What are PM2.5 particles?
Where do they come from, what are their dangers, and why is there such a fuss about them? Why do they deserve so much attention from everyone who cares about health? Watch out, it's a longread!

The Origin of the Particles

PM2.5 particles are suspended solid microparticles and tiny liquid droplets (10 nm to 2.5 µm in diameter) found in the air.

Why are they the focus of experts' attention? In contrast to large particles, they penetrate biological barriers and therefore pose a threat to human health.

Fine particulate matter remains in the air in the aerosol state and in the natural environment, but it is in megacities that they pose the greatest danger. In the urban environment there are more of them, the chemical composition of fine aerosols is much more toxic than in the wild. The composition of PM2.5 aerosol differs in different megacities, as do the parameters of the particles that make it up.
What is the origin of fine particulate matter? How do they appear?
Primary
They are small pieces of soot, car tires and asphalt, nitrates, sulfates, heavy metal oxides. Speaking of soot, charcoal is a good adsorbent. Toxic compounds are deposited on soot particles when the engine is running. As a result, soot takes on a "toxic" content.
Secondary
They appear due to the chemical reaction of oxides of nitrogen and sulfur with water vapor in the atmosphere. Acids are produced first, and then the salts of the nitrate and sulfate groups are precipitated from them.
In terms of the source fine particles have:
Anthropogenic (man-made) origin.
The main source of such particles is transport with internal combustion engines, as well as industrial plants, construction sites, mining, asphalt erosion, abrasion of tires and brake pads. Moreover, the emissions can be released into the air due to agricultural activities, since ammonia fertilizers are often used there.
Non-anthropogenic (natural) origin
Fine suspended particles come from soil erosion in steppes, deserts, and organic evaporation.

How much fine dust is in the air?

To assess the quality and danger of the air to human health, the mass concentration of PM2.5 particles is a key parameter. According to WHO standards, the average annual level of fine particles should not exceed 10 µg/m3 and should not exceed 25 µg/m3 a day.

So what is the cause of the fine particles fuss?

As far as China is concerned, everyone knows about fine particles. But in other parts of the world they are unfortunately becoming known as well. Fine suspended particles are dangerous, their threat to life and health is becoming increasingly apparent. The statistics are grim: 3 million people died between 1990 and 2010 from PM2.5 related causes. Inhalation of air with PM2.5 particles causes 3% of deaths from respiratory and cardiovascular diseases, and 5% of deaths from lung cancer.

Fine dusts are also dangerous because there is no immediate negative reaction when they are inhaled on a daily basis. They have a cumulative effect that leads to serious health problems. That is why the WHO report talks about the chronic effects of these particles on the human body, under which the inhabitants of megacities are every day.

City air contains particles of varying diameters, the larger ones settling to the ground over time. At the same time it is difficult for PM2.5 particles to overcome resistance of the air environment to settle down, and the smallest particles move in chaos of Brownian motion. Partially fine dust settles down after the rains, but in large cities these particles accumulate from various sources, and if there were no wind in the city, then literally "bricks" would fall from the sky. Only HEPA filters trap PM2.5 particles.
Effects on the body of fine suspended particles

As stated above, fine suspended particles are inhalable. It enters the lungs directly into the alveoli, where gas exchange occurs between the lungs and blood vessels. During the gas exchange PM2.5 particles get into the blood, which provokes the occurrence of cardiovascular diseases. Both the particles themselves and substances sorbed on the surface of the particles are affected.

Let us focus on two hypotheses of the effect of PM2.5 particles on the cardiovascular and respiratory systems put forward by scientist Peters A.

Hypothesis #1: the "immediate" response (2 hours later)

Based on the fact that in response to the particles hitting the respiratory system, a reflex is triggered, the intensity of breathing and heart rhythm changes. Thus, the nervous system reacts. Bottom line: instant response to a foreign stimulus.

Hypothesis #2: the "slow" response (24 hours later)

Fine particles are deposited in the lungs and blood vessels, leading to inflammation. This increases the number of cytokine proteins in the blood, triggering a chain of biochemical reactions. And this leads to thrombosis, coronary heart disease, heart attack.

To support the hypotheses with facts, studies were conducted on the behavior of fine dust in the body, which revealed six main mechanisms of harmful effects of PM2.5:
1
Respiratory rate, cardiac arrhythmia due to excitation of pulmonary receptors
This mechanism is based on a response to a stimulus entering the body. Thus, the bronchi are narrowed and coughing becomes more frequent, there is a scratchy throat and irritation in the chest cavity. As for cardiac arrhythmia, a correlation has been found between increased fine particulate matter and the incidence of such a disease in humans.
2
Destruction of the pulmonary epithelium
Fine dust entering the lungs destroys the surface of the alveoli, which are responsible for gas exchange, and this causes hypoxia or hypercapnia.
3
Inflammation as a response
When PM2.5 particles enter the body, the brain commands the bone marrow to get rid of the foreign bodies. In response to the signals from the brain, the bone marrow releases immune bodies - macrophages. They envelop the particles, causing inflammation in the lungs.
4
Blood clotting is increased
Fine dust can also cause this effect, because along with the bone marrow secreting macrophages, the liver secretes the protein CRP, which increases blood clotting, which in turn can lead to thrombosis.
5
Destabilization of lipid deposits
Because macrophages can break down vessel walls in addition to fine particles, the fibrous "covering" becomes thin, and an atherosclerotic plaque can escape into the vessel lumen, creating an even greater risk of thrombosis.
6
Vasoconstriction
Scientists have proven the relationship between fine dust and arterial vasoconstriction, which in turn leads to atherosclerosis.
So how is fine particulate matter detected?

There is a PM2.5 contamination index, which shows the content of these particles in the air. Many countries of the world already have special services of ecological monitoring of the air environment, for example, online monitoring The World Air Quality Index.

Information on the PM2.5 pollution index is available in Europe, USA and China, where the particle content is shown simultaneously with the weather forecast. In Russia, unfortunately, there is no freely available information on the presence of particles. However, this does not mean that there is no fine dust in the air. PM2.5 can be measured by portable air quality analyzers, which makes it possible to measure the particle level practically for everybody.

How is PM2.5 doing in cities around the world?

According to WHO, the US and China are the leaders in PM2.5 particle counts in the atmosphere, but Russia should not be written off either. Back in Soviet times, scientists were concerned about the amount of smog in the air. Now, if environmental monitoring data are not published, that does not mean there are no fine particles in the air in Moscow or St. Petersburg. Another thing is that in other Russian cities PM 2.5 is enough. Everything depends on the season and weather conditions. Smog also occurs in other major cities of the former Soviet Union: Kiev, Minsk, Bishkek, Baku, and others. Unlike China and the U.S., in Russia no significant measures are taken to inform the population, so it is rare to meet a person with a respirator mask here. But the harm from smog does not go anywhere, and the problem does not disappear.

Another thing is that while smog in Moscow is considered an emergency situation, in Beijing, Guangzhou, Shanghai it occurs almost every day. People have learned to protect themselves from exposure, but they can't change anything. The ecosystems of these cities are so overloaded with such particles that only favorable weather conditions can alleviate the situation.

Conclusion

To sum up, we would like to mention that the danger of PM2.5 particles lies in their cumulative gradual effect on the organism, because no one will risk testing the above hypotheses on a man. City dwellers, for the most part, inhale contaminated air, but at home or in nature the organism has time to be cleaned of PM2.5. Then all the above-mentioned problems can be avoided. If the PM2.5 particles accumulate faster than the purification occurs, however, there is a delayed effect.

Statistics show that each day a metropolis resident inhales 200 billion PM2.5 particles, half of which are deposited in the lungs. One such dose would not cause serious effects. If you are constantly exposed to them, however, you should consider protecting yourself from PM2.5 particles. Mask respirators in the street or HEPA filters in the supply ventilation system of houses can help to save you from them. You will hardly need a mask-repirator in Moscow, but you should not forget about it in Beijing, Tokyo, Los Angeles. It is not unreasonable to take care of the ventilation in the cities, because we spend most of our lives at home.

Follow the recommendations and take care of your health!