Environmental mercury that is absorbed in the diet can have serious effects on human health, including neurological and reproductive effects. The Pearl River Delta in south China has transformed from an agricultural centre to a manufacturing centre during the past few decades. It is now a hot spot for persistent toxic substances, including mercury, with implications for the health of the region’s residents.
Alexander Meins
What are the sources of mercury in the environment?
A major source of environmental mercury is coal combustion in coal-fired power plants. Mercury released into the atmosphere by combustion is deposited on land and water surfaces. Asia produces more than 50% of global emissions of mercury.1 China is the largest emitter in the world (followed by South Africa and India), producing most of its mercury emissions through nonferrous metal smelting, coal combustion and cement production.2,3
Mercury is mainly in the form of elemental mercury in the atmosphere and inorganic mercury (Hg2+) in water. Under anaerobic conditions, sulfide reduction bacteria in aquatic environments can convert inorganic mercury to organic mercury (methylmercury).
How does mercury enter the human food chain?
Methylmercury is readily absorbed by living organisms, including aquatic species and humans, and efficiently binds to fatty tissues.
Mercury and other persistent toxic substances accumulate in animal tissues (bioaccumulation). Mercury is found at higher concentrations in carnivores at higher trophic levels along the food chain, known as biomagnification. It can be detected in the human body through tissue and hair analyses.
What effects does mercury have on environmental and human health?
Methylmercury and other organo-metallic compounds, such as those of tin and lead, share some characteristics of the persistent organic pollutants listed on the control list of the Stockholm Convention: they are persistent, toxic, bioaccumulative and able to travel long distances through different media.
In mercury-contaminated wetland areas, elevated mercury levels in predatory fish have affected the reproductive success of fish-eating predators such as osprey, eagles, herons, kingfishers and songbirds.4 Adverse effects of mercury exposure on reptiles and mammals have also been reported.5
In humans, mercury is a neurotoxin: it affects the central nervous system, leading to neurological defects such as vision impairment, loss of coordination, muscle weakness and, in extreme cases, insanity, paralysis, coma and death. The developing fetus, especially during the first trimester of pregnancy, is particularly vulnerable. Fetal exposure to mercury, which can occur through placental transfer (during pregnancy) and lactation, can cause impairments in cognitive ability, attention span, memory and motor skills.
Acute or long-term exposure to mercury associated with consumption of contaminated fish has well-established adverse human health effects. The link between consumption of large predatory fish and mercury levels in human tissues has led to health authorities, including Food Standards Australia New Zealand and the United States Food and Drug Administration, recommending that the following groups should limit their consumption of fish such as tuna, swordfish and shark:
- women who are pregnant
- women who plan to become pregnant
- young children.
Case study: the Pearl River Delta of China
The Pearl River Delta in south China has been a centre for agriculture and aquaculture for centuries. In the dyke–pond system, fish and shrimp are reared in ponds adjacent to the sea, often in polyculture – that is, mixtures of freshwater fish species such as grass carp, mud carp, tilapia and grey mullet. Wastes such as digested pig manure and grass clippings are used as the major energy inputs.
During the past 30 years, the Pearl River Delta has become the most developed and industrialised region in China, and is now the world’s centre for manufacture of electronic equipment, textiles and pharmaceuticals. These activities have led to discharge of a wide range of toxins into the delta. Several studies have identified the Pearl River Delta as a hot spot for a number of persistent toxic substances, including mercury.6
Numerous pathways contribute to the mercury contamination of the region. Industrial ash, which is contaminated with heavy metals and persistent organic pollutants, is used as fertiliser, leading to contamination of crops. Increased coal combustion to generate electricity, often from power stations that lack emissions controls, results in increased mercury emissions, which enter the food chain when they are deposited onto soil and water from the air. The ‘trash fish’ (small fish with no commercial value) that are used as fish feed, and feed pellets containing fishmeal are often contaminated with mercury and other toxins, leading to contamination of the cultured fish.
Fortunately, the relatively short period of cultivation of most freshwater fish in fish ponds means that bioaccumulation of mercury in the food chain is not a major problem. However, levels of mercury in hair samples from residents of the area are correlated with the daily intake of mercury via fish consumption.
What can we do to minimise the risks?
Some success has been achieved in controlling mercury emissions. In the United States and Canada, stricter regulatory controls over emissions, including from coal-fired power plants, have led to reductions in mercury levels in fish. There is evidence that as levels of inorganic mercury decrease in the atmosphere, so does methylmercury accumulation in aquatic biota. Newly deposited mercury also appears to be methylated more efficiently than existing mercury in aquatic systems. This means that rapid reductions in mercury emissions – to minimise further deposition of mercury – should result in rapid benefits to human and wildlife health.
In China, efforts have been made to reduce mercury emissions from coal-fired power plants through improvements in energy efficiency and pre-combustion control measures.
The Global Mercury Partnership has started negotiations on a legally binding instrument to control global mercury pollution. In January 2016, 140 countries signed the Minamata Convention on Mercury, which aims to protect human health and the environment from human activities leading to release of mercury and its compounds.
At the global scale, reduction of mercury contamination will require sustained national and international commitments for decades to centuries. An effective strategy for reducing mercury exposure requires an examination of the complete life cycle of mercury, including regional sources and fates of mercury, and dietary exposure of coastal communities. In China, there appears to be an urgent need to establish a regional list of toxic chemicals for more efficient control, focusing on chemicals – such as mercury – commonly found in local food items.
This article is based on Ming-Hung Wong (2017). Chemical pollution and seafood safety, with a focus on mercury: the case of Pearl River Delta, South China. Environmental Technology & Innovation 7:63–76.
References
- Wong CSC, Duzgoren-Aydin NS, Aydin A, Wong MH (2006). Sources and trends of environmental mercury emissions in Asia. Science of the Total Environment 368:649–662.
- Pacyna EG, Pacyna JM, Steenhuisen F, Wilson SJ (2006). Global anthropogenic mercury emission inventory for 2000. Atmospheric Environment 40:4048–4063.
- Wu YS, Wang DG, Streets DJ, Hao J, Chan M, Jiang J (2006). Trends in anthropogenic mercury emissions in China from 1995 to 2003. Environmental Science & Technology 40:5312–5318.
- Depew DC, Burgess NM, Campbell LM (2013). Spatial patterns of methylmercury risks to common loons and piscivorous fish in Canada. Environmental Science & Technology 47:13093–13103.
- Schneider L, Maher W, Green A, Vogt RC (2013). Mercury contamination in reptiles: an emerging problem with consequences for wildlife and human health. In: Kim KH, Brown RJC (eds), Mercury: sources, applications and health impacts, Nova Science Publishers, Hauppauge, NY.
- Zhang L, Wong MH (2007). Environmental mercury contamination in China: sources and impacts. Environment International 33:108–121.