Particulate matter

In addition to gaseous components, a variety of contaminants in the form of airborne particles may be detected in indoor air. PM covers solid particles and liquid droplets found in air. Airborne particles cover a range of diameters—from a few nanometres to tens of micrometres—and are usually described based on their equivalent diameter. PM is usually categorized as PM2.5, PM10, and ultrafine particles (UFPs), which have an aerodynamic diameter less than 2.5 μm, 10 μm and 100 nm, respectively. The particle size and chemical composition of PM influence their transport and suspension in the air and their deposition in the lungs (Morawska and Salthammer 2003). From a regulatory point of view, the most common approach for assessing PM is to monitor the mass concentrations of PM10 and PM2.5 collected through a filter, but recently, the scientific focus is on measuring the UFP surface area and particle number concentrations (Cauda et al. 2012, Pacitto et al. 2018b). For determining the mean concentrations in an area, the common assumption is that each person in each region has the same exposure level. However, actual individual exposure is strongly dependent on personal time-activity patterns; therefore, the monitoring of mean concentrations could lead to significant errors and the development of individual- level monitoring is, therefore, recommended (Bo et al. 2017, Buonanno et al. 2014).

Outdoor particles

While gravity pulls larger particles (over 20 micrometers in diameter) down to the ground, smaller particles can remain suspended in the air for extended periods. These smaller particles can then enter indoor environments through ventilation systems.

PM is either directly emitted into the air or converted from gaseous precursors derived from anthropogenic and natural sources (Atkinson et al. 2010). Fine particles (PM2.5) are primarily a product of the combustion of coal, oil or gasoline, or released during the transformation of gases and organics (Srimuruganandam and Nagendra 2012). Coarse particles (PM10) are released through resuspension of road and street soil or industrial dusts, suspension of disturbed soils (e.g. during farming and mining), construction, coal and oil combustion, and ocean spray (Srimuruganandam and Nagendra 2012). Apart from dust, fly ash and oxides are also formed during various processes; additionally, coarse PM is also composed of pollen, bacteria and plant parts (Cheung et al. 2011). Road and vehicle-based dust formed 11% of total primary emissions of PM10 and PM2.5 in the European Union during 2017 (EEA 2019). Importantly, PM is one of the most severe pollutants with regard to health, especially in urban areas.

Indoor particles vary in size, form and chemical composition. They consist of ambient particles that infiltrate indoors and particles emitted or formed through various indoor processes and activities (Morawska et al. 2017). In indoor air, particles from different sources persist via deposition and resuspension. In urban environments, ambient particles originate mainly from traffic emissions, fossil fuel burning and resuspension, and also chemical and thermodynamic processes (Belis et al. 2013). Some indoor activities that contribute to indoor particles are, for example, cooking, smoking, cleaning, and candle burning, or chalk dust and art classes in school environments. Indoor particles have some differences in their composition and toxicity from outdoor aerosols; thus, it is essential to consider these separately (Oeder et al. 2011). In school environments, PM2.5 and PM10 have been shown to mainly be of indoor origin (as a result of resuspension, for example), whereas outdoor air seems to be the main source of UFPs (Morawska 2017, Oeder et al. 2011).

Human exposure to PM occurs mostly indoors, as a large proportion of time is spent indoors, and microenvironments are the major contributors with regard to personal exposure (Faria et al. 2020). Geographical location effects the daily particle dose, but culture and lifestyle, e.g. types of cooking, have a stronger effect on the microenvironments in which people spend their time. In a study by Pacitto et al. (2018b), the personal particle dose exposure was shown to be the lowest in Lund, Sweden, where the context is closer to Finland than to other European countries or Australia.

The main indoor sources for PM in school buildings are human activities, plants and building materials, especially mineral fibres (Chatzidiakou et al. 2012). In schools, PM2.5 and PM10 originate mostly from indoor sources; in particular, resuspension of particles brought in on children’s shoes and clothes is a highly significant source of indoor particles (Morawska et al. 2017). Therefore, taking off the shoes when entering the school building could significantly reduce the mass concentration of particles (Leppänen et al. 2020). Dynamic movement and the presence of a large number of people in a confined space increase particle exposure in schools (Fromme 2007). In addition, ventilation and infiltration introduce PM from outdoors, and vehicles are the main source of outdoor PM (Trompetter et al. 2018). A study of school children’s exposure to UFPs found that exposure during school hours was attributable more to urban background particles than traffic near the school, and no persistent indoor particle sources were detected (Mazaheri et al. 2014). Because of climate change, the contribution of natural sources to the ambient PM concentration levels is expected to increase in the future, through phenomenon with far-reaching effects, such as wildfires (Knorr et al. 2017). The adverse health effects of indoor PM in schools can be managed by providing sufficient and filtered ventilation, minimizing the sources of PM in the first place, ensuring cleanliness of the school, and building schools far from busy roads (Rivas et al. 2018, Morawska et al. 2017).

Source: Camilla Vornanen-Winqvist. 2020. "Indoor air contaminants, symptoms and effects of mechanical ventilation in school buildings"