[Encyclios]

Air pollution

The main culprits of air pollution are vehicles with internal combustion engines, industries, thermoelectric power plants, domestic heating, waste incineration plants, especially if they do not have suitable dust abatement and smoke purification systems.

Among the main pollutants are mentioned: Smoke, soot, ash, and dust emitted from heating systems, factory chimneys, waste incineration plants, cement plants, quarries, and mines; gaseous sulfur compounds, in particular sulfur dioxide, chosen as a parameter for assessing the degree of atmospheric pollution (which comes from the combustion of fossil coal, especially of poor or mediocre quality, of coke and heavy fuel oils, from the production of sulfuric acid, from the processing of many plastics, from the roasting of pyrites, from the desulfurization of natural gases), hydrogen sulfide (due to the desulfurization processes of petroleum in general and of gasoline in particular and of natural gases) and the malevolent thiophenes and mercaptans emitted by refineries and coke ovens; carbon monoxide, the most widespread gaseous pollutant that derives from the incomplete combustion of substances containing carbon, above all coal and hydrocarbons, while complete combustion forms carbon dioxide; nitrogen oxides, present in the exhaust gases of motor vehicles or due to the production of nitric acid and nitrates; hydrocarbons, unburnt or pyroscopic, present in the exhaust gases of motor vehicles; ozone, present in photochemical smog, around high-voltage electrical systems, and in the exhaust gases of idling internal combustion engines; lead in various volatile forms, released from the exhaust gases of motor vehicles fueled with ethylated gasoline, i.e., containing tetraethyl lead as an antiknock additive; various inorganic and organic acids (sulfuric, hydrochloric, hydrofluoric, bromidic, acetic, fumaric, tannic, etc.) released in combustions or from the manufacture of nitric acid and nitrates; and lead in various volatile forms, released from the exhaust gases of motor vehicles fueled with ethylated gasoline, i.e., containing tetraethyl lead as an antiknock additive.

Artificial radioactive products due, in addition to atomic explosions, to the processing of radioactive substances for the peaceful use of nuclear energy, to the use of radioactive nuclides in scientific research, industry, medicine and agriculture. To these man-made pollutants are added the normal natural atmospheric impurities such as pollen, spores, windblown dust, cosmic dust, volcanic ash or produced by forest fires, salt particles released from the sea surface, etc..

Unfortunately, the consequences of air pollution are hardly susceptible to timely control and are very often referred to different causes. Long-term high pollution levels, which often result in collective intoxication, often have catastrophic consequences on entire ecosystems. In particular, for humans and vertebrates, hydrocarbons have been shown to be potent carcinogens, promoting, among other things, the onset of lung cancer. Moreover, together with carbon monoxide and tetraethyl lead, they contribute, at a cellular level, to the enzymatic block of the respiratory chain.

As for the photochemical smog mentioned above, in addition to irritating and toxic effects on humans (eyes and respiratory tracts), it causes particularly serious damage to vegetation. The peroxyacetylnitrate is in fact responsible for the enzymatic block of photosynthesis and ozone, by enhancing the respiration of plants, causes an abnormal depletion of their nutrients; particularly insidious are also various gases and industrial exhaust fumes (such as solvents), as, at levels already toxic to the body, are not easily perceived by the senses.

Particularly alarming is finally the pollution of the atmosphere by radioactive products. Radionuclides, constituting the so-called fallout, are deposited on the ground, thanks also to atmospheric precipitations, and become part of the ecosystem food chain. It is believed that environments poorly supplied with nutrients more easily absorb radioactive nuclides; the latter, entering the biogeochemical cycles, are fixed by plants and from there pass, concentrating in tissues, to primary and secondary consumers, and so on, up to man with all the consequences that follow. It has been proved, for example, that some radioactive constituents of fallout (strontium 90 and cesium 137), absorbed by soil (concentration factor=1) and fixed by vegetation (concentration factor=21), are then found accumulated in high doses (concentration factor=714) in the bone tissue of herbivores such as sheep, deer, reindeer, thus constituting a danger for human populations consuming milk and meat of these animals.

Air pollution is measured by measuring the concentration of the main pollutants in special measuring stations, located so as not to be directly influenced by emissions from industrial areas or large urban agglomerations; the values measured are compared both with the maximum admissible values and with those of the “background concentration” measured in stations located at very high altitudes and far from the sources of pollution, according to OECD (Organisation for Economic Cooperation and Development) suggestions. The values measured in these stations, besides being very low, are almost constant. Other monitoring stations are installed in various points of the cities, where the concentration of pollutants produced by motor vehicles and combustion processes (SO₂, CO₂, NO₂, O₃) is very high. This concentration is often accentuated by particular conditions, such as the absence of wind and rain and thermal inversion.

Two alert levels are usually established, which should be matched by measures to bring the values within the established limits. Air pollution can be reduced by improving the quality of fuels (desulphurization); promoting the use of “clean” fuels (methane) and reducing dust and soot contained in the exhaust gases of thermal plants, before releasing them into the atmosphere; improving the thermal insulation of buildings, to reduce the amount of heat required; using district heating and cogeneration plants (combined production of electricity and heat) that allow a more effective control of polluting emissions. With regard to pollution produced by circulation, there is a tendency to reduce urban traffic by promoting the use of public transport; the amount of tetraethyl lead added to petrol as an anti-knock agent has been reduced, pending its complete elimination.