By: Wai Yan Chan, Andrei Masangcay, and Aleena Bacorro
Sources and Types of Heavy Metal Pollution:
There are 35 metals that may pose health/ environmental concerns. 23 of them are heavy metals (e.g. antimony, arsenic, bismuth, cadmium, cerium, chromium, cobalt, copper, gallium, gold, iron, lead, manganese, mercury, nickel, platinum, silver, tellurium, thallium, tin, uranium, vanadium, and zinc). Despite the health concerns that heavy metals pose, we need a small amount of these metals to maintain good health.
Arsenic
According to WHO guidelines, drinking water should contain less than 10 μg/ L. When arsenic level is 10-100 times over the standard, it can be a threat to human health (WHO 2019). Arsenic comes from both human activities and natural processes, including mining, processing of ores, and smelting. Arsenic is also naturally occurring in mineral deposits. In soil, bacteria can convert arsenic into harmful inorganic compounds through a process called biotransformation. These compounds are converted to mono-methyl-arsonic acid (MMA) and dimethyl-arsenic acid (DMA). Long-term exposure to MMA & DMA may cause cell damage. Other than these compounds that contain methyl functional groups, arsenic compounds (e.g. arsenite and arsenate) are highly carcinogenic (National Institute of Environmental Health Science 2024). Inappropriate disposal of arsenic affects the water quality. For instance, mining industries may discharge arsenic-containing water directly to nearby water bodies. Despite the health concerns posed by arsenic contamination, arsenic can be found in many products that we use every day, including paints, dyes, soaps, and semiconductors. Although some arsenic compounds remain in our bodies for a long period, most of them can be excreted through urine. A small amount of arsenic in the human body does not pose any severe threat. However, it is an issue in many countries, especially those in South Asia. Groundwater in Bangladesh and West Bengal (India) has the highest arsenic concentration, up to 3200 μg/L (320 times more than the standard).
Lead
Blood shouldn’t contain higher concentration of lead than 5 micrograms per decilitre (100 mL). Exceeding the limit can trigger a series of health issues (WHO 2021). The common sources of lead include, metal refining, wastes from battery production, soil, exhaust from vehicles, gasoline, paint in older homes, factories, fertilizers, pesticides (Washington State Department of Health 2024). Even at low concentrations, it disrupts plants’ stability in uptaking ions, affecting their photosynthesis process. Lead is toxic because it can replace other divalent ions (e.g. Ca2+, Mg2+, Fe2+) and monovalent ions (e.g. Na+), disrupting the metabolism of the cell by affecting the cell's biological process (e.g. inter/intra cell signaling, maturation, enzyme regulation, apoptosis, etc.) (National Library of Medicine [NIH] 2012). By replacing calcium, even at low concentrations, lead affects protein kinase C (PKC), which regulates memory storage, causing loss of memory. Lead in a pregnant woman’s body can pass to the fetus exposed. In the human body, lead is stored and accumulated in bones and teeth, and can distribute to the brain, kidneys, and liver (WHO 2024). Caribbean regions are affected by lead poisoning the worst, due to industrial emissions, battery recycling, paint, contaminated water, and food (NIH 1997).
Mercury
Among the heavy metals, mercury is one of the most bioaccumulative. Mercury is consumed by microorganisms that live in water bodies and transformed into methylmercury, eventually consumed by humans. Presence of mercury in human bodies is neurotoxic, can destroy microtubules and mitochondria (organelles), and peroxidate lipids. Hence, it interrupts intracellular activities, mainly targeting the brain. It affects transcription (a process of copying DNA), causing the disappearance of the Endoplasmic reticulum and activate natural killer cells. However, mercury also damages other organs, for instance, mercury vapor causes asthma and other respiratory problems (NIH 2014). Mercury pollution is a significant issue that affects many people globally. 8-10% of American women are estimated to give birth to children with neurological disorders. Mercury pollution in water mostly comes from anthropogenic sources, such as agriculture, regional/industrial waste discharges and mining. However, mercury is commonly used in the production of thermometers, barometers, paper industries, battery production, lamps, dental amalgam.
Cadmium
Global cadmium production has doubled between 1950 to 1990. It causes pulmonary and kidney disorders, potentially leading to cancer. Main source of emission non-ferrous metal (metal/ metal alloy that does not contain iron) production, fossil fuel combustion. Then soil absorbs cadmium through atmospheric deposition, phosphate fertilizers, and sewage sludge. Lastly, sources of intaking cadmium include cigarette smoke, shellfish, and organ meat are accounted for by cadmium exposure.
Heavy Metals & Water Pollution
Prolonged exposure to contaminated water can be risky, so it is crucial to be aware of the signs indicate that water contains heavy metals. It is important to distinguish between hard metals and heavy metal contaminated water. Water containing high concentrations of calcium and magnesium is called hard water. It is safe to drink, but hard water may be a sign of the presence of other toxic metals. If you see mineral stains on your clothing, feel slippery after washing hands with soap, rusting sinks or smell fishy, a water quality test is required. If your water is yellow, brown, or orange, it indicates the presence of toxic metals (e.g. high Fe, Mn, Cu), water purification is needed immediately (Croix Crystal 2024).
How do heavy metals enter the water system?
Mining, industrial, agricultural wastes, volcanic eruptions, wildfires are sources of heavy metals. Wastewater can contain heavy metals; proper treatment cannot eliminate them completely. Exposure to heavy metal is caused by leaching from galvanized steel or iron, particularly those installed in buildings before the 1960s, used for service lines and pipes (Government of Canada 2008). Agricultural discharges leach into soil, plants take up the heavy metals from their roots, and pass to the herbivores as they consume plants. This process is known as biomagnification. Phosphate fertilisers are derived from phosphate rock naturally containing heavy metals. Organic matters and clay materials that have charged surfaces attract heavy metal ions, increasing risk of plant uptaking. Surface run-off is also a common source of heavy metal in water, when fertilisers are excessively applied applied during heavy rainfall, water transport fertilisers that contain heavy metal into nearby water bodies. Pollutants can enter the water system through air. Vehicle exhausts, industrial emissions, and combustions release heavy metal into the atmosphere, and deposit it into water bodies. Once the heavy metals enter the ecosystem, they are likely to accumulate within the bodies of organisms. Since they are not organic pollutants, they are not biodegradable. Further effort is required to remove them.
Bioaccumulation
Borgå (2008) describes this as the accumulation of a pollutant such as heavy metals in an organism in connection to its level in its ambient medium. Additionally, the pollutants accepted for bioaccumulation are the ones that an organism can’t break down or excrete. An example of bioaccumulation is a heavy metal in a fish situated in a habitat near a field with active mining activities. Environmental pollution is considered the major gateway for heavy metals for bioaccumulation, Although heavy metals occur in the earth naturally throughout its crust, other organisms are affected by heavy metal bioaccumulation from human activity such as but not limited to mining, smelting operations, and other industrial processes (Tchounwou et al., 2012). In addition, Persistent, Bioaccumulative, and Toxic (BPT) Chemicals are a group of chemicals that can’t be easily degraded and can remain for a long time. Once an organism contains BPTs, these attach to the fat tissues, bones, and brain of any organism, while they persist in the environment for a long time, emitting dangerous chemicals (Check & Marteel‐Parrish, 2013). Biomagnification (also known as Food Chain Magnification or Biological Magnification) is also a leading cause of bioaccumulation, and is also known as its amplifier because it utilizes the food chain as a bridge for dangerous toxins to spread to organisms (Greenfield, 2024).
How Heavy Metals Poison the Developing Brain
Since the beginning of the industrial era, children have been exposed to heavy metals. Even after countless labor laws and regulations, children today are still susceptible to the harm caused by neurotoxic metals, such as lead. The brain, especially before it is fully developed, is extremely vulnerable, and thus, neurotoxins are extremely harmful to children and young adults. But what are heavy metals? How are we exposed to them? And, most importantly, how can we protect ourselves and our communities?
Heavy metals are elements that are toxic and carcinogenic in nature. For example, lead, mercury, cadmium, and (sometimes) chromium. Typically, they have a density greater than 5, but there are exceptions to this. All heavy metals, however, adversely affect bodily functions by producing free radicals–atoms, molecules, or ions that are missing a valence electron–that lead to severe health and environmental problems. The most well-known of these heavy metals is typically lead, with many warnings and regulations meant to protect against lead poisoning.
However, despite these regulations we are exposed to heavy metals in more ways than we expect. Heavy metals are commonly found in our environment and in our diet. While we can consume some heavy metals in small amounts in order to maintain good health, large amounts of heavy metals can lead to irreparable damage. In children, specifically, research suggests that the uptick in behavioral disorders found in children could be connected to heavy metal poisoning.
For example, prenatal exposure to mercury (measured through samples of cord blood) has been directly linked to ADHD symptoms in children and mercury content in air has also been correlated to the prevalence of autism in children. Children who grew up near former lead refinery plants were also said to be at higher risk for ADHD. These results hold similar for children exposed to most heavy metals. Thus, it is clear that there is a pattern in how heavy metals affect a child’s brain and behavioral development.
With all these negative effects, how do people mitigate their exposure to heavy metals? Further, what can people do to protect themselves? According to Clifford Villa, Deputy Assistant Administrator for Environmental Protection Agency (EPA)’s Office of Land and Emergency Management, “the first step in preventing these exposures is knowing about them.” In accordance with Villa’s words, the EPA published a resource guide for the public to access. While we may not have control over the air we breathe, these methods provide a way for us to protect ourselves from heavy metal poisoning and hopefully protect younger children from exposure to neurotoxins.
Global Regulations on Heavy Metals
Heavy metals are ubiquitous contaminants in the environment due to their spontaneous existence throughout the earth’s crust and their direct exposure to humans caused by anthropogenic processes such as mining, smelting operations, and many more industrial processes that promote the toxic activity of these heavy metals as they persist in the surrounding (Wu et al. (2016) & Tchounwou et al. (2012)).
Although this wasn’t previously seen as a concern, recently numerous reports of ecological degradation and diseases developed from excessive exposure to heavy metal ingredients added to cosmetic, aquacultural, and agricultural products, have surfaced, prompting global and local regulations on heavy metals to address the cause (Tchounwou et al., 2012).
Despite current regulations, a review of fifteen legislative acts, containing more than 67.2% of the global population conducted by Michalek et al. (2023) revealed that lower-income countries are less strict and lack standards similar to higher-income countries in regulating heavy metals. According to a report from the United Nations Environment Programme (2002), there has been a lack of global legislation and regulating policies that aim to mitigate the presence of heavy metals and other pollutants in products for daily use.
In essence, there have been numerous initiatives to aid in diminishing the existence of heavy metals in materials and products used daily, but this is not enough reason to ensure that we are completely safe from toxicity caused by heavy metals.
Citations
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