There are not only known sources of PM 2.5 production such as heat engines or boilers, but also many other aspects that we should know better

Although we have taken the path of environmental awareness , the enormous mass of interventions that we must do to make the air we breathe not excellent but at least less harmful, still needs to be unraveled.

With every step forward , such as the European regulations for the electrification of private and commercial mobility, the production of renewable energy from wind, hydraulic, solar and green hydrogen, it seems there is also a step backwards , due to the international crises that have undermined the energy independence of many states, with the use of electricity production with systems, such as coal, which were on the way to being dismantled.

Unfortunately, in the meantime, we have to record the continuation of negative environmental situations, especially in large cities, which make the air a killer for public health, causing numerous cancers for the population.

This is because smog contains agents recognized as carcinogens, such as fine particles of PM 2.5 and PM 10, polycyclic aromatic hydrocarbons, benzene, formaldehyde and heavy metals such as arsenic, cadmium and nickel.

Although the municipalities of large European cities and, above all, those in the Italian Po Valley, among the most polluted areas in Europe, are making many efforts to reduce the concentrations of pollutants in the atmosphere through the creation of pedestrian areas, traffic zones limited, the reduction of vehicle speed in the city, the strengthening of public transport, the encouragement of bike sharing, the creation of cycle paths where possible, the restrictions on boiler emissions and the facilitation of electric car traffic, what is still missing it is the mentality of citizens to actually do something that can help the community and their own health.

In many countries and cities, private traffic does not lose supporters, thus moving in unison with our private cars, creating congestion and unnecessary pollution.

What is PM 2.5

From a chemical point of view, particulate matter is composed of three main classes:

- inorganic ions: sulphates (SO42-), nitrates (NO3-), ammonium (NH4+)

- the carbonaceous fraction (TC) formed by organic carbon and elemental carbon

- the crustal material that may occur or be associated with atmospheric dust (Si, Ca, Al, etc.) or trace elements (Pb, Zn, etc.);

- an unidentified fraction which often corresponds to water but not only.

These components, which together constitute particulate matter, have different sizes and therefore contribute differently to the production of PM 2.5 or PM 10.

Speaking of PM 2.5 we can say that they are atmospheric particles with a diameter of 2.5 micrometers or less, these fractions, in fact, are extremely small and can be inhaled deeply into the lungs and even enter the bloodstream.

Where does PM 2.5 come from?

Private vehicle traffic and heating boilers are an important component for the generation of PM 2.5, but we must also consider industrial areas near cities, heavy and commercial traffic traveling close to and within them, and the age-old problem of air traffic which has a major impact on an urban area, as every large city usually has an airport nearby.

Then there are completely personal behaviors, such as cigarette smoking, which can accumulate with other risk factors that we have seen, making people's lives more precarious.

The synthesis of these problems on human health, linked to smog, can simply be summarized in the presence of PM 10 and PM 2.5 particulate matter that forms in the air, of which PM 2.5 is certainly the most dangerous.

There are also lesser-known polluting aspects to consider that affect fine particles in urban centres, in fact, when we talk about PM 2.5 produced by vehicular traffic we are immediately led to think of emissions from thermal engines, but there are also other sources of pollution that we must keep in mind.

Production of PM 2.5 from tires

The tires of cars or other means of transport are a sum of products of different nature which, through their rolling, allow a vehicle to move.

This rolling involves continuous abrasion of the tire surface releasing small or very small particles of compounds.

To understand what we can inhale from tires, in the form of fine wear dust such as PM 2.5, let's see how it is composed:

Rubber is the primary component of tires, which can be a mixture of natural rubber and synthetic rubber. Natural rubber offers elasticity and flexibility, while synthetic rubber can improve resistance to wear and aging.

Carbon black is a form of particulate carbon, added to the rubber mixture to improve the wear resistance and traction properties of the tire. It also serves as a strengthener and as a coloring agent.

Silica is used as an alternative reinforcement or in addition to carbon black, improving wet traction and rolling resistance, therefore bringing greater fuel efficiency.

In the "carcass" of the tire, i.e. the internal structure that gives shape and flexibility to the tire, textile materials such as polyester, nylon or rayon are often used.

In the "belt" of the tire, there are a series of layers placed between the casing and the tread, into which steel wires are often placed to provide reinforcement and stability.

Sulfide is used in the vulcanization process, helping to stabilize the molecular structure of the rubber, therefore making it more resistant and elastic.

Furthermore, various chemical additives are used to improve its general properties, among these we can mention antioxidants to prevent aging, plasticizers to improve flexibility, and accelerators that help in the vulcanization process. It is also possible to find other materials such as zinc, sulfur and other organic compounds

Production of PM 2.5 from vehicle brakes

The same considerations as for tires also apply to brakes, as the rotation of the brake disc on its calipers creates friction with the relative wear of the two parts in contact, which causes the release of PM 2.5 fine dust.

The ultra-fine particulate that is released by a series of braking could be released into the air and breathed by humans, which is why let's see how a braking system is made up to understand the waste it produces:

First of all we must consider that even the braking system, discs and pads, are made up of many different materials. As for the brake pads contained in the calipers, the components are bonded together by thermosetting resins, hard and resistant materials that have the ability to contain various products. When the pads are subjected to heat during the braking process, the binders help maintain their structural integrity.

Due to the high effort exerted by the braking system between pad and disc, it is necessary to use reinforcing products, these are often fibres, such as glass fibre, aramid fiber or carbon fibre. These strengtheners give greater mechanical strength to the pads and help prevent breakage and cracking.

Furthermore, components such as graphite or various types of metals such as copper, zinc or bronze are added to improve the friction performance of the pad. These materials help maintain a clean, rough surface on the brake disc.

During braking, friction generates heat, and this is why the use of a mixture of various metals, such as copper, zinc, aluminum or iron, is envisaged. Metals serve various purposes, including heat conduction, friction enhancement, and wear resistance.

As regards stabilization and noise reduction, materials such as molybdenum, disulphide or graphite are used.

Production of PM 2.5 from road surface wear

The road surface also undergoes a frictional effort from the tires causing wear and tear of the final mat, in the form of micro particles of bituminous compounds which can be released into the air, causing the possible breathing of PM 2 particles, 5 by man.

To understand what we can breathe near a road artery, let's see how a road surface is made to understand which components are released into the air.

Leaving aside the deeper stratification which hardly comes into contact with the tyres, let's focus on what is called the final mat, the surface where friction with the means of transport occurs.

The final mat is composed mostly of bitumen, a viscous, black, adhesive material derived from the distillation of crude oil. Bitumen acts as a binder, holding the other components of the road surface together and providing waterproofing.

This layer of bitumen incorporates a series of chemical additives such as:

Plasticizers: to improve the flexibility of the road surface

Stabilizers: to improve resistance to wear and deformation

Anti-aging agents: to increase the life of the road surface

Regenerating agents: Recycled materials, such as milled asphalt, that can be reintroduced into the mix

What are the effects of PM 2.5 on human health

Inhalation of PM 2.5 can cause irritation to the respiratory tract and aggravate chronic conditions such as asthma, bronchitis and other lung diseases.

In fact, depending on their ability to pass through the human respiratory system, fine particles can be broken down into:

- "inhalable fraction", which can reach the pharynx and larynx following inhalation through the mouth or nose, and includes practically all the particulate matter

- "thoracic fraction", which is able to reach the trachea and bronchi

- "respirable fraction" to indicate the class of smallest particles that are able to reach the alveoli and pass through them into the blood

Furthermore, there is evidence linking exposure to PM 2.5 to cardiovascular diseases, including heart attacks, strokes and other heart diseases, compromising the immune system, making people more vulnerable to infections.

But as previously described, PM 2.5 contains carcinogens, and long-term exposure has been linked to an increased risk of cancers, particularly lung and prostate cancer.

An impact on the nervous system of PM 2.5 pollution has been noted through some research aimed at considering the relationship of this pollutant with the increase in cases of Alzheimer's.

Even from the point of view of reproduction, there is evidence that such fine particulate matter, stored for long periods, can lead to an increase in premature births.

On the more negative side, some epidemiological studies have shown that areas with high levels of PM 2.5 tend to have higher mortality rates.

However, we can say that the effects of PM 2.5 on human health may vary based on age, general health and other individual factors. Children, the elderly and people with critical health conditions may be particularly vulnerable.