Technical Report No. 3
Environment Australia, May 2002
ISBN 0 6425 4781 5
The term 'heavy metals' has sometimes been restricted to metallic elements, which normally have atomic weights between 63 and 201 amu and specific gravity above 5.0 (DEST, 1996.) A more general classification is that they are metals or metalloids found in the periodic table of elements from Group IIA through VIA, including the semi-metallic elements: boron, arsenic, selenium, and tellurium (OHSA, 2000). Metals exist in elemental form or as ions dissolved in water, or as vapours, or as salts or minerals in rock, sand, and dust, and form a variety of inorganic or organic compounds. However, for simplicity, most environment regulators do not make distinctions between metal species, and refer to them as metals and their compounds (NPI, 1999 & 2001; EA, 1999).
Both natural and anthropogenic processes and sources emit metals and their compounds into the air. The processing of minerals, incineration of metallic objects, motor vehicle combustion of fuel containing metal additives, and the wearing out of motor vehicle tyres and brake pads result in the emission of metals associated with particulate matter. Metals occur naturally in soil and in rocks rich with minerals; thus weathering of the rocks, mining activities or even wind-blown dust can release these metals into air as particulate matter (Langmuir, 1997). Australian industries have reported the discharge into air of heavy metals and their compounds from power stations using fossils fuel such as coal, and from mines, mineral processing industries, refineries and brickworks (VIC EPA, 1996 & 1998a; NPI, 1999 & 2001).
Heavy metals are part of a large group of air pollutants called air toxics, which upon inhalation or ingestion, can be responsible for a range of health effects such as cancer, neurotoxicity, immunotoxicity, cardiotoxicity, reproductive toxicity, teratogenesis and genotoxicity (HEI, 1998; US EPA, 1999a). Plants and animals depend on some metals for nutrients, however certain forms of some metals can be toxic even at trace levels (AMAP, 1997). For most metals, the bound or complex species are less toxic than the elemental forms hence there is a loss in specificity when they are broadly classified as metals and their compounds.
When inhaled, very small particles containing metals or their compounds deposit beyond the bronchial regions of the lungs into the alveoli region (Brody et al., 1984). Epidemiological studies have established relationships between inhaled suspended particulate matter and morbidity/mortality in populations (Dockery et al., 1993; Pope et al., 1995; Abbey et al., 1995). Studies in occupational or community settings have established the health effects of exposure to heavy metals, such as lead and their compounds (Sibergeld, 1995; Sibergeld, 1996; Goyer, 1996). The localised release of some heavy metals from inhaled particulate matter has been hypothesised to be responsible for the lung tissue damage (Gilmour et al., 1996; Donaldson and MacNee, 1998).
The emissions of three heavy metals, lead, mercury and cadmium, are being regulated in Europe under the Convention on Long-range Transboundary Air Pollution. This convention is the first international, legally binding instrument to deal with problems of air pollution on a broad regional basis. It covers 42 countries in Europe and North America and the European Union. Since entering into force in 1983, the Convention has been extended by several protocols dealing with specific pollutants. Thus the Executive Body for the Convention, concerned that ' …emissions of certain heavy metals are transported across national boundaries and may cause damage to ecosystems of environmental and economic importance and may have harmful effects on human health…', adopted the Protocol on Heavy Metals in June 1998, in Aarhus, Denmark (UN/ECE, 2000). The Aarhus Protocol targets three particularly harmful metals: cadmium, lead and mercury, to set a framework for national legislation that will lead to the substantial decrease in the emissions of the three metals in Europe and North America. The protocol seeks to cut emissions of heavy metals from industrial sources, combustion processes and waste incineration. The Protocol will enter into force when ratified by sixteen signatory countries; as of 3 July 2000, only six countries had ratified the Protocol (EIS, 2000). Australia is not a signatory to the Convention.
Air toxics are not regulated under the National Environment Protection Measure (NEPM) for Ambient Air Quality, which addresses criteria pollutants in ambient air (NEPC, 1998). However, a program initiated by the Commonwealth Government, the Living Cities – Air Toxics Program (ATP) was aimed at addressing urban air quality issues by supporting the development of national approaches to the management of 'air toxics'. For the purpose of the Living Cities initiative, air toxics are defined as: '…gaseous, aerosol or particulate pollutants (other than the six criteria pollutants) which are present in the air in low concentrations with characteristics such as toxicity or persistence so as to be a hazard to human, plant or animal life…' (EA, 2000). The terms 'air toxics' and 'hazardous air pollutants' (HAPs) are used interchangeably (US EPA, 1999a). The Technical Advisory Group (TAG) for the ATP included the metals cadmium, chromium(VI), mercury, nickel and their compounds in the list of 28 priority air toxics identified in the ATP.
As a result of its presence in motor vehicle fuel, lead is the most routinely monitored heavy metal in ambient air in Australia. Lead has been measured in some Australian cities for over two decades. Unlike lead, the other metals have been measured in Australia only over the past few years in a few studies (Gras et al., 1992; ERDC, 1995; Gras, 1996; Ayers et al., 1998). These studies determined the concentration of several elements, including lead, in particulate matter. The concentrations of metals, metalloids and non-metals associated with suspended particulate matter were measured.
There are several organisations holding data from previous metals studies. Others have raw, unanalysed physical testing data on ambient air levels of heavy metals. There are also monitoring, modelling and emissions inventory data on heavy metals held by different jurisdictions. The following Australian reports have reviewed the results of some of these metals studies and those of several other toxic air pollutants: State of Knowledge Report: Air Toxics and Indoor Air Quality in Australia (EA, 2001); Hazardous Air Pollutants: A Review of Studies Performed in Australia and New Zealand. Volumes I & II (VIC EPA, 1999); State of Knowledge Airborne Particles in Australia and New Zealand (VIC EPA, 1998a). Unfortunately there has not been a review of the current methods used for sampling or analysis of metals in ambient air in Australia. There is also a need to consolidate the data from the different studies to identify any gaps in knowledge on heavy metals in air.
The objective of this project was to consolidate available data on heavy metals concentration in ambient air measured in Australia, analyse unpublished data from the States and Territories and attempt to identify any gaps in knowledge on the measurement of heavy metals and their compounds in ambient air.
The scope defined for the project was as follows:
- To review the currently available data on ambient air levels of heavy metals, including published literature and unpublished data, data held by State and Territory environment authorities and research institutions and any other relevant data available on this subject,
- To compile all data and literature sources used in the review as a separate bibliography/references section of the review,
- To critically analyse the scientific integrity of the data,
- Where possible, to reconcile the data for comparability and consolidate into consistent formats,
- For each heavy metal to identify sources, including point, area and mobile sources, and their comparative importance in terms of emissions and contribution to ambient air levels,
- For each heavy metal, to identify and discuss any long-term trends in ambient air levels found in the data, and
- To analyse physical testing data on metals from two sites in Queensland, which were collected over a period of five years, and to determine if there are any seasonal or annual trends in the concentrations of the species analysed. This analysed data were to be incorporated into the report.
The range of compounds to be reviewed included all metals and metalloids on the periodic table of elements, from Groups IIA to VIA, and their compounds. Data for sodium and potassium, which are not heavy metals, were also to be compiled since they are emitted from natural sources, such as sea-salt and sand, which contribute substantially to airborne particulate matter. However, it should be noted that sodium, potassium and some of the metalloids such as silicon are not considered to be air toxics, but their concentrations have been included in the review for comparison purposes.
Lead is a heavy metal, but since it is one of the criteria air pollutants regulated under the NEPM for ambient air quality, it is not one of the air toxics, under the definition used by the ATP. However, lead data have been included in this review to enable the comparison of ambient lead concentrations with the concentrations of other heavy metals.