Under the Canadian Environmental Protection Act, 1999 (CEPA 1999) the risk posed by a substance is determined by considering both its hazardous properties (its potential to cause adverse human health or ecological effects) and the amount of exposure there is to people and the environment. A substance may have hazardous properties; however, the risk to human health or to the environment may be low depending upon the level of exposure.
As a result of the draft screening assessment, the Government is proposing that thallium and its compounds may pose a risk to the environment, but not to human health at current levels of exposure.
About these substances
The screening assessment summarized here focuses on the thallium moiety. The assessment considers all thallium-containing substances that may release thallium as well as thallium in its elemental form, and thallium released in the environment in dissolved, solid or particulate forms. This includes 5 thallium-containing substances listed on the Domestic Substances List or the Revised In Commerce List, assessed under the Chemicals Management Plan (CMP).
The human health risks of thallium and its compounds were characterized and published in the Biomonitoring-based approach 2 science approach document, which compared human biomonitoring data (exposure) against biomonitoring guidance values (health effects) protective of human health.
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 environment and is therefore the focus of the ecological screening assessment summarized here.
According to information gathered by the Government, thallium substances are mainly used in semiconductors and the laser industry, fiber (optical) glasses, photoelectric cells, high temperature superconductors, scintillometers, and in the production of other chemicals.
Human and ecological exposures
As a naturally occurring substance, Canadians are exposed to thallium and its compounds from environmental sources (for example, air and drinking water) and primarily from food.
The human health exposure and risks of thallium and its compounds were characterized using a science approach based upon biomonitoring data, as described in the biomonitoring-based approach document. Human biomonitoring is the measurement of substances in blood, urine or breast milk through health studies or surveys, such as the Canadian Health Measures Survey. Finding the substance in the body does not necessarily mean that it is causing harm. Harmful effects will depend on the levels and the properties of the substances. The information on measured levels in humans is important to estimating exposure to Canadians.
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 releases of residues/by-products from various industrial activities. Detailed exposure scenarios were developed for the 3 sectoral activities that result in the highest releases of thallium to water through wastewater effluents, as determined from National Pollutant Release Inventory (NPRI) data. These include releases of thallium from metal mining activities, base metal smelting and refining, and coal-fired power generating facilities in Canada. Thallium exposure from wastewater treatment facilities was also considered.
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, loss of appetite, and abdominal cramps. 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.
Risk assessment outcomes
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 at current levels of exposure.
Based upon the information presented in the draft screening assessment, it was determined that there is risk of harm to the environment from thallium and its compounds. These substances may pose a risk to aquatic organisms through releases to water from multiple sectors near sites of discharges across Canada.
As a result of the draft screening assessment, the Government is proposing that thallium and its compounds are not harmful to human health at current levels of exposure.
The Government is also proposing, however, that thallium and its compounds may be entering the environment at levels that are harmful to the environment.
It is also proposed that thallium and its compounds meet the persistence criteria as set out in the Persistence and Bioaccumulation Regulations of CEPA 1999. No conclusion can be reached on the potential of thallium biomagnification in both aquatic and terrestrial food chains, due to limited and contradictory data.
If the proposed conclusions are confirmed in the final screening assessment, the Government will consider adding thallium and its compounds to Schedule 1 of CEPA 1999, also called the List of Toxic Substances, and 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: 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 risk management is appropriate.
Base metals smelting and refining: similar to the above action under the MDMER, for facilities that combine their effluent with metal mining operations. For facilities that do not combine effluent with metal mines, the Government will work with industry to gather additional data on thallium releases to the receiving water.
Information is being sought by the Government to inform risk management decision-making. Details can be found in the risk management scope, including where to send information during the public comment period, ending November 18, 2020.
Also, under the third phase of the CMP, the Government is conducting assessments on a variety of metals that may also identify metal mines and base metals smelting and refining facilities 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 CMP. This strategy is focused on releases rather than on single metals and will reduce the administrative burden on implicated sectors.