Air pollution

What is air pollution?

Air pollution is a form of environmental pollution that includes all physical, chemical, and biological phenomena that change the properties of the air with a consequent alteration of the geophysical balance.
Two types of air pollution can be distinguished, depending on the source:

• Anthropogenic: is pollution that derives from activities related to the presence of human beings
• Natural: is pollution that derives from natural phenomena

Industrial activities, households and mobility are the main sources of anthropogenic environmental pollution and have a significant influence on air quality, especially in large centres due to the high population density and, therefore, the high volume of harmful emissions in a small space.
Pollution of natural origin, on the other hand, includes phenomena that are difficult to predict and generally discontinuous in time, with high emissions for short periods, such as volcanic eruptions, forest fires, lightning storms and organic decomposition. As one can logically imagine, it is difficult (if not impossible) to limit emissions from these phenomena.

What are the main air and gaseous pollutants?

Several substances can have negative effects on air quality, but we generally consider the compounds with the highest emission volumes and, therefore, the predominant impact on air pollution. The best known are:

• carbon oxides (CO and CO2)
• nitrogen oxides (generally referred to as NOx)
• sulphur oxides (generally referred to as SOx)
• particulate dust (or particulate matter)
• volatile organic compounds (VOCs)

What causes air pollution?

The substances described above are emitted to varying degrees by human-induced activities and are mostly byproducts of desirable processes. Their effects can have a variety of consequences for the ecosystem, including the greenhouse effect, acid rain, and the spread of disease.

Looking more closely at the processes responsible for pollutant emissions, we find that essentially all oxides (carbon, sulphur, or nitrogen) result from combustion processes, and their formation is controlled by the presence (or absence) of carbon, sulphur, or nitrogen compounds, as well as by the appropriate pressure and temperature conditions.

When combustion uses a high carbon fuel, it produces higher levels of carbon oxides compared to nitrogen or sulphur equivalents, and conversely, when combustion uses high nitrogen streams, it produces higher levels of NOx. Given the great danger that nitrogen and sulphur oxides pose to the balance of the biosphere, over time increasingly stringent regulations have been imposed on their emissions and plant solutions have been developed to help remove nitrogen or sulphur compounds from processes that generate them before they are released into the atmosphere.

How can you tell if the air is polluted?

A scientific assessment of air quality must be based on measuring the concentrations of various pollutants in it, which is carried out according to criteria and guidelines defined by the technical regulations applicable at the location where the measurement is conducted. Fundamental elements for an evaluation of the air quality index are:

• particulate matter (PM 2.5 and PM10)
• ozone (O3)
• nitrogen dioxide (NO2)
• sulphur dioxide (SO 2)
• carbon monoxide (CO)

The World Health Organisation has updated its air quality guidelines, developed by more than a hundred medical and scientific societies worldwide, to set target levels for six major pollutants: PM2.5, PM10, ozone, nitrogen dioxide, sulphur dioxide and carbon monoxide.

The new WHO guidelines establish air quality levels necessary to protect human health and provide guidance for assessing public exposure to levels of pollutants that may cause health problems. The quantity and quality of studies demonstrating the adverse health effects of air pollution have increased over the past 15 years. For this reason, and after a thorough review of the data collected, the updated air quality guideline values (AQGs) are lower than those proposed 15 years ago. Specifically:

• the annual average of fine particulate matter (PM2.5) falls from 10 to 5 g/m3
• the annual average of inhalable particulate matter (PM10) from 20 to 15 g/m3
• nitrogen dioxide (NO2) drops significantly from 40 to 10 g/m3
• a daily limit of 4 g/m3 is suggested for carbon monoxide (CO)

The document also provides recommendations for good quality practices to manage certain types of particulate matter, such as elemental carbon, ultrafine particles and dust and sand storms, for which there are insufficient quantitative data to determine target values.

It is a wise idea to inform yourself about the legislation in force in your country, as it varies from country to country and often differs widely in terms of the applied technologies, emission limits and more.

Global air pollution: Real-time air quality index

Data from air quality monitoring stations can be easily accessed in real time to find out if and how our area differs from others, near and far. The World Air Quality Index, for example, uses real-time data from more than 10,000 stations around the world to create a comprehensive map of air pollution. The map begins with a calculation of the air quality index based on measurements of particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulphur dioxide, and carbon monoxide emissions.

What are the consequences of air pollution? What are the environmental effects? How does air pollution affect health?

There is much that could be written on the subject of ‘consequences’, certainly different types of impact can be identified depending on the type of pollutant and the concentrations detected in the air. For the purposes of this content, we will limit ourselves to mentioning:

Climate impacts: the greenhouse effect

The phenomenon known as the ‘greenhouse effect’ is based on the ability of molecules in the atmosphere to be transparent, that is, to transmit solar radiation.

The problem arises from the fact that solar radiation, after passing through the atmosphere, ‘bounces back’ to the ground, which absorbs only part of the radiation, but the molecules in the atmosphere, contrary to what they did on the ‘outward” journey, change their behaviour and are no longer transparent to the part of the radiation that comes back.

As a result, the returning solar radiation is absorbed by the molecules in the atmosphere, which send it back in all directions, increasing its energy content and thus its temperature. Carbon dioxide (CO2) is one of the gases responsible for this behaviour. It should be emphasised that compounds such as NOx or methane have a much greater ability to absorb and re-emit solar radiation than carbon dioxide, but again it is important to consider the different concentration and therefore much greater amount of CO2 in the atmosphere compared to the aforementioned compounds.

Impacts on the global ecosystem

The presence of chemical pollutants in the atmosphere can affect the health of animals and plants, thus compromising the balance of the world’s ecosystem.

The presence of nitrogen and sulphur oxides in the atmosphere can cause a drop in the pH of rainfall, which is therefore called ‘acid rain’ and can cause significant damage to vegetation and marine life.

The increased concentration of carbon dioxide in the air also causes acidification of the oceans, which can alter marine communities and reduce the amount of oxygen in coastal waters, thereby obstructing marine life.

Health impacts: the spread of diseases

The negative effects of air pollution have been widely documented and include respiratory problems such as asthma, cardiovascular diseases and mental health problems.

According to World Health Organisation (WHO) estimates, air pollution is responsible for approximately 7 million premature deaths worldwide each year. These premature deaths are mainly caused by cardiovascular diseases, lung cancer and other respiratory diseases.

Studies show that prolonged exposure to air pollution can also increase the risk of dementia, mental problems such as depression and other neurological disorders. In addition, it can also affect the cognitive performance of students and reduce the average hours of sleep per night in the general population.

Volatile organic compounds (VOCs) and fine dust can be responsible for the development of diseases, especially respiratory conditions and, in severe cases, can be carcinogenic, as is the case with many VOCs.

In addition, during the period of the Covid-19 pandemic, a relationship between high particulate concentrations and the spread of the virus was suspected, but no sufficiently comprehensive studies have yet been carried out in this regard.

How can air pollution be reduced?

How can I personally contribute to reducing air pollution?

According to the World Health Organisation (WHO), there are ten ways each of us can help combat air pollution:

I don’t drive during rush hours, I walk to work, I compost my waste, I recycle my rubbish, I don’t burn my waste, I use renewable energy to supply my home, I check air pollution levels daily, I turn off lights and unused electronic and electrical devices.

Pollution control is clearly more complex for manufacturing industries, which must necessarily adopt more sophisticated systems to ensure that emissions from their production processes meet the limits set by national legislation.

In this case, the only solution is the installation of pollutant abatement plants equipped with appropriate technologies that differ according to the type of pollutants to be treated, the volumes of air to be purified, and the limit concentrations to be respected.

Reducing industrial air pollution

There are numerous abatement technologies that are effective in reducing industrial air pollution. Filtration, purification and abatement plants are a particular category of equipment used to reduce concentrations of the main air pollutants generated as by-products of the most diverse industrial processes.

Air pollutant abatement plants

AIR PURIFICATION AND EMISSION ABATEMENT SYSTEMS

Air purification systems are technological devices that reduce the environmental impact caused by industrial activities, ensuring greater sustainability and reducing risks to human health. One of the most common systems is the dust collector, which uses a filtering system to capture airborne solid particles. This type of device is particularly useful in industrial activities that generate large amounts of dust, such as extractive industries.

Another type of air purification system is the fume collector, which removes toxic gases produced by industrial activities. These devices use different techniques, such as combustion, chemical absorption or catalysis, to separate and neutralise harmful gases.

In addition to dust and fume collectors, there are also other types of air purification systems, such as scrubbers, activated carbon filters, oxidation systems and solvent recovery. Each of these devices has specific applications depending on the needs of the industry. Below are the most popular technologies in industry.

POLLUTANT TYPE	ABATEMENT TECHNOLOGY	NOTES
Dust	Dust removal systems	Material moistening or water nebulisation systems so that dust particles, weighed down by the action of water, cannot be carried or lifted by air currents.
Fine dust	Dedusting systems	Usually bag filters or cartridge filters that capture dust-laden gases.
NOx – Nitrogen oxides	SCR devices  (Selective Catalytic Reduction)	Selective Catalytic Reduction (SCR for short) is a chemical process for the abatement of NOx in exhaust gases. SCR devices are used both in industrial combustion and in the internal combustion engines of mobile applications (such as motor vehicles).
SOx – Sulphur oxides	Wet abatement (Scrubber)	Scrubbers are purification technologies that use fluids of various types to achieve the removal of pollutants, typically water-soluble ones.
COV – Volatile Organic Compounds	Thermal oxidation or solvent recovery systems	Depending on the volatile organic compounds to be treated, either thermal oxidation technology (combustion) or recovery technology is chosen, which allows the solvent to be reused in the process itself.
CO – Carbon Monoxide	Thermal oxidation	The thermal process, through high-temperature oxidation, aims to transform harmful components into harmless substances: carbon dioxide (CO2) and water vapour (H2O).

Air pollutant recovery plants

Air pollutant recovery plants, so-called ‘zero-waste’ plants, are part of a production model based on the circular economy, whose fundamental principle is the recycling and reuse of resources, all the more so when, as in this case, these ‘resources’ are pollutants after their release into the atmosphere.

The main technologies for the recovery of air pollutants are based on solutions that separate the pollutant from the air stream and then recover it for use in the same or other cycles.

A typical example are solvent recovery plants (solvents are all members of the family of volatile organic compounds mentioned earlier), which allow companies that consume these products heavily to recover a large portion of them with significant cost savings and a large reduction in environmental impact.

In the field of solvent recovery, two types of equipment are available depending on the characteristics of the pollutant. The choice between solvent recovery with steam or with inert gas depends in particular on the degree of solubility of the solvent in water.

Energy-saving plants

Energy saving is, by nature, a way to reduce air pollution as all energy production technologies have an ecological cost. To confirm this, for decades the most important companies have been investing large sums in the recovery and reuse of thermal energy from the production cycle.

The market has developed the most diverse technological solutions, in the industrial plant engineering field we can mention: closed circuits, steam back, systems for heating thermal oil, steam production and air heating, trigeneration and others.

All of these solutions, basically, aim to use the heat (or frigours) generated as by-products of a given industrial process in favour of another process.