Therefore, REE operations are regulated under different regimes that are normally less robust and may not provide REE operations with the detail necessary to communicate, assess, and control the health risks.
In Canada, an acceptable assessment and control strategy for NORMs is described in the Canadian Guidelines for the Management of Naturally Occurring Radioactive Materials (NORM), published by Health Canada in 2011. The guidelines provide a more detailed set of graduated exposure levels at which increasingly stringent radiation protection measures are required (as shown in Table 2).
Examples of worker annual doses from NORM at REE operations can be found in Table 3, which compiles data from three mines. The table includes the most conservative estimates. It’s important to note that these annual doses may not be current and that they can vary greatly depending on the area of the mine, the job task, and the control measures in place. Different exposure pathways also provide significant contributions to overall dose. Table 3 shows the relative contribution of the different exposure pathways to overall dose and estimates the annual doses that could be expected at these REE operations. Based on this information, the workers would require a dose management program according to the Canadian Guidelines for the Management of Naturally Occurring Radioactive Materials.

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