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Airborne particles are a complex mixture of substances of different chemical composition and physical nature, varying 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 its toxicity depends on the specific composition, a high amount of airborne dust can already lead to health problems. Particularly particles smaller than 1 - 5 μm, which pass largely unfiltered from the respiratory tract into the alveoli of the lungs.
Generally the measurement groups all particles with diameters ranging from less than 0.1 microns to 50 microns. Larger particles are usually deposited by gravity in dust on soil and indoor surfaces. They are expressed as PM, (particulate matter) and the particle diameter as a subscript in μm: PM1, PM2.5 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 (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 it reduces the total annual values to 12 - 15 μg/m³. The EU sets maximum levels of 25 μg/m³ also for PM2.5, although with the prospect of increasing the restriction.
PM1 and PM2.5:
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 airborne particles in indoor air with outdoor air pollution is direct. Road traffic (mainly diesel vehicles) or industry (chimneys, coal, incinerators, quarries, certain agricultural work, etc.) are the main source of emissions that, depending on the ventilation and filter system, can be detected in indoor environments. However, building interiors have their own sources of particulate 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 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.
