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Mercury, Methylmercury, Fish tissue, Kidney toxicity

Mercury is an element which for the most part is indestructible and, therefore, should be considered a “forever chemical”. Roughly, up to 90% of methylmercury in a human body is from seafood intake [1]. Additional levels of methylmercury can come from breathing the air, especially for those living near coal burning powerplants. With that said, there are approximately 216 coal burning plants still in use in the United States with about 50 tons per year of mercury being released into the atmosphere [2]. Human exposure is greatest at the power plant facility, but about 80% of the reactive mercury (~40 tons) travels in the air and becomes deposited on the land/water within 1500+ miles of the power plant [3]. This is the main cause of methylmercury bioaccumulation and biomagnification in fish tissue that leads to high levels in human blood, especially in rural areas where families use locally caught fish to supplement their food supply/budget.

Long-term methylmercury exposure can lead to possible carcinogenicity and death, while even exposure to small amounts can cause developmental/reproductive issues in off-spring, neurotoxicity, digestive/immune dysfunctions, as well as problems with lungs, skin, eyes and kidneys [4, 5]. With respect to the latter, the National Institute of Health has determined that more than 1 in 7 adults in the United States [6] or roughly 47 million people based on 2022 US population demographics have some form of chronic kidney disease with the highest rates of prevalence in Americans with diabetes and high blood pressure; there are several papers in the literature that link methylmercury toxicity directly with both of these disease states [7-10]. Additionally, methylmercury blood levels in adult female participants from a 1999–2000 [11]. Suggests that more than 300,000 newborns each year in the United States are exposed in utero to concentrations that can be linked to neurodevelopmental effects. Methylmercury can also be found in human cord blood, placenta, and breast milk [12].

Virginia Department of Environmental Quality (VADEQ) has made accessible a significant amount of historical fish tissue data relating to methylmercury content [13]. The database included 44,380 fish samples collected/analyzed from 22 species of fish obtained from 45 Virginia bodies of water from 1995 to 2022 (Table 1); no data was available from 2009 through 2013.

Table 1: Bodies of Water and Fish Species Collected.

Mathematically, the total number of fish tissue samples collected along with the mean methylmercury levels, expressed in parts per billion (ppb), were calculated for each year of available data (Table 2).

Table 2: Historic Levels of Methylmercury in Fish Tissue (1995-2022).

The data was then used to calculate the daily human blood levels of methylmercury based on the amount present in fish tissue, portion size recommendations for men, women, and children made by either the Food & Drug Administration (FDA), the Environmental Protection Agency (EPA) and/or the Virginia Department of Health (VDH) [14, 15] (Table 3). The daily blood level of methylmercury was calculated using a 50-day half-life in the human body [19] which was then compared to the level known to cause kidney toxicity (0.01 ppb) based on the Agency for Toxic Substances and Disease Registry [12].

Results

Based on the data obtained (Table 4) using either actual or recommended methylmercury levels from regulatory agencies, the average amount of methylmercury in human blood per day over one 50-day half-life ranged between 0.04 ppb and 0.05 ppb or roughly 4 to 5 times the level known to produce kidney toxicity via sub-chronic oral exposure.

Table 4: Summary of Results.

Below is an example calculation used to determine daily methylmercury blood levels based on FDA/EPA allowable levels for Pregnant/Nursing Women.

Example calculation (pregnant/nursing women):

Amount of methylmercury per serving:

FDA/EPA Allowable Level for Pregnant/Nursing Women = 12 oz serving/week = 0.34 Kg times 150 ppb (allowable level in fish tissue) = 50 ppb of methylmercury in 3 - 4 ounce servings/week.

Amount of methylmercury metabolized daily:

FDA/EPA Allowable Level for Pregnant/Nursing Women = 50 ppb (amount of methylmercury per week) divided by 2 = methylmercury half-life total; divide initial dose (50 ppb) by 49 days = 0.51 ppb per day metabolized (approximation).

Amount of methylmercury/Kg:

50 ppb (initial methylmercury level) divided by 75 Kg (Pregnant/Nursing Women Body Weight) = 0.67 ppb/Kg

Amount of Available methylmercury per day:

0.67 ppb (amount of methylmercury/Kg) divided by 50 days (half-life) = 0.01 ppb Daily Blood Level (ppb/day)

Daily/average amount of methylmercury in human blood per day:

Over the course of 50 days half-life daily levels ranged from 0.01 to 0.07 ppb; Average blood level = 0.04 ppb or 4 times the dose known to cause kidney toxicity.

Using the initial and end half-life totals for the “allowable” methylmercury levels in fish tissue (Table 5) set by FDA/EPA/VDH would virtually double the level of methylmercury in the blood within approximately 1 year (350 days).

Table 5: Accumulation of Methylmercury Blood Levels after 1 Year - Pregnant/Nursing Women.

Discussion

Regulations relating to methylmercury ingestion from contaminated fish tissue range from 150 ppb to 500 ppb. As one may summarize, the recommended values do not appear to be based on the cumulative toxicity potential of methylmercury, but would appear to be based more on other concerns, possibly economic and/or nutritional. What is most concerning is that following these guidelines a pregnant or nursing mother would have a methylmercury blood level that is 4 times higher than the renal toxicity dose. Likewise, the recommendations for children would yield daily methylmercury blood levels 1 to 8 times higher than the dose that is associated with renal toxicity.

Another point of concern is how our regulatory agencies could be so far off the mark when it comes to determining a “safe” level of exposure to methylmercury. One possible explanation is the misuse of kinetic modeling. For example, the regulatory agencies appear to employ a “threshold kinetics” model, whereby, environmental toxicity is controlled by some type of balancing act between the known toxic and non-toxic doses of a substance. This type of modeling is excellent when there is only “one” chemical involved from “one” exposure source. Unfortunately, this is not the case; since our environment simultaneously exposes humans/animals to numerous chemicals from multiple sources. This causes an underestimation of the toxicity while ignoring potential synergistic interactions. The latter is a significant scientific concern when addressing toxicity associated with endocrine-disrupting chemicals which commonly cause toxicity at doses well below the level of “known” toxicity when mixed together [16-19].

Conclusion

It is unfortunate that from a public health perspective our “regulatory” agencies do not concur on the risks of exposure to numerous toxic substances. Nor do they listen to the findings of toxicology groups, like ATSDR who establish limits based on published scientific data. The concept of pumping the “maximum” amount of toxic chemicals into our water, land and/or air based on current regulatory practices is inappropriate and unsafe at best. “Dilution is the solution to pollution” is not only inaccurate, it is dangerous and defies the “precautionary principle” of do no harm as well as violates all basic principles of toxicology. Continuing these practices will continue to cause increases in disease/death to humans and to all things that live in the environment. How we handle exposure to toxic chemicals needs to change!

Acknowledgment

None

Conflict of Interest

None

Raw data available upon request at jmjdinardo@aol.com

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Appendix I: Daily Methylmercury Blood Levels: FDA/EPA Recommendations for Pregnant/Nursing Women

Appendix II:  Daily Methylmercury Blood Levels: Actual 23 Year Average of Methylmercury in Wild Caught Fish (Adults)

Appendix III: Daily Methylmercury Blood Levels: FDA/EPA Recommendations for Children

Appendix IV: Daily Methylmercury Blood Levels: VDH Recommendation for Adults