Fact file

Metals are present in trace amounts in the atmosphere. With the exception of mercury, metals occur in particulate form. They can be emitted into the atmosphere by natural processes (volcanoes, sea spray, erosion) or result from human activities, such as the combustion of fossil fuels (industry, heating, transport), metallurgy, waste incineration, etc.

Because of its industrial fabric historically associated with the steel industry, and the extraction and processing of non-ferrous metals, Wallonia is particularly affected by this type of pollution.

Walloon legislation regulates the levels in the air for 4 metals: arsenic (As), cadmium (Cd), nickel (Ni) and lead (Pb).

Origins of pollution

The majority of Walloon metal emissions (arsenic, cadmium, chromium, copper, mercury, nickel and lead) are attributable to the industrial sector (50% in 2014). However, these are on the decline; between 2000 and 2014, they fell by nearly 75% thanks to measures imposed in the sector. Another major source of heavy metals is the transport sector, which accounts for 31% of emissions in the Walloon Region (-12% between 2000 and 2014).

  • Arsenic. This comes mainly from the use of solid mineral fuels (coals) and from certain industrial processes such as the metallurgy of ferrous or non-ferrous metals or the production of glass.

  • Cadmium. This comes from waste incineration and industrial processes such as metallurgy.

  • Nickel. Emissions come from the combustion of fuel oil and coal, oil refining, waste incineration, and the production of special steels.

  • Lead. Historically, air pollution from lead is associated with traffic emissions. The lead added to petrol to improve its octane number was found in exhaust gases and then in the air. The steel industry was also a major source of lead emissions. Today, lead is still emitted in the industrial sector.


Health effects

Arsenic and nickel are trace elements and are necessary for life at low doses but can be toxic at higher doses. On the other hand, cadmium and lead have no known use and are harmful in all cases. 

  • Arsenic. Arsenic is well known to be a violent poison. The effects of chronic exposure are numerous: dermatitis, irritation of the airways, cardiovascular, haematological and hepatic effects, damage to the nervous system, etc. At lower doses, it is recognised in particular to be carcinogenic (respiratory tract, lungs, skin) and genotoxic.

  • Cadmium. This interferes with calcium metabolism, with an impact on the kidneys (renal failure) and the skeleton (loss of calcium, osteoporosis). It is also carcinogenic (lungs, prostate).

  • Nickel. Nickel is a skin allergen (dermatitis, eczema) and a respiratory system allergen. It is classified as carcinogenic (lungs, nasal cavity, larynx).

  • Lead. In the body, lead interferes with various enzyme systems and displaces other essential metals. It is mainly fixed in the bones, where it competes with calcium. It affects the nervous system (lead poisoning) and results in a reduction in the conduction speed of the nerves, causing neurological disorders (confusion, anxiety, memory loss, etc.). Children are particularly sensitive and can suffer from impaired brain development. It also disrupts the synthesis of haem-causing anemia. Lead is suspected of being carcinogenic.

Environmental impact

Metals can contaminate soils, surface water and groundwater. They accumulate in living organisms, where they disrupt biological mechanisms and balances. They can be found in foods, with an accumulating effect throughout the food chain.

The situation in Wallonia

Since the start of measurements at the end of the 1970s, metal concentrations have decreased. This is mainly due to two factors. First of all, there was a ban on petrol containing lead and then a reduction in steel activities.

Since 2016, there has been no exceeding of European standards. Only one industrial site still needs to be subject to special monitoring (a cadmium problem).


Measurement methods

The metals (PM10)  are sampled on filters (lasting 24 hours or 7 days, depending on the site) which, on return to the laboratory, are digested and then analysed by plasma torch spectroscopy coupled with detection by mass spectrometry.

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