Airborne particulate matter (PM) is a solid or liquid particle of various sizes. They are classified primarily by diameter:

• PM 10: particles smaller than 10 micrometers, which can reach the upper respiratory tract and sometimes the lungs.
• PM 2.5: particles smaller than 2.5 micrometers, which penetrate deep into the lungs, damaging the alveoli and overall health.
• PM 1: particles smaller than 1 micrometer, even thinner and more dangerous, capable of entering directly into the bloodstream through the lungs.

PM comes from natural sources (fires, volcanic eruptions, dust) and human activities (traffic, industry, fuel combustion).

PM 2.5 and PM 1 are particularly dangerous, causing respiratory, cardiovascular and lung damage problems, with PM 1 having even more profound systemic effects.

Even indoors, PM can accumulate, especially in poorly ventilated spaces or near sources of pollution such as smoke and wood-burning stoves. Reducing exposure and improving air quality is essential for health.

CO2 (carbon dioxide) is a colorless, odorless, non-flammable gas that occurs naturally in the atmosphere and is essential to biological and geological cycles.

It is produced by the respiration of living organisms and the burning of fossil fuels. CO2 tends to accumulate indoors, especially in crowded or poorly ventilated spaces, and excessive levels can cause headaches, drowsiness, difficulty breathing, dizziness, and mental confusion.

The ideal CO2 concentration in indoor environments should not exceed 1000 ppm. Good ventilation is essential for maintaining healthy air and preventing negative health effects.

The VOCs are carbon-containing chemicals that evaporate easily at room temperature. They are found in everyday products such as paints, solvents, detergents, building materials, and cosmetics.

Common VOCs include formaldehyde, toluene, benzene, and acetone. These compounds are hazardous to health, causing irritation to the eyes, skin, and respiratory tract, as well as headaches, dizziness, damage to the nervous system, and an increased risk of respiratory diseases.

To reduce exposure, it is important to ensure proper ventilation and choose products with low VOC emissions.

Temperature affects several aspects of indoor pollutant behavior:

• Pollutant volatilization: At higher temperatures, many volatile pollutants—such as Volatile Organic Compounds (VOCs)—tend to release more easily from surfaces into the air. For instance, paints, solvents, detergents, and furniture can emit greater quantities of VOCs as temperatures rise.
• Ventilation dynamics: Warmer air tends to rise and move toward windows or openings, influencing natural ventilation. This can alter how pollutants are distributed throughout the environment.

Humidity affects airborne particles and gases in various ways. High humidity can increase the weight of suspended particles, such as dust and allergens, causing them to settle; however, in very damp environments, dust tends to cling to surfaces.
Humidity also promotes the growth of mold and fungi, which release spores into the air—a serious biological pollutant. Furthermore, moisture can stimulate chemical reactions between pollutants, producing compounds that irritate the respiratory tract. Finally, humidity can alter the volatilization of VOCs, increasing or decreasing their concentration in the air.

Atmospheric pressure influences air movement and the dispersion of pollutants. Pressure fluctuations, such as strong winds or weather changes, can alter the airflow within buildings, modifying how pollutants are distributed.
When outdoor pressure decreases, indoor air tends to flow outward; conversely, under high outdoor pressure, air is pushed inward. This affects a building's ability to either retain or eliminate pollutants. For gases like radon, low indoor pressure facilitates infiltration into the structure. Pressure also impacts pollutant dispersion: low pressure tends to concentrate them, while high pressure can limit their spread.