By Prof. Alfred Poulos

There is increasing evidence that human and environmental health is affected by exposure to chemicals that are emitted as a consequence of the many human activities. Release of some chemicals, such as carbon dioxide, methane and other greenhouse gases, are believed to have contributed to the gradual increase in global temperatures (global warming) over the last few decades. The climatic consequences of global warming are already being felt.

But the release of greenhouse gases is really just the tip of the proverbial iceberg because many other chemicals are also being released. While the air we breathe consists of mainly of nitrogen, oxygen, carbon dioxide and water vapour, most people would be surprised to learn that both indoor and outdoor air also contain a complex mixture of other chemical substances, the amounts and composition varying according to location, proximity to traffic and industrial activities, and even time of year.

While many of the chemicals present in the air near a secluded beach, in a forest, or on the banks of a river in country areas, are natural substances, many of those present in the air along urban streets, and in indoor air in homes, hospitals, schools, and factories, are the result of human activities. Power generation and motor vehicles in particular contribute to air pollution producing copious amounts of chemical substances not normally found in significant amounts in country areas except via drift from urban areas.

Volatile and particulate matter

Air pollution is made up of volatile (i.e. gaseous material) and particulate matter. The gaseous material includes gases such as nitrogen dioxide (formed from the combustion of fossil fuels or wood), ozone (formed from the interaction of oxygen in the air with nitrogen dioxide and hydrocarbons from fossil fuels), sulphur dioxide (formed from the burning of fossil fuels, smelting, and paper production), carbon monoxide (formed by the burning of fossil fuels), methane (formed by the decomposition of plant matter and produced by livestock), and hydrocarbons (from motor vehicles). Of course, these are just the main gaseous pollutants, but there are many others that are released as a consequence of industrial activities.

The particulate matter (or PM) that is a component of air pollution is just as complex. While some is released into the atmosphere as a consequence of natural processes e.g. volcanoes, forest fires, pollen, and moulds, it is the particulate matter that is a byproduct of human activities, mostly the burning of fossil fuels and other industrial activities, that is of particular concern. These particular PMs, vary considerably in size.

For simplification purposes, the PMs have been thought to behave like spheres with varying diameters and masses. The PMs that have been most studied are those with a size of from 1-10 micrometres, where a micrometer is one millionth of a metre, or considerably smaller than the head of a pin. There is, as well, another type of PM resulting from the chemical reaction of some of the volatile pollutants, for example sulphur dioxide or nitrogen oxides, with each other to form what are referred to as "secondary aerosols". These may take the form of either liquid droplets or solid particles suspended in the air. The levels of these secondary aerosols vary greatly.^1,2 Because of their small size, the PMs are suspended in the air we breathe and hence can find their way into our lungs.

The chemicals that comprise the PMs have been well studied over the last few years. Analysis involves the trapping of the PMs on some sort of filter and then examination of the material that is trapped on the particles. There is an extraordinary mixture of different substances in PMs. They can broadly be divided into two categories – the solid particles and the bound material. The solids may include traces of salt, metals such as copper, iron, aluminium, zinc, manganese, chromium etc., which are normal components of the earth’s crust, or from human activities. The bound material varies according to where samples are taken and may contain substances derived from the burning of fossil fuels, in particular a group of chemicals termed "polycyclic aryl hydrocarbons" or PAH, and related substances that also contain the element nitrogen (termed nitro-PAH). Even pesticides, polychlorinated biphenyls (PCBs), and the flame retardants (PBDEs) have been found in PMs.^3-6

We are not free from PMs even in our homes, schools, shops, and hospitals because indoor air also contains particulate matter as well as volatile organic matter (VOC). The latter may include chemicals derived from many different sources including cooking, cleaning, furniture, carpets, paint, and personal care products.

From the lungs to the blood and then to different organs

Some of the chemical components of PMs have been detected in urine indicating that they can move from the lungs into the blood and then to different parts of our bodies including the kidneys where they are then excreted. But once they enter our bodies are they totally inert or can they contribute to disease? We know that exposure to PM, as well gaseous pollutants such as ozone and nitrogen dioxide can reduce the lung peak respiratory flow (PRF).⁷ The PRF is the measurement of the rate at which air is expelled from the lungs and is believed to correlate with inflammatory changes in the organ. Other researchers have demonstrated that PM exposure increases the production of nitric oxide, a gas formed by the lungs and associated with inflammation.⁸ What is particularly alarming is that chemical indicators of inflammation after exposure to air pollution are not only found in the lungs but also in the blood.^9,10

There is some evidence that air pollution can increase the risk of cardiovascular and other diseases.¹¹ Certainly some of the components of diesel exhaust can increase the formation of thrombus, the material that deposits in blood vessels and leads to blockage followed by either strokes or heart attack.¹² Indeed, the World Health Organisation has estimated that air pollution contributes to as many as 800,000 premature deaths per year.¹¹

Some governments are aware of the threats posed by global pollution – the global warming debate is not just about carbon dioxide – and have introduced measures to limit pollution e.g. alternative energy sources, stricter industrial emission controls, but really effective action is difficult because of the rapid industrialization of countries like India and China, the long lead times required to introduce newer less polluting technologies, and the perception that being green is synonymous with job losses. There is also the view, never expressed overtly, that air pollution is the price that we all have to pay if we want jobs and a better lifestyle.

Unfortunately, if the latter view continues to prevail, some major crisis will be required before governments are forced to confront the threat of air pollution.

References

1. Rhode RA and Muller RA (2015) Air pollution in China. Mapping of concentration and sources. PLoS 10 (8)
2. Guo S et al (2014) Elucidating severe urban haze formation in China. Proc Natl Acad Sci USA 111, 17373-8.
3. Wang X et al (2008a) "A wintertime study of polycyclic aromatic hydrocarbons in PM (2.5) and PM (2.5-10) in Beijing: assessment of energy structure conversion" J Hazard Mater 157, 47-56
4. Wang X et al (2008b) "Organochlorine pesticides in particulate matter of Beijing, China J Hazard Mater 155, 350-357;
5. Cheng JP et al (2007) "Polychlorinated biphenyls (PCBs) in PM10 surrounding a chemical industrial zone in Shanghai, China" Bull Environ Contam Toxicol 79, 448-453
6. Deng WJ et al (2007) "Distribution of PBDEs in air particles from an electronic waste recycling site compared with Guangzhou and Hong Kong, South China 33, 1063-106.
7. Correia-Deur JE et al (2012) "Variations in peak expiratory flow measurements associated to air pollution and allergic sensitization in children in Sao Paulo, Brazil. Am J Ind Med 55, 1087-1098.
8. Strak M et al (2012) "Respiratory health effects of airborne particulate matter: The role of particles size, composition and oxidative potential: The RAPTES Project." Environ Health Perspect 120, 1183-1189.
9. Dutta A et al (2012) "Systemic inflammatory changes and increased oxidative stress in rural Indian women cooking with biomass fuels" Toxicol Appl Pharmacol 261, 255-262
10. Brucker N et al (2013) Biomarkers of occupational exposure to air pollution, inflammation and oxidative damage to taxi drivers. Sci Total Environ 463-464, 884-893.
11. Anderson JO et al (2012) "Clearing the air: a review of the effects of particulate matter air pollution on human health" J Med Toxicol 8, 166-175.
12. Lucking AJ et al (2008) "Diesel exhaust inhalation increases thrombus formation in man" Eur Heart J 29, 3043-3051.

Published in Chain Reaction #133, September 2018. National magazine of Friends of the Earth Australia. www.foe.org.au/cr133