Air purification applications using photocatalysis

Abstract

Air quality is one of the critical factors that determine human health and longevity. The air that we breathe can be contaminated with various impurities. According to the typical size of airborne particles the air contaminants are divided into molecular impurities, consisting of diverse inorganic and organic compounds in the vapor state with a molecular size,10 nm, and mechanical impurities that have typical particles sizes ranging from 100 nm to 0.1 mm and consist of solid and liquid aerosols. Air pollution harms human health and the environment [1], is a local, pan-European, and hemispheric issue. A significant proportion of the world population lives in areas, especially cities, where violation of air quality standards occurs: particulate matter with a diameter ,2.5 μm (PM2.5), nitrogen dioxide (NO2), and ground-level ozone (O3) are now generally recognized as the three pollutants that most significantly affect human health. Long-term and peak exposures to these pollutants range in severity of impact, from impairing the respiratory system to premature death. Around 90% of city dwellers in Europe are exposed to pollutants at concentrations higher than the air quality levels deemed harmful to health. In addition to human health, biodiversity and ecosystems are also endangered by air pollution. furthermore, the damage of air pollution at a financial level reaches dire figures [2]. At the end of the last century, to the presence of particulate matter, nitrogen dioxide (NO2), sulfur oxides (SOX), carbon monoxide (CO), and ozone (O3) as air pollutants, a new group had to be added, the volatile organic compounds (VOCs). The discovery by Haagen-Smit and Fox [3] that ozone productionwas responsible for the so-called Los Angeles Smog, that it was caused by photochemical oxidation of mixtures containing VOCs and nitrogen oxides (NOX NO1NO2), provided the first evidence that VOCs can act as precursors of secondary pollution. Consequently, it became clear that both the increasing use of fossil fuels for transport and energy production and demand increase of new chemicals were associated with drastic VOCs increase in the air and with short- and long-term effects on human health and environment. Since then, the presence of these pollutants in the environment has continued to grow [4]. WHO recognizes that globally seven million deaths were attributable to the joint effects of ambient air pollution and household in 2016 [5]. These figures represent a total of 12% of deaths that occurred in the same year. By 2050, it is projected that India will have added 416 million urban dwellers, China 255 million, and Nigeria 189 million. The urban population of the world has multiplied from 751 million in 1950 to 4.2 billion in 2018 [6]. Because air pollution is much worse in large cities, it may be more reasonable to focus on the pollution controlling efforts on the most highly contaminated areas. It should be kept in mind that trying to reduce the concentration of a pollutant once it is already diluted is much more inefficient than when acting directly on the source.