Of the particles that are emitted into the atmosphere, the heaviest particles sediment more or less quickly to settle in the vicinity of their source. We call this sedimentable dust.
In reality, sedimentable dust is not characterised by a physical parameter but by reference to a sampling method. Sedimentable dust is thus defined as any particles found in a deposit gauge.
We not only determine the quantity of deposits but also analyse their metal content, some for obvious reasons of toxicity; others because they give indications on the source.
Origin of pollution
Wallonia is strongly affected by this type of pollution due to the presence of industries that emit a lot of dust, such as the steel industry, cement factories, lime kilns or quarries. These industries are often nested in residential areas. Given the limited range of sedimentable dust, the pollution is essentially local in nature and the measurement network is very clearly a local network.
Most often, this dust does not represent a direct toxic risk for human health. This is because the large particles do not enter the human respiratory system because they are filtered into the upper airways. However, the toxic elements possibly present in these deposits can accumulate in soils and ecosystems before finding their way into the food chain, and thus constitute an indirect risk to health.
Sedimentable dust, above all, creates dirt, thus creating a nuisance that is felt all the more strongly by local residents as it is visible. They cause damage to buildings, plants, the landscape and the living environment in general.
The deposits of dust on the leaves, by limiting gas exchange, can also have an impact on the development of vegetation.
The toxic elements that they sometimes contain are found in the environment, thus contaminating soils, surface water, groundwater, etc.
The situation in Wallonia
The deposit gauges constitute, above all, a network on a local scale, and each region is a particular case. However, for the majority of sites, the trend in fallout levels is downward. This is due to several factors, such as the closure of certain industrial activities, the economic slowdown in heavy industry, the measures taken to reduce dust emissions, etc.
The dust is collected by means of a deposit gauge which consists of a cylindrical container surmounted by a funnel 30 cm in diameter (Owen type gauge). The withdrawal period is 28 days (+/- 2 days).
Back in the laboratory, the mass of the dust is determined by gravimetry and the metals are determined by plasma torch spectrometry. For gauges installed in sites subject to this type of pollution, the fluorides are measured by ion chromatography.