Solid particles need to be less than 10 microns (10 μm) in diameter in order for us to be able to inhale them, so breathable particles are referred to as “PM10”. The smaller they are, the more deeply they can enter the lung. On the basis of different sizes, PM10 can be subdivided into three categories, these being coarse (2.5-10μm; extrathoracic deposition), fine (0.1-2.5μm; bronchial deposition) and ultrafine (<0.1μm; alveolar deposition). Ultrafine particles will have no problem entering the alveoli, and so may become involved in the process of respiration, allowing them to transfer into the bloodstream and so travel freely to other parts of the body such as the brain and heart. Air pollution is not, therefore, something only associated with the lung. Neither is it something we can easily see, unless levels of atmospheric contamination are so bad that the billions and billions of particles are visible as smog hanging over the city. Epidemiological and toxicological studies have found clear links between levels of air pollution and hospital admissions and deaths in patients, especially in children, elderly and sick people, and especially on smoggy days. Much research has already been done to identify and refine our understanding of urban air pollution sources.
Some examples of atmospheric particles are shown below. Particles like these are too small to be visible to the naked eye but are present everywhere in our cities. Some particles, such as the industrial fly ash and magnetite spheres, are individual, whereas others, such as the soot, comprise clustered aggregates of tiny, ultrafine PM. These high-magnification images are taken using an electron microscope, and the scale bar (white line) represents half a micron. Anything less than 10 microns in diameter can be inhaled by humans, and the smaller they are, the more deeply they can penetrate the lung.