Airborne particulate is a major component of urban air pollution. Anthropogenic sources include combustion within car engines, solid-fuel combustion in households, industrial activities (such as building, mining, manufacturing of cement, ceramics and bricks, and smelting), quarrying and mining.
Epidemiological evidence indicates a clear relationship between exposure to particulate matter and effects on health, particularly smaller particles that can reach the deep regions of the lungs.
Table 1 – Health effects attributed to exposure to suspended particulate matter
|Acute Effects|| Chronic Effects
|Lung inflammatory reactions||Increase in lower respiratory symptoms|
|Respiratory symptoms||Reduction in lung function in children|
|Adverse effects on the cardiovascular system||Increase in chronic obstructive pulmonary disease|
|Increase in hospital admissions||Reduction in lung function in adults|
|Increase in mortality||Reduction in life expectancy|
|Increase in medication usage|
Source: Health aspects of air pollution: Results from the WHO project “Systematic review of health aspects of air pollution in Europe” (WHO, 2004)
With particulate matter, the size of the particles is very important, determining both how long particles remain airborne to be inhaled, and whether they reach the deep regions of the lung where they can be absorbed. Only particles smaller than about 10microns (one thousandth of a millimetre) will reach the alveoli. Larger particles are deposited higher up in the respiratory system and removed on the mucocilliary escalator, but may then be swallowed and subsequently absorbed through the gastro-intestinal tract.
In practice, every individual will inhale a different fraction of a given dust cloud, and this can also vary depending on the particular environment and activity undertaken during exposure. However it is useful to define size selection criteria to use when sampling for airborne particulates, and this task has been undertaken by the International Standards Organisation in ISO 7708:1995 “Air quality — Particle size fraction definitions for health-related sampling”
“Inhalable” particulate is the fraction of airborne material which enters the nose and mouth during breathing. Having entered the respiratory system the particles can potentially be deposited anywhere within the respiratory tract. However only very small particles can penetrate deep into the lung . The “thoracic fraction”, describes the mass fraction of particles penetrating beyond the larynx and the fraction of particles reaching the deep lung is referred to as “respirable fraction”.
When sampling for environmental pollutants, two other fractions are used – PM10 and PM2.5.
Carry out a search on the Internet for “PM10” and you’ll find a large number of sites that mention and define the term. For example:
“The notation PM10 is used to describe particles of 10 micrometers or less and PM2.5 represents particles less than 2.5 micrometers in aerodynamic diameter.”
“PM10 is the term used to describe tiny particles in the air, made up of a complex mixture of soot, organic and inorganic materials having a particle size less than or equal to 10 microns diameter (10 microns is equal to one hundredth part of a millimetre).”
“PM10 stands for Particulate Matter of less than 10 millionths of a metre (10 micrometers or 10 um) in diameter.”
Unfortunately (and disappointingly), all these definitions are incorrect. PM10 and PM2.5 are defined by the International Standards Organisation as follows
PM10 – particles which pass through a size-selective inlet with a 50 % efficiency cut-off at 10 μm aerodynamic diameter. PM10 corresponds to the “thoracic convention” as defined in ISO 7708:1995, Clause 6.
PM2.5 – particles which pass through a size-selective inlet with a 50 % efficiency cut-off at 2,5 μm aerodynamic diameter. PM2,5 corresponds to the “high-risk respirable convention” as defined in ISO 7708:1995, 7.1.
So, in fact, PM10 is a standard size fraction where the median diameter is 10 microns. This means that 50% of the particles in this fraction have diameters greater than 10 microns. Similarly with PM2.5, 50% of the particles have a diameter greater than 2.5 microns.
I accept that trying to get the concept of particle size fractions across to the general public is not easy, and it’s often necessary to resort to simplifications. However, I think its inexcusable for the same erroneous definitions to be used in scientific publications and presentations and on websites aimed at environmental scientists and professionals.