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Fact file

Particles are solid or liquid entities suspended in the air, with which they form an aerosol. They can have very diverse compositions, densities, shapes, and sizes, depending on their mode of formation. Their size can vary from a few nanometres for the finest particles, i.e. 10-9m, or a billionth of a metre, to a micrometre for the largest aggregates, i.e. 10-6m, or a millionth of a metre.

Large particles contain elements abundant in the earth's layer and in sea salts, while fine particles contain sulphates, nitrates or ammonium, as well as carbon, organic compounds and metals, resulting, amongst other things, from the incomplete oxidation of fossil fuels.

We can distinguish the particles according to their origin. Primary particles are emitted directly, either via natural processes or via processes related to human activity.

On the other hand, secondary particles are not emitted but result from the condensation of gases, or even from chemical reactions between gases, giving rise to the formation of very fine particles.

The size and density of particles are essential characteristics that govern not only their behaviour in the atmosphere, but also their toxicity and environmental impact.

Fine particles behave similarly to gases and hardly sediment. They are then called particulate matter (in English, particulate matter abbreviates to PM). The smaller they are, the longer their lifespan in the atmosphere; their range can be long and their toxicity high.

Depending on its size, particulate matter is divided into several parts. PM10 consists of particles whose size is less than 10 µm (10 thousandths of a mm, or 6 to 8 times smaller than the thickness of a hair) while PM2.5 consists of particles whose size is less than 2.5 µm. Each part includes the previous one; thus PM2.5 is part of PM10. Currently, these two parts are the focus of all attention because they can enter the respiratory tract and cause damage to health.

These two pollutants are used for the calculation ofthe Belgian index of air quality, BelAQI


Origin of pollution

The sources of particulate matter are numerous and varied. Particles can be emitted by a series of natural processes (erosion, sea spray, pollen, volcanism, forest fires) or by human activities: there are sources linked to combustion processes (energy production, domestic heating, traffic), industrial processes (metallurgy, cement factories, production of fertilisers, extraction), agriculture, etc.

Certain human activities are indirect sources of particulate pollution. They emit pollutants that can give rise to secondary particles. Thus, spreading fertiliser can give rise to ammonia emissions, which can react with nitrates (from NOx) to give rise to ammonium nitrate aerosols.

In Wallonia, the majority of particulate emissions come from the residential sector (mainly heating), followed by the industrial sector, and finally, the transport sector and agriculture. Emissions have fallen sharply since 2000. This is mainly due to abatement measures in industry, the closure of steel plants, the generalisation of particulate filters, etc.

An increase in particulate emissions can be observed in residential areas. This is mainly due to an increase in wood consumption in this sector (consumption more than doubled between 2000 and 2018).  However, this increase must be qualified, as the evolution of heating installations has not yet been taken into account in the emission inventories. Emissions can be reduced by making optimal use of the equipment, especially the newer, more efficient and less polluting models. The use of quality fuels is also important.

For more information and to take the right actions to reduce emissions: The use of quality fuels is also important: pellets are preferred to logs because of their high calorific value and low humidity.


Health effects

Particulate matter is harmful to human health. Their health effects are noticeable both in the short term (inflammatory reactions of the lungs, increase in cardiovascular ailments, etc.) and in the long term (bronchitis, asthma, cancer, etc.). The health impact of particulate pollution is not only linked to the physical presence of particles in the body (causing inflammation) but also to their content in toxic substances, such as metals or certain organic compounds liable to pass into the blood. The smaller the particles, the deeper they can penetrate into the branches of the respiratory system. The finer ones can reach the capillaries and enter the bloodstream. According to the World Health Organization, there is no threshold below which particles have no impact on health.

Particulate pollution increases the risk of death, cardiovascular disease, and even lung cancer. Thus, our life expectancy is reduced by several months. Likewise, during pollution peaks, the level of hospitalisations and absenteeism increases. In addition, there is a loss of quality of life, especially for those at risk, such as asthmatics or people with cardiovascular disease.

Air pollution by particles thus appears, at the European level, as the environmental problem with the greatest impact in terms of public health and socio-economic costs.


Environmental impact

The most visible consequence is dirt on buildings, our historical heritage in particular, causing costs for their restoration. Dust deposits on leaves, limiting gas exchange, can also have an impact on the development of vegetation.

The content of toxic substances contained in the particles (organic compounds, metals) can be found in the environment and thus pollute surface water, groundwater, soils, etc.

Particles can have a complex effect on the climate: some particles contribute to cooling, while others, such as black carbon, contribute to warming.


The situation in Wallonia

With regard to PM10, all of the Walloon stations have complied with European requirements since 2008 as regards the annual limit value and since 2015 for the daily limit value. A station with a certain industrial character must still be the object of all attention since it flirts with the daily limit value.

Since the entry into force of the Directive, the annual limit value for PM2.5 has always been respected.

On the other hand, the recommendations of the World Health Organization are far from being followed. For PM10, only a few rural stations comply with the daily WHO recommendation, compared with 75% of stations for the annual recommendation (as of 2018).

The majority of rural stations and one urban station comply with the annual WHO recommendation for PM2.5 (as of 2018). On the other hand, no station respects the daily WHO recommendation.

Measurement methods

Particulate matter (PM10 and PM2.5) is measured continuously and in real time using monitors and using the principle of light scattering.


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