Dusts and mists

Dusts and mists constitute one of the classes of occupational health pollutants that are hazardous to the respiratory tract. The term 'particulate matter' commonly identifies the set of these substances characterised by a particle-like chemical-physical structure, which takes the form of liquid particles (mists) or solid particles (fumes and dusts) suspended in the atmosphere. The set of particles suspended in the atmosphere is defined as PM (from the English 'Particulate Matter').

How do these substances form?

The individual particles differ widely in size, shape, chemical composition and formation process.

Mists are microscopic droplets that are formed by processes of atomisation and condensation. Dusts are formed when a solid material is broken down into tiny fragments. Fumes are formed when a material that at ambient temperature and pressure is in a solid state (e.g. a metal) is vaporised by high heat. The vapour cools quickly, condensing into extremely fine particles.

The diameter of these particles is between 0.005 and 150 µm (the thickness of a human hair is about 100 µm).

Within this range, atmospheric dust can be divided into:

• coarse particles with a diameter greater than 10 µm
• fine particles (PM10) with a diameter of less than 10 µm;
• very fine particles (PM2.5) with a diameter of less than 2.5 µm.

Among solid particles, INAIL (Italian National Institute for Insurance against Accidents at Work) identifies three types of substances, differentiating them as follows:

• fumes resulting from condensation, combustion and vaporisation processes, which have a different composition from the source material and dimensions of less than a micron;
• dusts originating from the mechanical action on a solid body (grinding, cutting, polishing, etc.) and generally having a composition similar to the source material, or derived from processes of crystallisation of supersaturated vapours or gas-particle conversion processes;
• fibres, of natural or synthetic origin, consisting of elongated particles whose length is at least three times their diameter. The Who (World health organisation, 1988) defines fibres as all particles with a length greater than 5 µm and a diameter smaller than 3 µm.

How are they hazardous to health?

Due to their small size, particulates tend to remain suspended in the air and be breathed in by exposed individuals when not adequately protected.

As the diameter of inhaled particles varies, so do the possible clinical effects.

• Coarse particles stop at the first respiratory tract.
• Fine particles (PM10), also known as inhalable dust, penetrate the upper airway or extrathoracic tract (nasal cavities, pharynx and larynx).
• Very fine particles (PM2.5), also called respirable dust, can reach the lower parts of the respiratory tract or tracheobronchial tract (trachea, bronchi, bronchioles and pulmonary alveoli).

Clinical effects of mists

Possible effects caused by exposure to mists include:

• Rhinitis, pharyngitis, laryngitis, bronchitis;
• Irritative and allergic dermatitis;
• Lung tumours.

Clinical effects of fumes

Here are some of the possible acute effects caused by exposure to welding fumes:

• Upper airway irritation and bronchitis;
• Welding fume fever;
• Bronchopneumonia;
• Pulmonary oedema.

Here are some of the possible chronic effects caused by exposure to welding fumes:

• Welder's lung;
• Tumours (lung, paranasal sinuses);
• Chronic gastritis up to gastroduodenal ulcer.

To learn more about the risks to respiratory health caused by welding operations read our dedicated article: Health risk work sectors - Welding 

Current regulations

Current legislation sets out different conventions depending on whether the risks are to workers or to the general population.

As INAIL mentions in Conoscere il rischio - Altre polveri e fibre (Knowing the risk - Other dust and fibres), in fact, "to prevent or reduce harmful effects on human health and the environment as a whole, legislative decree 155/2010 (as well as the technical standard UNI EN 12341) refers to two conventions, denoted PM10 and PM2.5 respectively, where PM stands for particulate matter and the numerical values in-dicate the aerodynamic diameter of the particles (in µm) for a penetration efficiency of 50%".

The conventional PM10 curve is very similar to the conventional thoracic curve, whereas the PM2.5 curve represents a very fine fraction, which is considered to be able to reach the pulmonary alveoli in the case of the population consisting of children or infirm adults (the group considered 'high risk'). The limit values for human health, defined for PM10 and PM2.5, are set out in the same legislative decree (and subsequent updates), and represent much more restrictive conditions than those allowed for exposure in the workplace.

Stay informed, stay safe.