When we breathe in particle matter, most coarse particles stay in the upper respiratory tract, while fine and ultrafine particles are able to travel all the way down into the tiny lung sacs called alveoli.

As far as we know today, the parameters that play an important role in affecting health are the size and surface area of particles, their number and their composition.

Since they can absorb and transfer a high number of pollutants, the composition of PM varies. However, metals, organic compounds, material of biologic origin, ions, reactive gases, and a carbon core are the major components.

Health effects of PM include:

• Reduced lung function and higher risk of lung infection
• Worsening of respiratory diseases
• Chest pain, fatigue etc. for people with heart disease
• Higher risk of lung cancer

PM uptake in the human body

The human body takes up PM through our respiratory system as we breathe. Although the respiratory system has an advanced way of cleaning itself, PM can deposit and build up, creating damaging effects on our health.

The human respiratory system

The human respiratory system is a series of organs responsible for taking in oxygen and expelling carbon dioxide. Its primary organs are the lungs, which carry out this exchange of gases as we breathe.The respiratory system is divided into two parts:

UPPER RESPIRATORY TRACT:

This includes the nose, mouth, and the beginning of the windpipe, the trachea, through which we draw in air as we breathe.

LOWER RESPIRATORY TRACT:

This consists of the trachea and the lungs, which contain the bronchi, which in their turn branch off into the broncheoli, followed by the smaller alveoli.

The Trachea – a tube shaped windpipe connects the throat to the bronchi.

The Bronchi – the trachea divides into two bronchi (tubes), each leading to one lung. Inside the lungs, each of the bronchi divides into smaller bronchi.

The Broncheoli - the bronchi branch off into smaller tubes called broncheoli, which divide into the pulmonary alveoli.

Pulmonary alveoli – these tiny sacs are lined by a membrane covered by blood capillaries.

The exchange of gases that we call “breathing” takes place through the membrane of the pulmonary alveoli, where oxygen (O2) is absorbed from the air into the blood capillaries, and the beating of the heart circulates it through all the tissues in the body. At the same time, carbon dioxide (CO2) is transmitted from the blood capillaries into the alveoli, travelling out through the bronchi and the upper respiratory tract.

Due to the round structure of the air sacs of the alveoli, the inner surface of the lungs where the exchange of gases takes place is very large, thus helping to take up a large amount of oxygen.[1]

Figure 2F shows the human respiratory system and the most vital parts of it.

PM Deposition in the Respiratory System

Apart from particle size, other factors affect the process of particles accumulating within the lung, such as physiological and environmental factors, lung anatomy, temperature, humidity or breathing patterns.

When particles settle in the respiratory tract they do so through five different mechanisms, which are known as inertial impaction, gravitational sedimentation, Brownian diffusion, interception and electrostatic precipitation.

Inertial impaction takes place in the nose, throat and upper respiratory tract as well as the passages between them, as inhaled particles stick to the walls of the airways.

Gravity is the most important factor in making particles with a diameter ranging between 0.5 and 5 micrometer move down the small airways and alveoli, by a process called gravitational sedimentation.

The larger the particle size and weight, and the longer it has spent time in the airways, the more likely that it will settle here.

Brownian diffusion happens when particles collide with gas molecules, pushing them in different directions. Unlike impaction and sedimentation, the chance of particles settling through Brownian diffusion increases the smaller they are.

Particles less than 0.5 micrometer in diameter are the most likely to settle in alveolar region of the lung through this process. For even smaller particles, less than 0.01 micrometer in size, diffusion also often takes place in the nose and mouth as well as the passages behind and between them.

Finally, electrostatic precipitation happens when charged particles cling to the surfaces of the airways through electrostatic force. Less than 10% of particles settle in the airways of lungs in this way and it usually happens with particles that have a large ratio between length and diameter. Fibers such as asbestos is an example of this type of particle.[2]

Fig 2G and Fig 2H shows where in the respiratory system different sized particles deposit, and due to what kind of deposition mechanism.

Defense Mechanism of the Respiratory System

The respiratory system uses a series of defense mechanisms to clean and protect itself. In the uppermost part of the respiratory tract, particles are easily cleared by coughing and sneezing. Only extremely small particles, less than 3 to 5 microns in diameter, are able to penetrate the deep parts of the lung. Further down the respiratory tract, the lung has two major clearance mechanisms: the mucociliary and the phagocytic system.

MUCOCILIARY SYSTEM

One of the respiratory system’s defense mechanisms involves tiny, muscular, hair-like cilia, which grow on the cells that line the airways. The trachea is covered by a liquid layer of mucus that is carried back and forth by the cilia. These tiny muscles constrict more than 1,000 times a minute, moving the mucus upwards about 0.5 to 1 centimeter per minute.

Particles and bacteria that are trapped in the mucus layer are coughed out or moved to the mouth and swallowed. Particles that are approximately under 5 micrometers in size are taken care of this way.

PHAGOCYTIC SYSTEM

Particles of a fine or ultrafine size of below 0.1 micrometer mainly collect in the bronchioles or alveoli. Since the alveoli need to perform their job of gas exchange, they cannot be covered by mucus and cilia, which would slow down the process. Instead, another defense system is used.

Macrophages are a type of white blood cells that eat fragments of cells, foreign substances, microbes, cancer cells, and anything else that does not have the types of proteins specific to healthy body cells on its surface.

Alveolar macrophages, also known as dust cells, on the surface of the alveoli seek out intruding particles and bind themselves to them, before they swallow them, kill any that are living, and digest them in a process called phagocytosis. They then travel on to dispose of the waste material through the lymphatic system or the mucus “escalator” in the airways.[3]

Since the alveolar macrophages are located at one of the major boundaries between the body and the outside world, their activity is relatively high. However, depending on the concentration of inhaled particles, they may suffer from overload and stay in the alveoli instead of leaving to get rid of the waste. If the inhaled particles are highly toxic, they can also cause direct damage to the alveolar macrophages.

How PM Injures the Human Body

Exactly how particulate matter hurts our bodies is still an emerging area for research. However, evidence suggests that PM is closely connected to cardiovascular diseases. Studies have also shown that PM causes inflammation in our respiratory system as well as oxidative stress caused by free radicals.

Free Radical Peroxidation

Metals, organic components and even the surface of the PM2.5 particles can trigger production of free radicals. A free radical is an atom, molecule, or ion that has an unpaired valence electron. These unpaired electrons make free radicals highly reactive to other substances, which can cause serious health issues.[4]

As an example, it has been shown that some water-soluble particles can produce hydroxyl radicals, which are known for causing damage to DNA. Damaged DNA can lead to mutations and cancerous growths in addition to other permanent conditions.[5]

Oxidative stress is a term for stress on the body caused by antioxidants being unable to stop damage from radicals building up. Ongoing oxidative stress may be involved in respiratory illness like asthma or chronic obstructive pulmonary disease (COPD), but it is also believed to be a cause of other diseases including neurodegenerative disorders (Alzheimer’s disease, Parkinson’s disease, Huntington’s disease) and aging.

The balance of oxidants and antioxidants in our system is an important factor for cellular homeostasis, i.e. the proper and efficient function of a cell. Antioxidants can be created by the body itself or enter it through food or dietary supplements. The lung has a well-developed biological defense system with efficient antioxidant mechanisms against damage from free radicals.[2]

INFLAMMATORY INJURY

Several studies have shown that PM2.5 is related to inflammatory cytokines in the bronchiole and alveoli. Cytokines are a small group of proteins and peptides that are responsible for carrying electrical signals. If production of inflammatory cytokines continues on a large scale, it can contribute to inflammatory diseases, which have also been linked to diseases such as arteriosclerotic vascular disease and cancer.[5] Inflammation and oxidative stress are considered the main reasons for side effects on the lungs caused by breathing in specific types of particles.[2]

INTRACELLULAR CALCIUM IMBALANCE

PM2.5 triggers a significant increase in production of free radicals, resulting in a higher number of Ca2+ concentrations inside the cells. While calcium plays an important role in the cellular functions, too much of it causes inflammatory reactions, leading to inflammation and cell damage.[5]

PM2.5 AND CARDIOVASCULAR DISEASES

There is substantial evidence that long-term exposure to air pollution increases the risk of blood vessel damage, which is connected to many cardiovascular diseases, including heart attack and stroke.[6]

Cardiovascular diseases (CVD) include disorders of the heart and blood vessels, particularly those carrying blood to the brain. Together, these disorders are the leading cause of death across the globe, with low and middle income countries most heavily affected. According to the World Health Organization (WHO), each year about 17.3 million people die of cardiovascular disease, adding up to 30% of all deaths. In addition to risk factors connected to genetics and behavior, the inhalation of air containing fine particulate matter is associated with CVD.[7]

Inhaled particles may cause inflammation of the respiratory tract. PM2.5 inhalation especially can lead to local inflammation in the lungs, which then spreads to the general circulation and affects the entire body’s system, increasing the risk of cardiovascular stress.

Under normal circumstances, the rhythm of the heart is controlled by autorhythmic cells within the heart. Exposure to PM2.5 can also stimulate the autonomic nervous system (ANS) and increase the risk of arrhythmia and acute problems with the heart and blood vessels, which can have serious consequences for elderly people.[7]

Are Smaller Particles More Dangerous?

A growing number of recent studies in toxicology, epidemiology and other related fields have shown a close link between breathing in particles and illness.

While PM already affects more people than any other type of pollutant, there is strong evidence to support that ultrafine and fine particles are more dangerous than larger ones when it comes to mortality and cardiovascular and respiratory effects.

PM between 0.1 μm and 1 μm in diameter can remain in the atmosphere for days or weeks, and are able to move a significant distance through the air in that time. There have been studies that show that exposure to traffic pollution, where ultrafine particles (UFPs) are common, is associated with cardiovascular problems. However, it is not clear whether these issues are caused directly by UFPs, or by some other combination of PM sizes or other pollutants.[8,9]

What has been shown is that an increase in PM2.5 concentrations in the air by 10 micrograms per cubic meter raises the overall death rate by 4%, and deaths from lung cancer by 15-27 %.[5]

There are a number of reasons why smaller particles have a higher potential to damage our health. Their size allows smaller PM to slip past the body’s filter of nose hairs when we breathe in, reaching all the way to the end of the respiratory tract, where they may build up and damage other parts of the body.[5]

Also, though PM2.5 have smaller diameters, together they have a large surface area. Compared to an equal amount of larger particles, the larger surface area of the ultrafine and fine particles can not only carry more toxic matter, but also have an effect on a larger area of lung tissue than an equal mass of larger particles.[10]

In addition, metal components, polycyclic aromatic hydrocarbons (PAHs) and other organic components such as endotoxins present in the particle can make PM2.5 even more toxic.[11] Certain materials that make up ultrafine particles are strongly associated with death from heart attacks. These include copper, iron, other metals, and elemental carbon, commonly known as soot. For several of these materials, they are more likely to cause death by heart attack when they are found in ultrafine particles than in slightly larger fine particles.[12]

There is some evidence that inhaled solid particles below 300 nm can enter the bloodstream from the alveoli and then move on to organs, and even enter cells all the way to the cell nucleus. However, not all studies have been able to show this movement. In the scientific community, many are of the opinion that current scientific evidence shows enough issues with exposure to UFPs that more should be done to protect public health (2015).[8]

FAQs

What are the health effects of exposure to particulate matter?

Exposure to high levels of PM can cause short-term effects like irritation of the eyes, nose, and throat, coughing, wheezing, and shortness of breath. Long-term exposure can cause chronic respiratory and cardiovascular diseases, lung cancer, and premature death. 

What are the sources of particulate matter?

Particulate matter can come from natural sources like dust and wildfires, as well as human-made sources such as vehicles, industrial activities, and burning of fossil fuels. 

What are the different types of particulate matter?

Particulate matter can be classified based on their size, ranging from coarse particles (PM10) to fine particles (PM2.5) and ultrafine particles (PM0.1). Read more about Particulate Matter (PM).

How can we reduce particulate matter

pollution?

Measures to reduce particulate matter pollution include using cleaner fuels, implementing strict emission standards, promoting sustainable transportation, creating green spaces, and educating the public on the effects of particulate matter. 

Conclusion

Particulate matter is a major contributor to air

pollution and poses potential threats to human health and the environment. While particulate matter is present in the atmosphere at all times, high levels of PM

can have serious health effects, particularly on vulnerable populations like children, the elderly, and those with pre-existing respiratory and cardiovascular conditions.

Therefore, it is important to take measures to reduce particulate matter pollution, including using cleaner fuels, implementing strict emission standards, promoting sustainable transportation, creating green spaces, and educating the public on the effects of particulate matter. By taking these steps, we can create a cleaner, healthier, and more sustainable environment for ourselves and give clean air to our next generation.

If you want to learn more about Particulate Matter, read our article "The air we breathe - Particulate Matter (PM)".