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Size, shape and solubility matter
The size of the particle is a main determinant of where in the respiratory tract the particle will come to rest when inhaled. Larger particles are generally filtered in the nose and throat via cilia and mucus, but particulate matter smaller than about 10 micrometers, can settle in the bronchi and lungs
and cause health problems. The 10-micrometer size does not represent a
strict boundary between respirable and non-respirable particles, but has
been agreed upon for monitoring of airborne particulate matter by most
regulatory agencies. Because of their small size, particles on the order
of 10 micrometers or less (coarse particulate matter, PM10) can penetrate the deepest part of the lungs such as the bronchioles or alveoli; when asthmatics are exposed to these conditions it can trigger bronchoconstriction.
Similarly, so called fine particulate matter (PM2.5), tend to penetrate into the gas exchange regions of the lung (alveolus), and very small particles (ultrafine particulate matter, PM0.1)
may pass through the lungs to affect other organs. Penetration of
particles is not wholly dependent on their size; shape and chemical
composition also play a part. To avoid this complication, simple
nomenclature is used to indicate the different degrees of relative
penetration of a PM particle into the cardiovascular system. Inhalable particles penetrate no further than the bronchi as they are filtered out by the cilia. Thoracic particles can penetrate right into terminal bronchioles whereas PM0.1, which can penetrate to alveoli, the gas exchange area, and hence the circulatory system are termed respirable particles.
In analogy, the inhalable dust fraction is the fraction of dust entering
nose and mouth which may be deposited anywhere in the respiratory
tract. The thoracic fraction is the fraction that enters the thorax and
is deposited within the lung's airways. The respirable fraction is what
is deposited in the gas exchange regions (alveoli).
The smallest particles, less than 100 nanometers (nanoparticles), may be even more damaging to the cardiovascular system.
Nanoparticles can pass through cell membranes and migrate into other
organs, including the brain. Particles emitted from modern diesel engines (commonly referred to as Diesel Particulate Matter, or DPM) are typically in the size range of 100 nanometers (0.1 micrometer). These soot particles also carry carcinogens like benzopyrenes
adsorbed on their surface. Particulate mass is not a proper measure of
the health hazard, because one particle of 10 µm diameter has
approximately the same mass as 1 million particles of 100 nm diameter,
but is much less hazardous, as it is unlikely to enter the alveoli.
Legislative limits for engine emissions based on mass are therefore not
protective. Proposals for new regulations exist in some countries, with suggestions to limit the particle surface area or the particle count (numerical quantity) instead.
The site and extent of absorption of inhaled gases and vapors are
determined by their solubility in water. Absorption is also dependent
upon air flow rates and the partial pressure of the gases in the
inspired air. The fate of a specific contaminant is dependent upon the
form in which it exists (aerosol or particulate). Inhalation also
depends upon the breathing rate of the subject.
Another complexity not entirely documented is how the shape of PM can affect health, except for the needle-like shape of asbestos
which can lodge itself in the lungs. Geometrically angular shapes have
more surface area than rounder shapes, which in turn affects the binding
capacity of the particle to other, possibly more dangerous substances.
Health problems
The effects of inhaling particulate matter that has been widely studied in humans and animals include asthma, lung cancer, respiratory diseases, cardiovascular disease, premature delivery, birth defects, low birth weight, and premature death.
Inhalation of PM2.5 – PM10 is associated with elevated risk of adverse pregnancy outcomes, such as low birth weight. Maternal PM2.5 exposure during pregnancy is also associated with high blood pressure in children. Exposure to PM2.5 has been associated with greater reductions in birth weight than exposure to PM10.
PM exposure can cause inflammation, oxidative stress, endocrine
disruption, and impaired oxygen transport access to the placenta, all of which are mechanisms for heightening the risk of low birth weight. Overall epidemiologic and toxicological evidence suggests that a causal relationship exists between long-term exposures to PM2.5 and developmental outcomes (i.e. low birth weight). However, studies investigating the significance of trimester-specific exposure have proven to be inconclusive,
and results of international studies have been inconsistent in drawing
associations of prenatal particulate matter exposure and low birth
weight. As perinatal outcomes have been associated with lifelong health
and exposure to particulate matter is widespread, this issue is of
critical public health importance and additional research will be
essential to inform public policy on the matter.
Increased levels of fine particles in the air as a result of anthropogenic particulate air pollution "is consistently and independently related to the most serious effects, including lung cancer and other cardiopulmonary mortality." A large number of deaths and other health problems associated with particulate pollution was first demonstrated in the early 1970s
and has been reproduced many times since. PM pollution is estimated to
cause 22,000–52,000 deaths per year in the United States (from 2000) contributed to ~370,000 premature deaths in Europe during 2005. and 3.22 million deaths globally in 2010 per the global burden of disease collaboration.
A 2002 study indicated that PM2.5 leads to high plaque deposits in arteries, causing vascular inflammation and atherosclerosis – a hardening of the arteries that reduces elasticity, which can lead to heart attacks and other cardiovascular problems.
A 2014 meta analysis reported that long term exposure to particulate
matter is linked to coronary events. The study included 11 cohorts
participating in the European Study of Cohorts for Air Pollution Effects
(ESCAPE) with 100,166 participants, followed for an average of 11.5
years. An increase in estimated annual exposure to PM 2.5 of just 5 µg/m3 was linked with a 13% increased risk of heart attacks.
In 2017 a study revealed that PM not only affects human cells and
tissues, but also impacts bacteria which cause disease in humans. This study concluded that biofilm formation, antibiotic tolerance, and colonisation of both Staphylococcus aureus and Streptococcus pneumoniae was altered by Black Carbon exposure.
The World Health Organization (WHO)
estimated in 2005 that "... fine particulate air pollution (PM(2.5)),
causes about 3% of mortality from cardiopulmonary disease, about 5% of
mortality from cancer of the trachea, bronchus, and lung, and about 1%
of mortality from acute respiratory infections in children under 5
years, worldwide.". A 2011 study concluded that traffic exhaust is the single most serious preventable cause of heart attack in the general public, the cause of 7.4% of all attacks.
The largest US study on acute health effects of coarse particle
pollution between 2.5 and 10 micrometers in diameter. was published 2008
and found an association with hospital admissions for cardiovascular
diseases but no evidence of an association with the number of hospital
admissions for respiratory diseases. After taking into account fine particle levels (PM2.5
and less), the association with coarse particles remained but was no
longer statistically significant, which means the effect is due to the
subsection of fine particles.
Particulate matter studies in Bangkok Thailand from 2008
indicated a 1.9% increased risk of dying from cardiovascular disease,
and 1.0% risk of all disease for every 10 micrograms per cubic meter.
Levels averaged 65 in 1996, 68 in 2002, and 52 in 2004. Decreasing
levels may be attributed to conversions of diesel to natural gas
combustion as well as improved regulations.
The Mongolian government agency recorded a 45% increase in the
rate of respiratory illness in the past five years (reported in
September 2014). Bronchial asthma, chronic obstructive pulmonary disease
and interstitial pneumonia were the most common ailments treated by
area hospitals. Levels of premature death, chronic bronchitis, and
cardiovascular disease are increasing at a rapid rate.
A study In 2000 conducted in the U.S. explored how fine
particulate matter may be more harmful than coarse particulate matter.
The study was based on six different cities. They found that deaths and
hospital visits that were caused by particulate matter in the air were
primarily due fine particulate matter.
Effects on vegetation
Particulate matter can clog stomatal openings of plants and interfere with photosynthesis functions.
In this manner, high particulate matter concentrations in the
atmosphere can lead to growth stunting or mortality in some plant
species.
