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Airborne particles are a complex mixture of substances of different chemical composition and physical nature that vary in size from 0.005 to 100 μm and cannot be detected with the naked eye. The composition of these particles is a heterogeneous mixture, ranging from low volatile compounds, asbestos, fungal spores, bacteria, all kinds of allergens, or even heavy metals.
Although the assessment of their toxicity depends on the specific composition, a high amount of airborne dust can already lead to health problems. Especially particles smaller than 1 - 5 μm, which pass largely unfiltered from the respiratory tract into the lung alveoli.
Generally, the measurement groups all particles with a diameter of less than 0.1 micron up to 50 microns. Larger particles are usually deposited by gravity in dust on floors and indoor surfaces. They are expressed as PM (particulate matter) and the particle diameter as a subscript in μm: PM1, PM2.5, PM4 or PM10 are the most frequent in indoor air quality measurements.
The larger the particle size, the shorter the time they remain in suspension. Particles larger than 10 μm settle quickly in dust; PM10 particles (with diameter ≤ 10 μm) can remain suspended for hours. PM2.5 particles (with diameter ≤ 2.5 μm) can remain airborne for weeks and are more susceptible to travel through ventilation systems. Current EPA (US Environmental Protection Agency) standards, recommend maximum values for PM2.5 of 35 μg/m³, although following the latest revision of the NAAQS (National Ambient Air Quality Standards) in February 2024, it proposes a reduction in total annual values from 12 to 9 μg/m3. The EU sets maximum levels of 20 μg/m³ also for PM2.5.
PM 1.0, PM 2.5 and PM 4.0:
PM10:
The suspended particulate matter is assessed on the basis of the weight of particles of each size per volume of air (μm/m³).
The association of suspended particles in indoor air with outdoor air pollution is direct. Road traffic (mainly diesel vehicles) or industry (chimneys, coal, incinerators, quarries, certain agricultural works, etc.) are the main source of emissions that, depending on the ventilation system and filters, can be detected in indoor environments. However, building interiors have their own sources of particulate matter emissions such as combustion appliances or tobacco smoke, or of biological origin such as pollen, spores, bacteria or fungi.
In the absence of known indoor sources, particulate matter concentrations in indoor air are very similar to those in outdoor air.
A low level of particulate matter means a low level of airborne dust and therefore an absence of irritants, potential allergens and pollutants. Air free of airborne particles is clean air and is characteristic of a healthy indoor environment.
Particles in indoor air can be respiratory irritants and potentially allergenic. The health effects depend on the type of particle present and how easily it penetrates the body. People with weakened respiratory systems or previous respiratory pathologies such as asthma are more directly affected. The associated symptoms range from irritation of the respiratory tract and eyes, increased incidence of respiratory and cardiovascular diseases, increased incidence of cancer in the long term, or even aggravation of infectious diseases or weakened immune systems.
