In Focus
February 2013

Airborne particulates: Size does matter

According to the World Health Organization (WHO), airborne particulates affect more people than any other pollutant. Composed of sulfates, nitrates, ammonia, sodium chloride, carbon, mineral dust and water, particulate matter is a complex mix of solids and liquids of organic and inorganic substances. Airborne particulates are classified into two categories: those smaller than 10 microns (PM10) and those smaller than 2.5 microns (PM2.5). The distinction is important.

"Particle size is the most important factor in determining where particles are deposited in the lung," said Nasir Hassan, a Manila-based environmental health team leader for WHO's Western Pacific Regional Office.

Very large particles or those larger than 10 microns end up in the nose. Those between 2.5 and 10 microns lodge in the upper respiratory tract and large airways. PM2.5 particles, in contrast, can make it into terminal bronchioles and alveoli, the smallest passages which are found deep in the lungs.

There, the particles can interfere with the exchange of gases, the process by which life-giving oxygen makes its way into the body and waste carbon dioxide is expelled. Studies have shown that fine particles deposited into the farthest reaches of the lungs increase the possibility of morbidity arising from pneumonia and asthma. They also lead to decreased lung growth and function.

There also are other effects. Data from Brazil, Central and Eastern Europe and China suggest that exposure to particulate matter may be associated with decreased birth weights, according to Hassan.

Information compiled by WHO indicate that cities in Southeast Asia averaged 97 micrograms per cubic meter for particles smaller than 10 microns. That is far above the WHO's recommended maximum of 20 for PM10 and 10 for PM2.5 (See Tables 1 and 2).

Table 1: Guideline Values for Particulate Matter Exposure


10 μg/m3 annual mean

25 μg/m3 24-hour mean


20 μg/m3 annual mean

50 μg/m3 24-hour mean

Source: World Health Organization

Table 2: Guideline Values for Particulate Matter Exposure

Afr: Sub-Saharan Africa; Amr: Americas; Emr: Eastern Mediterranean; Eur: Europe; Sear: South-East Asia; Wpr: Western Pacific; HI: High income; LMI: Low and middle income; PM10: Fine particulate matter of 10 microns or less.

Source: World Health Organization

Data from China's capital city Beijing showed varying PM2.5 levels in 2012. An official monitoring station reported levels below 50, while the U.S. Embassy put the figure often in excess of 300. According to private air quality monitor BeijingAir, the 24-hour average typically was more than 200. In January 2013, severe smog sent the official tally to more than 900, and this bout of extreme pollution lead to a 30% increase in the number of patients in hospitals reporting breathing problems, according to news accounts.

Particulates come from a variety sources, said Axel Friedrich, a Hong Kong-based international transport consultant. Previously, he was head of the transport division for UBA, Germany's environmental agency.

Wood ovens, for instance, emit large quantities of ultrafine particles; there are also other sources, such as construction. But, there is one source that is responsible for the bulk of the particulate matter in the air.

"In most cities, the share of diesel particle emissions on the PM2.5 concentration are in the range of 25 to 30%," Friedrich said. "The vast majority of particles are emitted by diesel engines."

In contrast, petrol engines are a much cleaner technology. Particulates were included as part of regulations for the first diesel European emission standard, Euro 1, when it was rolled out in 1992. Particulate matter standards for petrol engines, on the other hand, were not included until 2009, with the introduction of Euro 5. With the launch of Euro 6 in 2014, particulate controls will need to be in place for all petrol engines.

Some recent studies have shown that the total number of particles is also important in terms of public health, perhaps more so than particle size. If that is the case, then some current emission control strategies may be found wanting. A common approach is to reduce sulphur levels in fuel. Even with sulfur levels at 10 parts per million (ppm), which is considered ultra low, the impact on particle size is minimal, with a 10% reduction in mass and size. There is no change in the number of particles at all.

Diesel particulate filters could prove extremely helpful. In fact, improvements in air quality demand this technology be in vehicles. Otherwise, an increase in the number of diesel vehicles will lead to an increase in the overall particle load. That includes those particles smaller than 2.5 microns, Friedrich said.

"Dieselization of the fleet, without cleaner fuels and advanced emissions control devices, will surely affect PM2.5," said Bert Fabian, transport program manager for Manila-based Clean Air Asia. Reaching and maintaining WHO PM2.5 guideline values will require several initiatives beyond cleaner fuels, he said.

May Ajero, air quality program manager for Clean Air Asia, is in the process of putting together a comprehensive list of what 21 nations across Asia are doing with regard to PM2.5 in the areas of standards, monitoring air quality and reporting results. Some countries have all these elements in place, while others may only have one or two of the three.

As of mid-January, a comprehensive tally of the current status has not been released. With regard to PM10, all three elements are on much firmer ground, a result of the order in which standards were initially put in place by more advanced economies, Ajero said.

"In the course of history, standards for PM2.5 were adopted 10 years later than for PM10," she said.

Clean Air Asia would like to see Asian countries adopt air quality standards within the next few years that are based on WHO targets for daily and annual levels for particulate exposure. There are indications that steps are being taken to make this happen. For instance, in 2012 South Korea conducted a survey of PM2.5 levels, three years before the planned implementation of national standards in 2015. Also, in early 2012 China announced a PM2.5 monitoring plan, which will encompass all cities at the prefecture level and above by 2015.

The benefits of success could be substantial. Today, seven out of 10 cities in Asia don't meet basic WHO air quality targets for PM10, according to Clean Air Asia. Further, each year 800,000 premature deaths in Asia are attributed to air pollution and that number is expected to double in coming years. Thus, reducing airborne particulate matter, including PM2.5, could save millions of lives.

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