Doubting the dangers of mercury poisoning

         I haven't posted in a while. That's because two weeks ago I received my PhD. This was much to my relief and also to those around me who wondered if I'd ever finish (though scary enough, six years to completion is becoming the norm). Along the way I had many, many doubts about my project, myself, and whole host of other personal introspective moments. In the end they were healthy ones, much in the same way that challenges to your views on life will make you a little more robust. This is not always the case, since many people can break down permanently through self-doubt. Like sports, some forms of stress are good, others bad. No-one ever said breaking your leg will make your bones stronger. Neither do people become mentally tougher thanks to relentless psychological torture.

Ostensibly my PhD project was meant to investigate the physical chemical process that led to mercury's movement through the environment. My part in the investigation was a small piece of the puzzle; I only looked at the interaction of ozone with mercury near surfaces. When I had done some preliminary experiments it was time to write an introduction to my unfinished manuscript. Here are some examples of opening sentences found in environmental mercury papers:

Mercury is one of the most toxic elements in nature...

Mercury is a potent neurotoxin, which enters remote ecosystems primarily through atmospheric deposition.

There are [coal] flue gas components for which no simple cleanup process exists today. One such component  is elemental mercury.

The atmosphere is a major pathway by which mercury pollution reaches ecosystems.

Some papers seem uncommitted themselves to the dangers, but cite action by others who are 'concerned'

In recent years, a number of international reviews and policy agreements have been made concerning atmospheric Hg including the heavy metals protocol of the UNECE convention on long-range transboundary air pollution.

After reading enough of these opening blurbs I did some background research and discovered -to my amazement- that mercury poisonings were on the whole quite rare. This led to a small crisis where I questioned the entire point of the project. This was a crisis because mercury research ostensibly (there's that word again) is about reminding us this liquid metal is dangerous. 

It's not the first time I've questioned my research aims. Years ago my summer research project at Carleton "last chance" U was to investigate the lability of the O-H bond in vitamin C. The goal of the project was to find a molecule that could be readily designed into an improved antioxidant. Turns out that idea was fundamentally flawed for a whole host of reasons. Too many to go into here, but needless to say we have so little idea of what an anti-oxidant really does anyone should be ashamed of trying to make 'improvements' on one. Pure foolishness. But that's also how new things get discovered sometimes. Scientists pretend to be doing one thing, whilst researching anther more manageable question. So I didn't quit the project, but the attempt at self-deception left a foul taste in my mouth.

Back to mercury. After I discovered to my bemusement that mercury wasn't as obviously dangerous to the general public as I had first assumed, I decided to write a short article. I have never published it anywhere because it doesn't fit an obvious research theme. Here it is:

_____________________________________

The Dangers of Mercury

     If chance curiosity has lead you to read reports environmental mercury, you may already be familiar with headlines like: “Mercury in Alaska fish: Something else to worry about” ¹, “State needs to address mercury issue: Commissioners worried about impacts of contamination on local tourism” ², or “Study finds factories a factor for mercury levels in fish” ³. Each of these headlines was taken from a local newspaper from a small city in North America (Prince William Sound, Alaska; Ketchum, Indiana; and Edmonton, Alberta; respectively), and each is concerned that industrial mercury pollution is contaminating otherwise pristine lakes and wildlife. The articles appear alarmist, but they cite credible studies of mercury analysis. Many reports state, quite correctly, that mercury pollution arrives to them through a global chain of emission sources, rather than locally.  In fact this pollution is so invasive that researchers have found contamination in the Arctic Circle^4-6. In Canada, we have seen elevated levels of mercury in Algonquin Park⁷, remote lakes in Ontario⁸, and southern Quebec⁹.

         Researchers say we are contaminating everything from rural lakes to mountain glaciers with mercury emissions, and the two most significant culprits (accounting for more than 1/3 of emissions) are coal combustion and waste incineration¹⁰. This contamination begins in the air (but not yet at dangerous levels), then precipitates over lakes, converted to methylmercury by beacteria, and spreads through the aquatic food chain. Most notably methylmercury accumulates in fish at levels sometimes exceeding 1 part per million (farm-raised canned tuna is not immune either¹¹). Hence we humans, near the “top” of the food chain get our methyl mercury doses mainly through fish consumption. Studies show elevated blood, hair, and urine mercury levels in those who rely on fish in their daily diet^12-14.

         We have established that, through various forms of combustion, mercury blankets the planet. But how dangerous is mercury at this level of contamination? If we can measure it, does that mean it is dangerous? I will argue there remains some doubt as to the environmental significance of mercury. Certainly in high enough doses mercury is very toxic, and we discuss this point first. Knowing the toxicity of mercury permits acceptable threshold values for air, water, and fish contaminated with mercury. The crucial point of this paper will come in comparing threshold values to actual consumption levels. We highlight the most widely accepted reports on the effects of mercury in fish-eating populations. We also glance at the measures taken in the coal power industry to reduce mercury in the environment. An appendix provides additional detail for the atmospheric pathway of mercury.

The toxicity of mercury:

        Paracelsus write in the 14^th century “All things are poison and nothing is without poison, only the dose permits something not to be poisonous”. Many chemicals, be they cyanide, vitamin D, or table salt have a known threshold dosage where they become dangerous (i.e. it must show an onset of negative symptoms or cause death). A toxin like mercury has no benefit like vitamin D, but like all chemicals it has a certain threshold level of toxicity.

         Mercury (Hg), methylmercury (MeHg⁺), and dimethylmercury (Me₂Hg) are poisonous. The insidious aspect to mercury poisoning is that even at lethal doses an onset of symptoms has been shown to take weeks or months to develop. The subsequent onset of symptoms is thereafter quite rapid¹⁵. For example, there is the well-known case of a professor who accidentally spilled a few drops of dimethylmercury on her hand¹⁶. Six months after exposure a rapid onset of neurological degeneration developed, leading to her death four months later.  Dimethylmercury had accumulated in her brain, including the frontal lobe, the cerebral hemispheres, and cerebellum. Such cases are extremely rare, but demonstrate the extreme danger of working with purified methylmercury compounds.

         On a larger scale, several mass mercury poisonings occurred in Iraq¹⁵ and Japan¹⁷ wherein thousands died due to more or less direct exposure. In Minamata Japan, a chlor-alkali plant located upstream had been dumping methylmercury waste into the city’s drinking water, contaminating local fish. The Iraq poisonings occurred when methylmercury fungicide was accidentally sprayed on grain destined for use as bread. Cases of severe mercury poisoning are generally non-atmospheric in nature, but have provided researchers with the concentration of mercury in those with visible signs of poisoning. The Iraq data has in fact become the US-EPA baseline for establishing acceptable threshold limits of mercury consumption in fish, also known as a TLV.

          A threshold limit value (TLV) is the maximum allowable ‘safe’, or acceptable ‘background level’ of a chemical exposure. In table 1 we list several of the more deadly airborne toxins. Acceptable concentrations of carbon monoxide, a potentially lethal gas, is one thousand times higher than for mercury compounds. 

Table 1: Threshold limits for some toxic airborne contaminants¹⁸

Substance	40-hour work week limit (mg/m3)
Dimethyl mercury (Me2Hg)	0.01
Mercury (Hg)	0.025
Strychnine (C21H22N2O2)	0.15
Cyanide salt (CN-)	5
Carbon monoxide (CO)	29

The art of determining safe exposure levels is difficult. Symptoms associated with lower than lethal mercury concentrations include memory problems, fatigue, depression, irritability, and slow reflexes¹⁹. These symptoms are then noted along with the actual mercury present their bodies.

         We measure the mercury contamination in people by measuring its concentration (and type) in blood (MeHg), urine (inorganic Hg), and hair (MeHg). Blood concentrations are difficult to measure as they decrease days after exposure (through absorption into organs and excretion). Mercury in urine also decreases with time, but will demonstrate how much mercury is immediately present in our body. Mercury in hair is the most common form of testing; hair is easily acquired and when long strands are available its will provide a historical record of contamination. To remain healthy, mercury levels in hair should be shown to remain below 14 ppm Hg²⁰ . One study found evidence of reflex loss if hair mercury contamination is above 10 ppm²². Typical populations show levels well below 1 ppm²¹.   

         A question we might ask is ‘Can we absorb a dangerous level of mercury through air or drinking water’? The evidence shows us direct exposure is not an issue. Mercury concentrations in the troposphere range between 1.3 - 1.7 nanograms per m^{3 2324}. If we compare this range to the TLV data in table 1, it is 1/10,000^th what is considered safe to breathe²⁵. Drinking water has a threshold consumption level of about 2 ng/mL mercury, but most lake water contains below 0.002-0.02 ng/L ^26,27. Hence water and air are generally safe from dangerous levels mercury contamination. Some exceptions to water safety are found at sites contaminated by nearby gold and silver mines since almost half a million tons of Hg have been used for this purpose worldwide²⁸.

         Fish, by contrast, are the principle (and almost singular) source of the mercury contamination found in humans. The FDA recommends a threshold consumption value of 1 ppm in fish (and some may put the limit at 0.5 ppm). Many fish come close to this limit while others exceed it (e.g. swordfish). Through bioaccumulation of Hg in the food chain, mercury in fish is a million-fold more concentrated that the surrounding water. The mode of accumulation goes like this:

Water(Hg) => Water(MeHg) => phytoplankton(MeHg) => zooplankton(MeHg) => fish(MeHg)

We have reached the meat of the matter, so to speak. Whether mercury is dangerous for humans rests entirely on what are the acceptable levels found in fish.  A review of methylmercury in daily nutrition²⁹ found no (regularly consumed) fish types were generally advised against. To balance the benefits of eating (most kinds) of fish with the potential dangers, there are limits to this consumption. The World Health Organization (WHO) recommends a reference dose (RfD) limit of 2 mg-Hg per kilogram bodyweight per day ³⁰, while the U.S. EPA recommends 0.1 mg-Hg kg-bw^-1 day^{-1 31}. Cans of light and white-style tuna sold in the U.S. contain about 0.12 ppm and 0.41 ppm methylmercury, respectively ¹¹. In one week, a 75 kg person can eat white tuna containing 1 mg of total Hg according to the WHO, hence 2.5 kg (90 oz) of tuna. The US EPA regulations are far stricter and would allow for only 4.5 oz of the same tuna per week. Given that 6-12 ounces of fish are generally recommended per week, it remains quite feasible to meet a reasonable intake assuming the EPA restrictions are a cautious upper limit. Fish remains an important component in our health; its dietary inclusion significantly reduces the chance of suffering coronary heart failure³² typically offsetting the dangers of mercury. In terms of how vital fish are in our diet, less than 10% of the total protein intake in industrialized countries is derived from sources of fish³³.

Some important case studies

         For those living in coastal or island communities there may be increased risks in fish consumption since it comprises a substantially larger portion of their diet ³³. Two studies on methylmercury exposure in island communities should be noted in particular, one from the Republic of Seychelles ^13,34, the other from the Faroe islands ³⁵. The former study found “no detectable adverse effects” on children whose mothers consumed 12 meals of fish per week during pregnancy. The latter study found “adverse” effects on children routinely consuming whale meat and blubber. It has since been considered plausible, however, that PCBs (poly chlorinated biphenyls) simultaneously contaminated the Faroe islanders’ food, enhancing the toxic effects of MeHg ¹⁴.
         Further studies in other remote, fish-eating regions analyze PCBs and MeHg simultaneously (both bio-accumulate in remote areas), found some association with physical effects from the former, but statistically less conclusive results for the latter ^36,37. Saint-Amour et al. concluded that the “clinical significance of the results observed in the present study is difficult to assess” ³⁶. A well-cited report concerning the effects of mercury on a remote population analyzed New Zealand mothers who consumed mercury-contaminated food while later testing the aptitude of their offspring³⁸. But the data was not conclusive; a re-analysis of this same data ³⁹ demonstrated a single child’s aggregate test scores caused substantial changes to the regression. This data point made the difference between concluding there was “an association between high prenatal mercury exposure and decreased test performance” and finding that “[they] did not find significant associations between mercury and children’s test scores”. The evidence for detrimental mercury effects in Kjellstrom’s study was finally concluded as “marginal” ³⁹. Similarly ambiguous results were obtained by Weil et al ⁴⁰, where a group of 474 elderly Americans were observed for blood mercury and neurobehavioral correlations and no statistically significant links were found.

         In studying an Amazonian river-dwelling population, Dolbec et al ²² noted decreasing motor skills with increasing mercury in blood and hair samples (median hair mercury levels: 9 ppm). Mercury hair levels were found to account for 8-16 % of motor skill variance. There was no correlation with mercury exposure and muscle strength. A separate study in the Amazon discovered rates of malaria were four times higher in those with direct exposure to mercury from gold mining ¹². It is certainly possible that mercury weakens the body for other diseases, though this has not been generalized.

         There are of course other isolated populations which can be affected, especially those near heavy industry. It must be remembered that mercury is often rests only one order of magnitude below dangerous consumption levels. To some degree we are playing with fire. But no matter our fears the  data as reported must have the final say; here observe that under normal dietary conditions, and even among those with significant aquatic-based food sources, there are not enough adverse effects to declare such peoples suffer from significantly poorer health. Analytical techniques are clearly able to measure mercury well below toxic threshold levels, but the mere act of discovering mercury in one’s hair, blood, or urine is not unto itself a cause for alarm. 

A side note: Mercury in coal power plants

         In the past 15 years there has been a more concerted effort to remove Hg⁰(g) from coal stack emissions. The U.S. EPA requires under the March 15, 2005 Clean Air Mercury Rule to permanently cap and reduce mercury emissions from coal-fired power plants. Emissions must be drastically reduced, providing incentive to discover high temperature catalytic oxidation reactions ^43-45. The heterogeneous chemistry of atmospheric mercury continues to be the subject of investigation both experimentally ⁴¹. Norton et al ⁴⁵ provide a comprehensive survey of fly ash mixtures optimally suited to mercury removal, noting that NO₂, HCl, and SO₂ enhance oxidation when inserted in a flue gas line with catalytice beds. An efficient and cost-effective method for removing >69% of coal fire mercury is sought by the U.S. Department of Energy ⁴⁶.

         Mercury removal designs, if successful, are potentially profitable, perhaps demonstrated by the numerous patents filed for mercury removal systems, i.e. U.S. Pat. no. 5409522, 6136281, 6322613, 7141091, 7288233, 7037474, and 7217401. Corporations such as ADA Technologies Inc. have been working with the U.S. EPA and the DOE concerning Hg⁰ removal ^47,48, then partnered with CH2M HILL to form Amended Silicates^TM. This program is aimed at mercury removal using Hg⁰(g)-adsorbing chemicals over a silicate substrate ⁴⁹. The cost of removing mercury is currently very high; estimates exceed $29,000 USD/lb of mercury in coal plants^47,50. This potentially makes the removal of gaseous mercury worth its weight in gold. Amended Silicates may have designed a system to reduce this cost to 1,000 USD/lb Hg, however this is not yet implemented.

Conclusion:

         It is a well-established fact that the most geographically widespread mercury contamination comes from the atmosphere. More precisely, about 1/3 comes exclusively from coal fire power plants. Deposited mercury then reaches lakes where it is absorbed into the food chain. Yet contamination in water and air is not a direct threat to us. Fish and seafood are known to contain much higher levels of mercury than their surroundings, but for a normal diet 1ppm mercury is of no concern. Communities with an almost exclusive fish protein diet show little sign of mercury poisoning. PCB contamination, separate from mercury, can also be a contributing cause. Coal fire mercury emissions are being reduced, but the challenge is not yet economically feasible. Options for burning cleaner coal must be considered together with options for burning less coal.

         For those whose source of food is not a luxury of choice, we may expect such coal decontamination to be the most beneficial. For those in regions still practicing artisanal mining, solutions will be of a much different nature.              

            The public is often torn between conflicting reports of fish food health benefits and the dangers of mercury. Anthropogenic mercury emissions have not yet reached dangerous levels; both the USEPA and WHO guidelines show it is possible to consume fish on a regular basis without ill effect. Of greater concern is the sustainability of oceanic fish stocks themselves, the subject of a very different debate.

       
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