The Hazards of Critical Minerals
Assessing Occupational Health Risks in the Rare Earth Element Mining Industry
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Pictured above: The Mountain Pass Rare Earth Mine and Processing Facility in California. Photo by Wikimedia Commons user Tmy350 and licensed under the Creative Commons Attribution-Share Alike 4.0 International license.
The world is undergoing a major transition toward cleaner energy technologies, which demand substantially different resources than those required by the current energy system that is powered by traditional hydrocarbon materials. Clean energy technologies rely on the supply of “critical minerals,” including rare earth elements, or REEs. The current supply of these minerals falls well short of what is needed for the transition to cleaner energy. These minerals come from a relatively small number of producers, creating vulnerabilities in their supply chains. Because of their importance to the clean energy transition, a secure and increasing supply of these minerals is essential to international efforts to tackle climate change. Countries including the United States and Canada have identified such minerals as essential for economic security and are attempting to accelerate their production and supply.

As with any major economic transition that depends on the manufacture of new products, the extraction and processing of raw materials presents a new mix of hazards. The health risks associated with REE mining are currently understudied, and the industry lacks regulations or best-practice guidance to protect workers against these unique health hazards. As a result, risks in REE mining are likely underestimated or even overlooked. The early assessment of risks and the adoption of best practices to limit harmful exposures will help contribute to the safe and responsible extraction of REEs required for the clean energy transition.
WHAT ARE RARE EARTH ELEMENTS? REEs are a group of 17 elements on the periodic table, comprising the lanthanide group as well as yttrium and scandium, as these tend to occur in the same ore deposits and exhibit similar chemical properties as the lanthanides. The term “rare” can be misleading because REEs are abundant in Earth’s crust. But they’re not found in concentrated deposits like other resources, such as gold.
REEs are sometimes referred to as “industrial vitamins.” Although they are used in small quantities, they are vital to the technologies in which they are used. Their unique magnetic and optical properties are required to produce wind turbines and electric vehicle motors. They are also crucial in other technologies like fiber-optic cables, laser-guided weapons, and smartphones.
Governments and industry have recognized that technological innovation is essential to combating climate change. The International Energy Agency (IEA) has predicted that if the climate goals adopted in 2015 by the Paris Agreement are met, the demand for REEs will be seven times higher in 2040 than today. Recycling can help secure the supply of critical minerals such as nickel and cobalt, but almost no global recycling capabilities currently exist for REEs. According to an IEA report published in 2022, the recycling rate for REEs is less than one percent. One country, China, dominates the REE supply chain; IEA notes that China extracted 60 percent and processed 85 percent of the global REE supply in 2019. But since Canada and the U.S. have each entered international partnerships to secure their own supply of REEs, there has been an increase in REE exploration, production, and processing. Of the 128 advanced REE exploration projects globally, 10 are in the U.S. and 16 are in Canada, according to a 2023 article in Ore Geology Reviews. The Mountain Pass REE mine in California supplies around 15 percent of the global supply of REEs, and Canada is currently developing its first REE processing facility in Saskatchewan.
UNIQUE RISKS TO WORKER HEALTH Two occupational hazards are unique to REE mining: the REEs themselves and the ionizing radiation from naturally occurring uranium and thorium, which are often present in REE deposits. Other hazards found on REE mine sites are typical of mining operations in general; these include crystalline silica, noise, whole-body vibration, and thermal stress caused by extremely hot or cold temperatures. Naturally occurring asbestos, lead, arsenic, and other toxic contaminants may also be found in REE ores.
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.
Table 1. Concentrations of Uranium and Thorium Oxide in Rare Earth Element Ore Bodies
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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.
Table 2. Radiation Protection Classification System Based on Annual Dose
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Table 3. Worker Annual Doses at REE Operations from NORMs
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The following steps are recommended when conducting a baseline exposure assessment of ionizing radiation from NORM at an REE mining operation: Collect measurements. Collect personal or area samples for external gamma radiation, radon progeny, thoron progeny, and long-lived radioactive dust. Results from personal samples more accurately reflect exposure risk, but they may not be necessary for the initial assessment if risk is expected to be low. Estimate annual dose. Estimate worker dose from the sum of all exposure pathways measured. Compare the estimated annual dose with the annual dose limits in Health Canada’s NORM guidelines or other similar best-practice guidelines. Implement control measures and devise future sampling plans based on risk. For external gamma radiation, controls may include time, distance, and shielding from the source. For radon, thoron, and long-lived radioactive dust, controls may include ventilation systems and dust management practices. For ingestion of radioactive materials, personal hygiene and housekeeping practices should be implemented. Detection of surface contamination can be used to verify whether these practices are effective. PROACTIVELY ASSESSING RISKS The safe and responsible extraction of REEs is vital to meeting global clean energy targets. OEHS professionals must be proactive in their approach to assessing risks in new and emerging industries like REE mining and processing where there is a lack of research or guidance on exposure assessment. Following a proactive approach will reduce the risk of workers developing long-term health effects, such as lung disease and cancer, that may only be detectable decades after initial exposure. COURTNEY GENDRON, MPH, CIH, is a lead HSE consultant with professional services firm WSP and secretary of the AIHA Mining Working Group. Send feedback to The Synergist.
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Environmental Reviews: “The Potential Environmental Risks Associated with the Development of Rare Earth Element Production in Canada” (2021).
Health Canada: Canadian Guidelines for the Management of Naturally Occurring Radioactive Materials (NORM), (PDF, 2011).
International Atomic Energy Agency: “Radiation Protection and Safety of Radiation Sources: International Basic Safety Standards, General Safety Requirements Part 3” (2014).
International Atomic Energy Agency: Safety Reports Series No. 68: “Radiation Protection and NORM Residue Management in the Production of Rare Earths from Thorium-Containing Minerals” (2011).
International Energy Agency: “World Energy Outlook Special Report: The Role of Critical Minerals in Clean Energy Transitions” (2022).
NIOSH: Health Hazard Evaluation, HETA 88-166-1944, Hoeganaes Magnetic Materials (PDF, 1989).
Ore Geology Reviews: “Global Rare Earth Elements Projects: New Developments and Supply Chains” (2023).
Toxicology and Applied Pharmacology: “The Acute Mammalian Toxicity of Rare Earth Nitrates and Oxides” (1963).