The ins and outs of the mercury soil guideline value withdrawal
On November 21st the Environment Agency announced they would be withdrawing the Soil Guideline Value (SGV) for mercury (including the associated TOX Report and Supporting Information Document) following discussions with Public Health England (PHE). The discussions arose from a 2012 European Food Safety Authority (EFSA)1 report that considered methylmercury and inorganic mercury in food and the associated public health risk. In summary, the EFSA identified an oral Total Daily Intake (TDI) for both the methyl and inorganic forms of mercury that were lower than the Health Criteria Values (HCV) used to create the SGV 2.
Soil Guideline Values (SGV) are designed to assist professionals in the assessment of long-term risk to health from human exposure to chemical contaminants in soil. SGV are ‘trigger values’ for screening out low risk areas of land contaminated. They give an indication of representative average levels of chemicals in soil below which the long-term health risks are likely to be minimal. Exceeding an SGV does not mean that remediation is always necessary, although in many cases some further investigation and evaluation of the risk will be carried out.
The Environment Agency did confirm that they would not be providing an update to the SGVs as guideline creation is no longer their brief. However, they will continue to recommend that relevant bodies providing public health assessments be consulted where alternative guideline criteria have been published or are being developed for mercury (including elemental mercury).
An example of a suitable alternative would be the S4ULs (Suitable 4 Use Levels) produced by LQM/CIEH, which are an update to their previous Generic Assessment Criteria (GACs). The authors of the guidance consider the S4ULs to be the equivalent to the SGVs and therefore relevant for use in generic quantitative risk assessments under both planning and Part 2A regimes3. In addition, Section 8 of the S4ULs, which relates to mercury, considered the EFSAs report as part of the process to derive the oral TDI.
Why is Mercury a problem?
Whilst some metals are essential for a person’s growth and development, mercury is similar to other heavy metals in that it does not aid any essential biological function 4. However, the toxicity of mercury can vary widely as is it is found in three forms: elemental, inorganic and organic. Elemental and inorganic forms are absorbed particularly effectively via inhalation and ingestion respectively, however the organic form of mercury can be absorbed via all available routes into the body. The reason for this is as follows5:
- Elemental mercury is most easily absorbed via inhalation because it vaporises at room temperature. It can pass directly into the bloodstream through the lungs, giving it access to all parts of the body. However, ingestion rarely results in severe poisoning as it has a low absorption rate through the gut.
- Inorganic mercury is water soluble, easily absorbed through the gut. These salts don’t vaporise easily, although aerosols can be inhaled and enter the bloodstream.
- Organic mercury is soluble in fats, which means it can enter cells easily. As a result, it is able to readily enter the nervous system and cross into the brain.
In addition to direct effects mercury bioaccumulates and biomagnifies. Bioaccumulation is where substances are retained and therefore accumulate. Biomagnification is a step further, with the chemical becoming increasingly concentrated with the food chain6.
What is the treatment?
Mercury poisoning can be partially or fully treated by the process of chelation therapy**, which is also used to treat other types of heavy metal poisoning. A larger compound is used to bind the mercury, a chelating agent, in this case EDTA (Ethylene Diamine Tetra-acetic Acid). This stops the biological action of the metal and also enables the body to remove it in urine7.
** Defined as “the process by which a chelate is formed”, which is itself the combination of a metal ion with a chemical compound (a chelating agent) to form a heterocyclic ring (a ring containing at least two elements). 8
What does this mean for the future?
The removal of the SGV for mercury will leave professionals without a freely available source of ‘trigger values’ for use in their contaminated land risk assessments. While others are available, this lack of a standard could lead to a varied approach between sites. Conversely, what does this mean for sites that have already been assessed for methyl and inorganic forms of mercury? As the SGV was withdrawn primarily due to the HCVs used to derive them being too high, does this mean that sites assessed and considered to be suitable for their current use based on these values will need to be reassessed to ensure they don’t in fact pose a risk to human health?
- EFSA Panel on Contaminants in the Food Chain (2012). Scientific Opinion on the risk for public health related to the presence of mercury and methylmercury in food. EFSA Journal 2012;10(12):2985.
- CL:AIRE (2018). Withdrawal of the SGV and associated reports for Mercury (online). UK, https://www.claire.co.uk/home/news/1118-withdrawal-of-the-sgv-and-associated-reports-for-mercury, accessed 27th November 2018
- P. Nathanail, C. McCaffrey, A. Gillett, R. Ogden and J. Nathanail (2015). The LQM/CIEH S4ULs for Human Health Risk Assessment. Copyright Land Quality Press, Nottingham.
- 6. M. Gochfeld (2003). Cases of mercury exposure, bioavailability, and absorption. Ecotoxicology and Environmental Safety 56(1): 174-179.
- N.J. Langford and R.E. Ferner (1999). Toxicity of mercury. Journal of Human Hypertension 13: 651-656.
- P.K. Nag, S. Kumar, R.R. Tiwari, S. Patel, N. Joshi, H.V.K. Bhatt and S.K. Ghosh (2007). Mercury: Exposure & Effects. ENVIS-NIOH Newsletter 2(2).
- Healthline (2005-2018). What Does Chelation Therapy Treat? (online). USA, https://www.healthline.com/health/chelation-therapy, accessed 27th November 2018.
- The Free Dictionary By Farlex (2003-2015). Chelation (online). Huntingdon Valley, PA, USA, http://www.thefreedictionary.com/chelation, accessed 23rd January 2015.