UNEP Prelims 2023 Q59: Mercury Pollution Sources — Explained
Consider the following statements regarding mercury pollution:
1. Gold mining activity is a source of mercury pollution in the world.
2. Coal-based thermal power plants cause mercury pollution.
3. There is no known safe level of exposure to mercury.
How many of the above statements are correct?
- Only one
- Only two
- All three
- None
Answer: (b) Only two
Statement-wise explanation
- 1: correct — UNEP's Global Mercury Assessment 2018 found that artisanal and small-scale gold mining (ASGM) produced almost 38% of man-made mercury emissions to air in 2015. That makes it the largest single source in the world. Miners mix liquid mercury with ore to bind the gold, then burn off the mercury. The vapour goes into the air and the waste goes into rivers and soil.
- 2: correct — Coal contains traces of mercury. Burning coal in power plants releases it through the flue gas. UNEP's 2018 assessment puts stationary combustion of fossil fuels and biomass at about 24% of global man-made emissions, with coal burning alone at about 21%. That makes coal the second-largest source after ASGM.
- 3: incorrect — International bodies do set tolerable intake levels for mercury. In 2003, the FAO/WHO Joint Expert Committee on Food Additives (JECFA) set a Provisional Tolerable Weekly Intake for methylmercury of 1.6 µg per kg of body weight. The phrase 'no known safe level' is what WHO says about lead, not mercury. WHO lists mercury among its ten chemicals of major public health concern, but a defined tolerable intake exists.
How to eliminate
Statements 1 and 2 can be settled from basic knowledge. Mercury is used to extract gold, and coal power plants are a standard example of mercury emissions. Both are true, so the answer is two or three. That rules out 'only one' and 'none'. Everything then depends on statement 3, and it can only be settled by knowing one fact: 'no known safe level of exposure' is WHO's wording for lead, while mercury has a tolerable intake. JECFA's figure for methylmercury is 1.6 µg/kg body weight per week. The sweeping phrase 'no known safe level' should make you check which metal it really belongs to. Remember the pairing: lead has no safe level, and methylmercury has a tolerable weekly intake.
Know more
Mercury: sources, pathway, and global control
Mercury is a heavy metal that occurs naturally. Human activity has raised the amount of mercury in the atmosphere by about 450% above natural levels (UNEP, Global Mercury Assessment 2018). In 2015, man-made emissions to air were about 2,220 tonnes, roughly 20% more than in 2010.
| Source (2015, UNEP GMA 2018) | Share of global man-made air emissions |
|---|---|
| Artisanal & small-scale gold mining | ~38% |
| Stationary combustion of fossil fuels and biomass | ~24% (coal ~21%) |
| Others: non-ferrous metal production, cement, waste, chlor-alkali | remainder |
Pathway to humans. Mercury released into air settles in water. There, microbes convert it into methylmercury. Methylmercury builds up in living tissue and becomes more concentrated at each step up the food chain, so large predatory fish carry the most. Most human exposure comes from eating contaminated fish. It harms the nervous system, kidneys and immune system, and is most dangerous to the developing foetus and young children (WHO).
Methylmercury vs ethylmercury. Ethylmercury, used as a vaccine preservative (thiomersal), is broken down quickly and does not accumulate in the body. Methylmercury does accumulate.
Minamata disease. Named after Minamata Bay in Japan. In the 1950s, people there were poisoned by eating fish contaminated with methylmercury from a chemical factory's wastewater.
Minamata Convention on Mercury
- Adopted in 2013. In force since 16 August 2017.
- India ratified it in 2018, with flexibility to keep using some mercury-based products and processes until 2025.
- Controls mercury mining, trade, mercury-added products (thermometers, some lamps, batteries) and emissions from coal plants, and requires national plans for ASGM.
- A 2023 amendment banned the manufacture and export of more mercury-added products after 2025.
India's emission norms for thermal power plants control SO₂ and NOₓ. Installing flue-gas desulphurisation (FGD) units to meet them also cuts mercury emissions as a co-benefit.
NCERT Concepts to Revise
- Water pollution: Mercury from mining and coal ash enters water as an inorganic pollutant, where it becomes methylmercury, the form that reaches people through fish.
- Ashio mine pollution: A classic case of a mine poisoning a river with metal waste, the same pathway at stake when mining is called a mercury source.