Look beyond the visible air pollution

According to the World Health Organisation (WHO), more than 750000 Chinese people die every year from air pollution-related diseases. A number of times already this year, residents of China’s major cities were repeatedly advised to stay indoors or wear masks while outdoors. Unfortunately, air pollution is not limited to China. Both developing and developed countries face dangerous levels of air pollution, but the most dangerous kind of air pollution is invisible. 

Deceptively clean-looking air can carry toxic trace elements, tiny particles that people breathe in or that penetrate the skin and spread through the body. When they are inhaled, small toxic air particles find their way into people’s lungs and enter the blood. If they are absorbed through the skin, these small particles also find their way into a person’s blood. Once in the blood system, air pollutants bind themselves to either red blood cells or plasma proteins and are transported to the body’s organs, such as the heart, kidneys and bones. 

But how can we legislate and regulate pollution that we cannot see? Our research group at Stellenbosch University — in collaboration with iThemba LABS and scientists from the Joint Institute of Nuclear Research (JINR) in Russia — has found a new way to reveal these invisible pollutants in the Western Cape area using mosses and lichens. 

Mosses, which mostly grow in moist dark areas, are generally green and have a small leaf-like form, whereas lichens are usually a grey or pale white, and grow in disk shapes on trees or rocks. 

These non-vascular plants (non-vascular means they don’t have xylem and phloem, the plant equivalent of our veins and arteries) act as natural air filters and trap toxic trace elements. Like humans, plants breathe in the air around them, so the trace elements that people inhale are also accumulated by the plants. 

To find out what elements are in the plants, we bombard them with neutrons, in a technique called neutron activation analysis. This makes the accumulated trace elements in the plants radioactive. Once the elements are radioactive, they start emitting gamma rays. Each element has its own unique gamma-ray signature, meaning that scientists can not only say whether an element is there, but how much of it is there.

Trace elements are not always a bad thing. We have many vital trace elements in our bodies, such as oxygen, hydrogen and calcium. But others can be toxic: lead, cadmium and mercury, for example, can cause brain damage, kidney failure and infertility. There are many causes of trace elements in the air (including natural ones such as volcanoes), but humans are the major culprits, through cars, industry and urbanisation. This means that if you live in a city, you are at the greatest risk. 

Global air pollution levels have risen significantly in recent years. As people move to cities, the world experiences a rapid rise in industrialisation and there are more cars on the roads. Yet governments and policy makers have failed to respond decisively. 

There is also the problem that air pollution disperses rapidly, so it is difficult to point a finger at a single source. In South Africa, factories are required by law to declare any processes that could potentially release pollutants. However, due to the cumulative effects of traffic volume, high density housing and total emissions by clusters of factories, the reporting does not necessarily reveal the collective high levels of toxins from all sources in a particular area. 

Since the establishment of air quality standards by the United Nations in 1972, South African air pollution studies have focused mainly on classical pollutants — such as carbon monoxide, the size of toxic air particles (known as particulate matter) and ozone — that affect visibility. Heavy metals and other toxic chemical elements have not received enough attention, despite having been identified as the most harmful active air pollutants by the WHO.

So, while the naked human eye might not be able to see these harmful elements that we are inhaling, we can detect them using plants as well as nuclear and related techniques. Now that we can “see” them, we can hopefully start doing something about them.

Ntombizikhona “Zina” Ndlovu is a PhD candidate at the University of Stellenbosch.

This publication is the culmination of a six-month-long Mail & Guardian project, called Science Voices, to teach postgraduate science students how to turn their academic writing into something the public can read and enjoy.

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