Particle pollution: Size does matter

Every millilitre of air that we breathe contains a few thousand very small, solid particles. At rest, we inhale about 700 millilitres of air with every breath. That is over a million particles in every breath. When the body is exerted, our breathing rate increases, and the number of particles inhaled may be up to three times higher.

These atmospheric particles, or aerosols, originate from a number of natural sources, including ground dust, sea salt, pollen and volcanic eruptions, forest fires, as well as from anthropogenic sources such as motor vehicles, industrial processes and wood burning.

During recent years, the contribution to the aerosol budget by human activities has grown exponentially. Today, it is estimated that the main source of particulate pollution in urban environments is from motor vehicles. This poses significant and growing air pollution and health threats. Standing by the side of a busy road, a person may inhale about 40 million particles with each breath. And, if you happen to get a whiff from the exhaust plume of a passing lorry, that number could go up by a factor of ten! In some cities, like Hong Kong, Los Angeles and Mexico City, the situation has reached such proportions that emergency legislation is being enforced to arrest the problem.

How serious is the problem here

Today, we have around 2.5 million motor vehicles plying 12,000 km of tarred road. This includes 300,000 cars, 80,000 buses and nearly 250,000 goods vehicles, lorries and vans. Petrol, used in cars and motor cycles contain lead compounds, which are cumulative poisons as they slowly build up within the body. Many countries are now adopting unleaded petrol to mitigate this problem.

Most of the particulate pollution comes from medium and heavy duty vehicles that operate on diesel. Diesel emissions are often seen as thick black clouds behind vehicles on our roads. These soot particles are coated with a noxious cocktail of dangerous chemicals like benzenes, formaldehydes and a particularly nasty group of chemicals that are classified as polynucleic aromatic hydrocarbons or PAH’s for short. Also present are other volatile organic compounds as well as inorganic compounds like sulfuric acid.

Many of these are known carcinogens or cancer-causing substances. These dangerous chemicals coat the particles which are then inhaled deep into our lungs. Particulate pollution has been consistently associated with increased cardiovascular and respiratory diseases and mortality. Although the association is well established, the mechanisms responsible for these adverse health effects have not been well-elucidated.

Mass versus size

Standards for emission control are based on particle mass. That is, there are regulations that restrict the mass of particle pollution emitted by a vehicle. For example, currently, European Union emission standards for heavy-duty vehicles limit particulate emission to 20 milligrams per kilowatt-hour of energy expended by the engine.

This may sound technical to the layman; but the point that I am making is that the legislative standards are based on particle mass and say nothing about particle size or number. There is an important distinction here. Look at the accompanying diagram. If you put together one thousand small water drops, they will form a single water drop 10 times larger in diameter. In other words, the mass of one large water drop is the same as the combined mass of a thousand drops ten times smaller in diameter. However in this example, the combined surface area of the one thousand small water drops is a hundred times larger than the surface area of the large drop.

The same thing applies to aerosol particles. A vehicle may emit a certain mass of particulates which may consist of either a small number of large particles or a large number of small particles. Using a particulate mass sensor to monitor emissions from two vehicles, we may deduce that Vehicle A is the cleaner emitter if it emits less particulate mass than Vehicle B. However, Vehicle A may in fact be emitting many more particles than Vehicle B. Which then is the ‘cleaner’ vehicle?

Why size matters

The size of a particle is important in two ways. First, for a given mass of particulates, a large number of small particles present a much larger surface area than a small number of larger particles. This allows a larger surface area for the dangerous chemicals to adhere to. Secondly, when we inhale, larger particles are trapped in the nostrils and upper airways and are prevented from entering our respiratory system.

However, the smaller the particle, the deeper it can penetrate into our lungs, carrying any toxic material with them. Once they get in there, they can cause a lot of adverse health effects, like pulmonary inflammation. They may provoke the release of toxic free radicals; and they can even pass through interstitial layers to enter the bloodstream. Leaving aside the technical jargon, all it means is that they can do a whole lot of damage to your health!

Diesel particles

The diagram shows what we call a number-size distribution of the particles present in the emissions from a diesel bus. The number of particles is shown as a function of particle size, which is given in nanometers (nm). A nanometer is a millionth of a millimetre. Gas molecules are a tenth of a nanometer to 1 nm in diameter, while virus particles are a few tens of nanometers across. Typical vehicle emission particles range in size from a few nanometers to a hundredth of a millimetre. Most of the soot particles that we see are a few hundred nanometers in size.

The diesel particle size distribution is bi-modal, that is it has two peaks. The peak on the right, at about 60 nm is composed of black carbon soot. Note the second peak that occurs at smaller sizes, at around 10 nm. This second peak is often observed when a vehicle is accelerating hard or labouring under a heavy load. A substantial number of the total particle count is found here, and they are known as nanoparticles. They are too small to be seen with the naked eye.

The existence of nanoparticles in vehicle emissions was not known until recently, after scientists had developed instrumentation that could measure such small particles.

Today we can measure nanoparticles in the air down to a size of about 1 nm.

Because of their small size, nanoparticles do not contribute substantially to the total mass of particulates and they can go largely undetected by instruments that measure particulate mass. As vehicle emission standards are governed by particulate mass, they are not controlled by legislation. Nanoparticles easily pass through conventional face masks and filters designed to trap germs and viruses. They penetrate deep into the human lungs and cause adverse health effects that are of increasing concern to environmentalists and the medical profession.

For this reason, some countries, notably Switzerland, are today advocating strict emission control guidelines incorporating particle number and size, to complement existing measures to control emissions of particulate mass.

(The writer is attached to the International Laboratory of Air Quality and Health, School of Physical Sciences, Queensland University of Technology, Brisbane, Australia, and has been working on environmental related research for many years)