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In the first of a new series on core professional knowledge, Marek Bidwell looks at how pollutants impact habitats and species
Despite developments in environment management and regulation, pollution incidents remain headline news. Singapore recently had particulate levels 10 times higher than the exposure guidelines set out by the World Health Organisation, due to smog caused by the burning of forests in Indonesia.
Pollution is defined in the Oxford dictionary as “the presence in or introduction into the environment of a substance which has harmful or poisonous effects” and can be categorised in different ways:
- Planetary boundaries – In 2009, a conference hosted by the Stockholm Resilience Centre defined nine significant environment issues as “planetary boundaries” and identified safe operating limits for humanity for each. Two of these are associated with direct pollution: atmospheric aerosol loading and chemical pollution, which includes radioactive compounds, heavy metals and organic compounds. Others are associated with indirect pollution: interference with nitrogen and phosphorus cycles; climate change; stratospheric ozone; and acidification of the oceans. The remaining three are largely tied to increasing population and consumption and are: freshwater use; land use; and biodiversity loss. There are many complex interactions between the nine boundaries and often disregard for one exacerbates the others.
- Medium affected – Pollution is frequently defined as affecting air, water or land, sometimes with the addition of noise and vibration, odour, solid waste and light. Categories may be subdivided, such as water into rivers, estuaries, sea and groundwater, for example. Many pollutants migrate from one medium to another through pathways and cycles and may also be taken up by crops into food chains.
- Spatial impacts – Different types of pollutants have an impact across different areas. Effects can be local, regional, transboundary or even global. Often those pollutants that have highly-localised impacts – such as noise, dust, and odour – are of greatest concern for residents and land users.
- Harmful to habitats or organisms – Pollutants can be divided into those that are directly toxic to organisms and those that primarily affect the physical environment.
The renaissance physician Paracelsus coined the phrase “the dose makes the poison”, and it is worth remembering that even common substances, such as salt, can cause pollution in high concentrations.
Putting a population of an organism (such as the freshwater amphipod Gammerus pulex) into containers and increasing the dose of a toxic substance until half is dead is one way of testing its acute effects. This is known as the median lethal concentration (or “LC50”). The precise mechanism that causes death will vary depending on the substance and species.
Certain heavy metals, such as copper, are essential for invertebrates at very low levels, but in excess they alter the normal function of enzymes, damaging cells and disrupting the organism’s water balance. Toxic effects can depend on the organism’s size; surface area; stage in its life cycle; level of stress (such as oxygen levels or food availability); sex; the presence of other pollutants; and any evolved tolerance (such as bacteria’s resistance to antibiotics).
It is estimated that there are 80,000–100,000 chemicals in the global market, but that toxicity data exists only for several thousand. It is not surprising, therefore, that some have unintended consequences.
A vivid example of the unanticipated toxic effects of a substance is provided by Tony Juniper in his book What has nature ever done for us? Juniper describes the collapse of vulture populations in India – which fell 97% between 1993 and 2002 – due to a drug called “diclofenac”.
The anti-inflammatory was widely used to treat livestock until it was banned in 2006. Dead animals are eaten by vultures, providing a valuable environmental cleansing service in climates where they rapidly putrefy. However, diclofenac is highly poisonous to vultures, causing renal failure and death, and its veterinary use has devastated populations of the birds across Asia.
In her classic text Silent spring, Rachel Carson described how the spraying of the insecticide dichlorodiphenyltrichloroethane (DDT) in America not only killed pests, but other insects, birds and fish. Since then, a number of toxic substances (known as persistent organic compounds) have either been banned, or usage restricted – and alternatives found, such as narrower-spectrum insecticides. Unfortunately some of the replacements have also had unintended consequences. Neonicotinoids, for example, have been linked to declining bee populations.
The environmental and economic effects of major marine oil spills, such as Deepwater Horizon, are well reported. Oil physically smothers birds, freshwater and marine mammals, and also has a soluble element that is toxic to organisms. The toxic element bioaccumulates in species, and they can become dangerous for human consumption.
The final impact of an oil spill is difficult to predict because, in addition to the size of the spill, the impact will vary, depending on the: oil type and fraction; degree of mixing in the water; weather conditions; proximity to shore; sensitivity of the ecosystem; and presence of bacteria that can breakdown the oil. Certain chemical dispersants may be more toxic than the oil itself, and make the oil more bioavailable to organisms.
While providing valuable quantitative data, the study of the lethal dose of pollutants does not consider the long-term sublethal effects pollutants have on organisms’ reproductive success, behaviour, immune systems and, ultimately, life expectancy.
In this area, analysis of the environmental impact is more complex. Additional factors need to be considered, including: how the substance is used and disposed of; its tendency to disperse; its level of persistence; its level of interaction with other pollutants; and its sublethal modes of action.
Substances including polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and certain brominated flame retardants have been found at elevated levels in wildlife and human populations in remote areas of the planet; including the Arctic. These compounds are designed to be persistent and bioaccumulate within organisms over time because they are not readily broken down or excreted. This build-up of toxins is compounded when predators eat organisms containing the harmful substances. The concentration of these substances can then increase exponentially – a process known as biomagnification.
There is increasing concern over the endocrine-disrupting properties of ubiquitous chemicals, such as phthalates which are used in plastic bottles, medications and food packaging. Phthalates are readily released into air, water, food and drink, and academic studies have found people worldwide with multiple phthalates in their urine. While their use has been restricted in children’s toys in the EU, determining “safe” levels of exposure for such substances is difficult.
Levels of toxic substances like lead have been successfully reduced in developed countries, but still have a significant impact on human health in other countries – where lead is still used in petrol, and practices such as artisanal mining persist.
The Blacksmith Institute, which helps to cleanup pollution hotspots around the world, estimates that pollution acutely affects the lives of 200 million people in developing countries. Pollution remediation, however, can be very cost effective in saving lives – relative to other types of public health interventions.
Air pollutants commonly emitted from factories and transport (such as particulates, nitrogen dioxide, solvents and ozone) have chronic effects on human health (such as respiratory diseases, asthma and cancer). The World Health Organisation publishes guideline levels for each pollutant, but these are frequently exceeded in cities around the world.
Some substances – while they may be toxic at high concentrations – are better known for the physical or chemical changes that they cause in the environment. In this way they can have profound effects on whole ecosystems and can be thought of as indirect pollutants. Principal examples are: nutrients (causing eutrophication); acid gases (causing acid rain); greenhouse gases (causing climate change); and carbon dioxide (causing ocean acidification).
In his book The god species, Mark Lynas said: “Thanks to Fritz Haber [the German chemist who synthesized ammonia], humans are the only species on the planet, apart from Rhizobium bacteria, that are able to fix their own nitrogen directly from the atmosphere.” It is something we do on a massive scale to fertilise crops, but the side effect of fertiliser runoff is the nutrification of rivers, lakes, estuaries and seas. The ecological balance of many water bodies around the world has been substantially altered.
An abundance of normally limited nutrients causes algae blooms and/or rapid growth of plants. In the first instance, the plants and algae shade out other species and when they die the amount of decomposing organic matter is significantly increased, removing vital oxygen from the water and killing other organisms, including invertebrates and fish. This process is known as eutrophication.
Other organic pollutants, including sewage, paper mulch, milk and beer contribute to this process because they are nutrients and are consumed by aerobic bacteria in the water, which in turn consume oxygen. As oxygen levels drop, aerobic bacteria are replaced with anaerobic bacteria that continue to feed on the organic material, and produce toxic by-products such as methane and ammonia.
Anyone who has kept tropical fish in a tank, or even a goldfish, has conducted a real-life experiment on the toxic effects of ammonia on a freshwater ecosystem.
Another group of pollutants that affects whole ecosystems are acid gasses, such as oxides of sulphur and nitrogen, which are released during combustion of fossil fuels, dispersed in the air and deposited downwind through acid rain and dry deposition. Acid rain was a key environmental focus of the 1980s.
Studies found that about 18,000 lakes in Norway had a pH of less than 5.5, reducing fish stocks substantially. Action was taken in Europe to curb the emission of such gases and studies have shown that lakes are recovering in Norway and other areas: water is slowly neutralising and populations of fish, such as brown trout, are rising again. This is a positive example of how environmental problems can be identified, analysed and reduced.
Reasons to be cheerful?
Despite continuing significant pollution events, such as the high particulate levels in Singapore, there are reasons to be optimistic that serious pollution problems have been identified and solved (or substantially reduced) in particular areas or regions. Success has been greatest in controlling point-source emissions, and where there are readily available technological fixes. Diffuse pollution is a growing problem, however.
There is an urgent need for proven pollution-prevention measures to be transferred to developing countries so that pollution is not “offshored” – either becoming someone else’s problem in the case of local or regional pollutants, or having no net benefit whatsoever for global pollutants. At the same time, although innovation and development can solve many problems and improve individuals’ quality of life, it must go hand in hand with careful assessment of the potential risks.
Key pollution terms and concepts
Source-pathway-receptor model – The impact of pollution is dependent on: its toxicity; how it disperses in the air, land, or water; and the sensitivity of the receptor. For example, two solvent spills from a factory would have different impacts if one was on to a permeable sandy soil, while the other was over impermeable soil that blocks percolation.
If pollution reaches a receptor – such as a river or lake – the impact will depend not only on the nature and quantity of the pollutant, but also the capacity of the ecosystem to assimilate the pollution. In this respect enclosed bodies of water are particularly sensitive.
Containment v dilute and disperse – Until late 20th century, the focus was on dispersal of pollution away from the source, the hope being that sufficient dilution would render it harmless. This philosophy applied to the design of landfill sites and factory chimneys. As the background effects of pollution began to manifest – such as the transboundary effects of acid rain – efforts have moved towards pollution prevention and containment.
Point source v diffuse pollution – Pollution from chimneys and pipes in factories is referred to as “point source” and can be more easily monitored and regulated than nutrient runoff from fields or oily runoff from highways, which is described as “diffuse” pollution.
Abundance v diversity – A heavily-polluted environment will not necessarily be devoid of life, but it will favour particular species that are resilient to the pollution. Examples of such species include: blue-green algae that thrive in nutrient-rich water and tubificid worms, which are tolerant to a range of toxic substances, including lead and zinc. On the other hand, there are also “indicator” species, such as crayfish or mayfly, which can signify clean environments with a high diversity of species.
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