The risks posed by a substance are determined by both its hazardous properties (potential to cause adverse human health or ecological effects) and the amount or extent of exposure to people or the environment.
When needed, the Government implements risk management measures under CEPA and other federal acts to help prevent or reduce potential harm.
The Government concluded that thallium and its compounds may pose a risk to the environment, but not to human health, at levels of exposure considered in the assessment.
These substances may pose a risk to aquatic organisms through releases to water from a small number of facilities in the metal mining and base metals smelting and refining sectors.
Risk management actions are proposed to reduce human-caused releases of thallium to water from the metal mining and base metals smelting and refining sectors.
The scope of the assessment was on the thallium moiety. For the assessment, moiety means a part of a molecule and is the chemical entity expected to have toxicological significance. The assessment considered all thallium-containing substances that may release thallium, as well as thallium in its elemental form, and thallium released to the environment in dissolved, solid or particulate forms. This included 3 thallium-containing substances listed on the Domestic Substances List, 1 on the Revised In Commerce List (R-ICL), and 1 substance that was removed from the R-ICL in 2022. They were assessed under the third phase of the Chemicals Management Plan (CMP).
Thallium is found in the earth's crust and is typically associated with sulfide ores of various metals including zinc, copper, iron, and lead. It is also present in coal. Thallium is present in many natural minerals as well as in meteorites, volcanic rocks, plants, and trace amounts in most living organisms.
Thallium exists in 2 oxidation states in the aquatic environment, called monovalent thallous Tl(I) and trivalent thallic Tl(III), respectively.
The highly soluble and weakly reactive Tl(I) ion is the more bioavailable thallium species in both aquatic and terrestrial environments and is therefore the focus of the ecological assessment.
According to information gathered by the Government, thallium substances are mainly used internationally in fiber (optical) glasses, semiconductor material, infrared radiation detection and transmission equipment, photoelectric cells, gamma radiation detection devices (scintillometers), mercury-thallium alloys, crystalline filters, high-density liquid for sink-float separation of minerals, and in the production of other chemicals.
Other uses include use in mercury arc lamps, alloys with other metals, jewellery, fireworks, and pigments and dyes. Additional uses of thallium in Canada are in products available to consumers, for example, as a radiopharmaceutical addition to human drug products and in natural health products licensed as homeopathic medicines. Thallium may also be used as a component in the manufacture of food packaging materials.
Human and ecological exposures
As a naturally occurring substance, people in Canada are exposed to thallium and its compounds from environmental sources (for example, air and drinking water) and primarily from food.
Thallium compounds may be released into the environment naturally from weathering or breakdown of soils or rocks, and through forest fires.
Sources of thallium entering the environment due to human activity are primarily associated with incidental production and releases of residues or by-products from various industrial activities. This includes smelting and refining processes, metal mining, wastewater treatment systems, and fly ashes of coal-fired electrical power generation.
Thallium releases to the aquatic environment from metal mining, base metals smelting and refining, coal-fired power generating facilities, and wastewater treatment systems in Canada were analyzed. Data from the National Pollutant Release Inventory (NPRI), the Environmental Effects Monitoring (EEM) provisions under the Metal and Diamond Mining Effluent Regulations (MDMER), and industry environmental assessment reports were used.
For the assessment of risk to human health, thallium and its compounds were evaluated using a scientific approach based on human biomonitoring data, entitled Biomonitoring-based Approach 2. In the approach, human biomonitoring data (as a measure of exposure) are compared against biomonitoring guidance values (health-based exposure guidance values) that are protective of human health.
Human biomonitoring is the measurement of substances in blood, urine or human milk through health studies or surveys, such as the Canadian Health Measures Survey (CHMS). The information on measured levels in humans is important to estimating exposure to Canadians.
Key health and ecological effects (hazard)
In humans, critical health effects from chronic (long-term) exposure to elevated levels of thallium include headaches, nausea, vomiting, fatigue, and hair loss. Neurological symptoms occur with chronic exposure to elevated levels of thallium.
Thallium causes mortality as well as growth and reproductive effects to both aquatic and terrestrial organisms, at very low concentrations.
Consideration of subpopulations who may have greater susceptibility or greater exposure
There are groups of individuals within the Canadian population who, due to greater susceptibility or greater exposure, may be more vulnerable to experiencing adverse health effects from exposure to substances.
Certain subpopulations are routinely considered throughout the assessment process, such as infants, children, and people of reproductive age. For instance, age-specific exposures are routinely estimated and developmental toxicity studies are evaluated for potential adverse health effects.
For thallium and its compounds, Canadian human biomonitoring data in children, pregnant women, and specific Indigenous communities were used to inform consideration of these populations and take them into account in the risk assessment outcomes.
Risk assessment outcomes
Assessments focus on information critical to determining whether substances are harmful to human health or the environment under CEPA. This is done by considering scientific information, including information, if available, on subpopulations who may have greater susceptibility or greater exposure, vulnerable environments and cumulative effects and by incorporating a weight of evidence approach and precaution. The potential for cumulative effects was considered in the assessment by examining cumulative exposures to the moiety of Thallium.
Based upon a comparison of levels of thallium measured in human urine, and levels associated with health effects, it was found that thallium and its compounds are considered to be of low concern to human health.
Based upon the information presented in the assessment, it was determined that there is risk of harm to the environment from thallium and its compounds. The analysis for the metal mining and base metals smelting sectors indicated that thallium releases from most facilities are limited, but there is a potential for harm to the aquatic environment as a result of thallium release from a small number of facilities in these sectors.
Assessment conclusions
The Government concluded that thallium and its compounds are not harmful to human health at levels of exposure considered in the assessment.
However, the Government concluded that thallium and its compounds may be entering the environment at levels that are harmful to the environment.
It was also determined that thallium and its compounds meet the persistence criteria but not the bioaccumulation criteria as set out in the Persistence and Bioaccumulation Regulations of CEPA.
Preventive actions and risk reduction
The Government intends to propose the addition of thallium and its compounds to Part 2 of Schedule 1 to CEPA. Adding a substance to Schedule 1 does not restrict its use, manufacture or import. Rather, it enables the Government to take enforceable risk management actions under CEPA, using a 2-track approach to manage risks.
Toxic substances that pose the highest risk (that is, meet certain criteria) are added to Part 1 of Schedule 1. These are prioritized for total, partial or conditional prohibition.
Other toxic substances are added to Part 2 of Schedule 1 and are prioritized for pollution prevention.
Regulations specifying criteria for the classification of substances that pose the highest risk or that are carcinogenic, mutagenic or toxic to reproduction may be developed. When criteria are available, some substances considered for addition to Part 2 of Schedule 1 may instead be considered for addition to Part 1 of Schedule 1.
Publication of the risk management approach aims to inform stakeholders of proposed risk management actions and continue discussion about their development. The Government will consider the following actions to reduce human-caused releases of thallium to water from the following sectors or activities, to address ecological concerns:
Metal mining: by reviewing information received from regulated mines in response to environmental effects monitoring requirements under the Metal and Diamond Mining Effluent Regulations (MDMER), to determine if additional regulatory or non-regulatory risk management is appropriate. For mining facilities not covered by the MDMER, by working with industry and/or provinces to gather additional information from facilities not covered by the MDMER on thallium concentrations in surface waters.
Base metals smelting and refining: For facilities that combine their effluent with metal mines: in the same ways as described above for metal mining. For facilities that do not combine their effluent with metal mines: by working with industry to gather additional data on thallium concentrations in wastewater effluent, effluent receiving waters, and surface waters.
Information is being sought by the Government to inform risk management decision-making. Details can be found in the risk management approach, including where to send information during the public comment period, ending May 29, 2024.
Also, under the CMP, the Government is conducting assessments on a variety of metals that may identify similar or additional sectors as sources of risk. The Government is considering the risk management actions for thallium as part of a more comprehensive strategy to manage metals concluded as harmful to the environment under the third phase of the CMP. This strategy is focused on effluents rather than on single metals and will reduce the administrative burden on implicated sectors.
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Related resources
Thallium and its compounds may be found in products available to consumers. People in Canada should follow any safety warnings and directions related to the product and dispose of products responsibly.
Assessments conducted under CEPA focus on risks of exposure of the general population. Hazards related to chemicals used in the workplace are defined within the Workplace Hazardous Materials Information System (WHMIS). If a substance is harmful to the general population, it could also be of concern for individuals in situations where a higher volume of the substance is used or where the substance is used for a longer duration (for example, the workplace). The Government of Canada recognizes that it is the responsibility of the federal, provincial and territorial occupational health and safety organizations to coordinate legislation for the safe use of chemicals in the workplace. We are working to support this role by integrating the information, tools, and technical expertise of the CMP and Health Canada's Workplace Hazardous Products Program.