Table of Contents
Background
Zinc (Zn) is a naturally occurring element that occurs in the environment mainly in the form of mineral components and dissolved form in water. It has high significance for plants and animals as it is an essential micronutrient and plays important role in many biological processes in the cell. Zn can be found naturally in drinking water in small concentrations and is not considered a health concern generally. However, elevated Zn levels in drinking water can have negative effects on human health. Further, long-term exposure to high Zn levels is associated with various diseases. Zn can contaminate drinking water in many ways. It can be through natural sources as a result of natural processes such as weathering of rocks and minerals containing Zn. Human activities play a key role in the spread and contamination of drinking water from Zn.
This includes industrial sources (mining and smelting), agricultural (use of Zn-containing fertilizers), corrosion of water supply pipes (galvanized steel pipes), landfill leakages, urban runoff, and point sources such as sewage treatment plants and industrial discharges. From these sources, Zn can become part of ground and surface water reservoirs through leaching and runoff processes resulting in contamination of drinking water if not properly treated before its supply to the consumers. Importantly, the levels of Zn in drinking water may vary based on the drinking water source and local geology. In the US, Zn levels in drinking water are regulated under Safe Drinking Water Act (SDWA). EPA has set the MCL of 5 mg/L in public drinking water systems based on the best available technology, treatment techniques, and costs and the levels should not exceed the prescribed MCL for public health safety.
Additionally, WHO also recommends that the Zn levels in drinking water should not be above 5 mg/L. The EPA guidelines also require regular monitoring of Zn in the public water system including the provision of an annual consumer confidence report (CCR) to the customers. According to the EPA, drinking water in the US generally contains low Zn levels and does not pose any health concerns to consumers. However, there are some areas where Zn can be in high concentrations due to local geology or industrial activities. Importantly, EPA does not regulate private well water therefore, it’s the responsibility of the well owner to test the drinking water and take necessary measures in case of high contaminant levels are found. Here it is important to mention specifically NYC And NJ where the Zn levels in drinking water are typically low and not considered a significant health problem. DEP and NJDEP are the main departments that regularly monitor for elevated Zn levels in the drinking water supply in both states respectively and ensure that the supplied water has Zn levels under the MCL.
Elevated exposure to Zn through drinking water may negatively affect human health. Chronic exposure to high Zn levels has been associated with stomach cramps, nausea, and vomiting along with an increased risk of developing neurological, and behavioral disorders and delayed development in children. Further, Zn can also lead to copper deficiency anemia a condition in which Zn competes with Cu for absorption in the body resulting in a high intake of Zn causing decreased Cu absorption. Although, Zn presence in drinking water does not always means it exceeds the MCL and the detected levels are harmful to human health. However, its long-term exposure should be taken into account and if you are concerned with the Zn levels in your drinking water, it is better to get your water tested through a local utility or contact EPA for additional information. If the detected concentration is higher, steps should be taken to reduce the exposure to Zn by applying suitable treatment approaches.
Zn has high metabolic significance for the human body as it plays vital roles in many biological activities (Frassinetti et al. 2006). It is important that the daily consumption of Zn mush be within recommended daily allowances and should be taken through consultation with a doctor or nutritionist. Several stages are involved in the Zn metabolism involving absorption in the small intestine in the form of free ions via Zn transporters such as ZIP4 and ZnT1 (Reis et al. 2020). The absorbed Zn is then transported to the liver through metallothionein proteins (Elinder and Nordberg 2019). The liver then distributes the metabolized Zn to various tissues and organs in the body. Storage of Zn takes place in various body tissues comprising the liver, pancreas, kidney, bones, and muscles (Grüngreiff et al. 2016).
Once entered into the body, Zn can result in health problems through various cellular and molecular mechanisms. Its high levels in the cell can lead to the production of reactive oxygen species (ROS) and cause oxidative stress. Such conditions lead to DNA, proteins, and lipids damage and hence contribute to disease development (Marreiro et al. 2017; Sharma et al. 2012). High Zn levels can also trigger inflammatory responses in the body leading to immune cell activation and the release of inflammatory mediators (Prasad and Bao 2019). These inflammatory responses contribute to various disease development such as cancer and cardiovascular diseases. Important cellular processes including programmed cell death (apoptosis) may also be affected by high Zn levels. Apoptosis is important in maintaining a balance between cell growth and death and high Zn levels disrupt this balance leading to cancer development (Franklin and Costello 2009). Being an important component of many metabolic pathways in the cell, Zn plays a critical role in cellular homeostasis as well as the synthesis of DNA, proteins, and cell division.
High Zn exposure results in the disruption of these metabolic pathways leading to the development of various diseases (Chasapis et al. 2020). Moreover, it has also been reported that Zn can affect gene expression through various mechanisms including epigenetic changes (DNA methylation and miRNA) that result in changes in overall cellular behavior (Meng et al. 2022). However, there is limited epidemiologic evidence available related to Zn exposure through drinking water and associated molecular mechanisms of disease development, and further research is necessary to explore this research question by conducting population-based studies showing correlations between drining water Zn and its impacts on cellular and molecular biomarkers.
Detection Methods and Removal Strategies
There are several methods to detect and quantify Zn in drinking water. Preference to select a suitable detection method is based on the desired results and operating costs along with the availability of instruments. Atomic absorption spectrophotometer (AAS) coupled with suitable detection systems such as flame or graphite furnaces (FAAS/GFAAS) is a widely used method to quantify Zn due to its relatively low costs (Siraj and Kitte 2013). The method utilizes light absorption by Zn atoms to measure Zn concentration in a water sample. The method has high sensitivity and can detect Zn levels upto µg/L levels.
For more sensitive results in ng/L levels, ICP is the preferred method coupled with OES/MS detector (Silva et al. 2009). However, the limitation of ICP includes its high operating costs and can only be preferred where highly sensitive results are required. Colorimetric methods are also commonly used to detect Zn by using color reactions. However, these methods are simple, quick, and have less sensitivity compared to AAS or ICP-based methods. It is important to mention that Zn levels in drinking water should always be tested by a certified laboratory using suitable methods and equipment. The results must be compared to the EPA guideline values to determine if the detected levels of Zn in drinking water are safe for human consumption.
There are several methods to detect and quantify Zn in drinking water. Preference to select a suitable detection method is based on the desired results and operating costs along with the availability of instruments. Atomic absorption spectrophotometer (AAS) coupled with suitable detection systems such as flame or graphite furnaces (FAAS/GFAAS) is a widely used method to quantify Zn due to its relatively low costs (Siraj and Kitte 2013). The method utilizes light absorption by Zn atoms to measure Zn concentration in a water sample. The method has high sensitivity and can detect Zn levels upto µg/L levels.
For more sensitive results in ng/L levels, ICP is the preferred method coupled with OES/MS detector (Silva et al. 2009). However, the limitation of ICP includes its high operating costs and can only be preferred where highly sensitive results are required. Colorimetric methods are also commonly used to detect Zn by using color reactions. However, these methods are simple, quick, and have less sensitivity compared to AAS or ICP-based methods. It is important to mention that Zn levels in drinking water should always be tested by a certified laboratory using suitable methods and equipment. The results must be compared to the EPA guideline values to determine if the detected levels of Zn in drinking water are safe for human consumption.
Public Perspective
Following frequently asked questions (FAQs) try to address some general public concerns in the US, especially the NYC and NJ region.
The daily requirement of Zn for adult humans is 15-22 mg/day to meet nutrition requirements.
Elevated Zn levels in drinking water may cause the water to have a milky, chalky, or turbid appearance and a metallic/astringent taste. High Zn levels in drinking water may lead to symptoms of Zn poisoning includin “low blood pressure, urine retention, jaundice, seizures, joint pain, fever, coughing, and a metallic taste in the mouth”.
Zn is an essential mineral that plays a key role in multiple aspects of human health due to its key roles in many biological processes. Supplementing with Zn may improve immunity, blood sugar levels, and eye, heart, and skin health.
Certain kits have been developed recently with the potential to detect Zn in drinking water on the spot. For more sensitive results, Zn can be quantified using the laboratory-based method from a certified water testing laboratory.
Zn is required by our body in small amounts. The FDA permits bottled water to contain up to 5.0 milligrams per liter of Zn.
Zn deficiency can happen in people who have problems absorbing nutrients, for example, older people and those who have some gut diseases. Some medicines can also increase the loss of Zn through urine from the body.
Zn is best absorbed when taken with a meal that contains proteins.
Generally speaking, boiling water can help to kill harmful bacteria in drinking water but does not remove metals and minerals including Zn. For this, the prescribed treatment approach should be adopted.
A coating of Zn protects galvanized pipe, but when corrosion occurs, it deposits high levels of Zn and Fe into the tap water leading to their high levels in drinking water supplies.
The human body only absorbs 20-40% of the Zn present in food, consequently, many people drink mineral water rich in Zn to meet their body requirements.
Conclusion
Zn is a naturally occurring mineral that can be found in drinking water in dissolved form. It is an essential micronutrient and is involved in many metabolic processes in humans. However, its elevated levels in drinking water can result in various health abnormalities through various mechanisms. In the US, the levels of Zn in drinking water are found in low concentrations and below the EPA-prescribed MCL hence considered safe to drink. However, its regular monitoring should be taken into account to ensure that the levels are not exceeding the limits due to human activities and proper treatment approaches must be adopted to lower the high Zn levels in order to ensure a safe drinking water supply to the consumers.
References
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- Yasir A. Rehman, Ph.D.
Dr. Rehman was born in Rawalpindi, Pakistan. He completed his MSc from PMAS – Arid Agriculture University Rawalpindi in 2011 where his thesis comprised a health risk assessment of subjects living in the vicinity of wastewater channels in urban settings and its relationship with the incidence of Malaria.
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OTHER RESEARCH ON WATER CONTAMINANTS BY DR. YASIR
