Table 1.

Basic terms.

Terms	Meaning
Particulate Matter (PM)	PM is a well-known proxy indicator for understanding pollution levels in air, and is commonly used by specialists when measuring ambient air quality (US EPA, 2021; Navarro et al., 2021). As indicated by the United States Environmental Protection Agency (US EPA, 2021): “PM stands for particulate matter (also called particle pollution): the term for a mixture of solid particles and liquid droplets found in the air.” PM contains various substances including organics, metals, elemental and organic carbon, sulfates, and nitrates (Watson, 2002; Seigneur, 2019; Maleki et al., 2021). Ambient PM is categorized into various sizes: inhalable ultrafine particles with a diameter ≤0.1 μm (PM0.1); inhalable submicron particles with a diameter ≤1 μm (PM1), which is the sum of PM1 and PM0.1; inhalable fine particles with a diameter ≤2.5 μm (PM2.5), which is the sum of particles with a diameter ≤2.5 μm; and inhalable coarse particles with a diameter ≤10 μm (PM10), which is the sum of all particles with a diameter ≤10 μm (see related review in Maleki et al., 2021; Navarro et al., 2021). Exposure scientists and industrial hygienists use the same size classification for exposure to aerosol particles (see related review in Milton, 2021; Navarro et al., 2021). Fig. 1 represents the size distribution of atmospheric particles typical of a polluted urban area (reprinted from (Seigneur, 2019)).
Air Pollution	Air pollution in the Cambridge Dictionary is defined as: “harmful substances in the air, often consisting of waste from vehicles or industry.“. Maleki et al. (2021), in a systematic review of the association between atmospheric PM pollution and the prevalence of SARS-CoV-2, defined air pollution as “a mixture of natural or anthropogenic compounds in indoor or ambient air including solid particles (such as particulate matter (PM), bioaerosols), liquid (droplets) and gases (carbon monoxide (CO), nitrogen (NOx) and sulfur (SOx).”
Aerosol	An aerosol is defined in the Merriam-Webster dictionary (Merriam-Webster.com, 2021) as “a suspension of fine solid or liquid particles in gas.” In the context of air pollution, an aerosol commonly refers to the PM in the air” (Merriam-Webster.com, 2021; Seinfeld and Pandis, 1998).
Bioaerosol	A bioaerosol is an airborne collection of biological material (bacterial cells and cellular fragments, fungal spores and fungal hyphae, viruses, pollen grains and products of microbial metabolism). It is assumed that the particle size of particulate material in bioaerosols is generally 0.3–100 μm in diameter; larger particles tend to settle rapidly and are not readily transported in the air (Stetzenbach, 2009).
Infectious Aerosol	Fennelly (2020) defines infectious aerosols as “particles with potentially pathogenic viruses, bacteria, and fungi suspended in the air, which are subject to the same physical laws as other airborne particulate matter.” The term ‘aerosol’ (aerosol particles) can have different meanings for specialists from different fields. Clearly, the terms ‘bioaerosols’ and ‘infectious aerosols’ have a definite meaning for medical staff and epidemiologists; however, these terms cannot be directly associated with air pollution or atmospheric aerosol particles or PM, especially if the terms are used for analysis of the epidemiological process or the transmission of infections.
Droplet Nuclei and Virus-laden Particles/Droplets/Aerosols	Viruses and bacteria are not usually air dispersed as free particles, but are attached to soil, dust or organic aggregates, determined by the medium from which the viruses or bacteria were aerosolized (seawater, soil, bird feces, etc. (Reche et al., 2018)). The viruses involved in respiratory infections (including COVID-19) are also generally not transmitted on their own, but are embedded within much larger mucosalivary particles or droplets generated during respiratory activities (Cox et al., 2020; Pόhlker et al., 2021). Initial respiratory droplets contain water and varying amounts of saliva and mucus, comprising inorganic and organic ions and glycoproteins (Nicas, 2005; Gralton, 2011; Thomas, 2013). It is well known that the respiratory droplets generated by a cough, sneeze, speech, or breath (e.g., d < 20 μm) can be airborne and remain airborne for a long time (Vuorinen et al., 2020: “… 20 μm droplets … can linger in still air for 20 min −1 h”). Airflow may carry these aerosols for long periods, although the duration depends on the context, which is true even without taking account of droplet drying, and is particularly true when drying is considered (drying strongly influences the airborne duration). It is known that droplets with a radius of less than 20 μm evaporate rapidly to 20–50% of their initial size and stay sedimented as so-called droplet nuclei for a long time. Droplet nuclei are dry particles that may include viruses and other pathogens and tend to remain airborne, and thus can be distributed over a greater area than initial droplets (Wells, 1934; Nicas et al., 2005; Ishmatov et al., 2013; Netz, 2020; Lieber et al., 2021). *It is important to note that there are a few myths concerning the airborne transmission of SARS-CoV-2. Recently, Tang et al. (2021a) dismantled six popular myths, the most popular being the delusion that all virus-laden particles larger than 5 μm cannot stay in the air for a long time and fall within 1–2 m of the source. This is an obvious outdated misconception and relates to the outdated and inaccurate use of the terms ‘airborne’, ‘aerosols’, ‘droplets’, and ‘droplet nuclei’ (Atkinson et al., 2009; World Health Organization, 2014). As was rightly noted by Tang et al. (2021c): “… the authors continually cite in support of their definition of these terms an 18-year-old WHO document on the 2003 SARS-CoV-1 outbreaks, rather than more recent articles that redefine these terms in a way that is more consistent with actual mechanisms of transmission …” Moreover, many people, including scientists, still mistakenly believe in the myth that “the virus is only 100 nm in size so filters and masks will not work,” although, as explained, this is a delusion and reflects a lack of understanding of the principles of transmission of respiratory viruses.