Summary - Hg

Mercury (Hg) is a metal that is released into the environment from both natural and anthropogenic sources. After release into the environment, it undergoes complex transformations and cycles between atmosphere, land and aquatic systems. During this biogeochemical cycle, humans, plants, and animals are exposed to mercury, potentially resulting in a variety of health impacts (EFSA, 2008; EFSA, 2012)[1-Link,2-Link].

The three chemical forms of mercury are (i) elemental or metallic mercury (Hg⁰), (ii) inorganic mercury (mercurous (Hg₂²⁺) and mercuric (Hg²⁺) cations) and (iii) organic mercury.

In its elemental form, mercury is a liquid at ambient temperatures and pressures and it volatilises strongly. In general, elemental mercury is the predominant form of mercury in the atmosphere (Selin, 2009)[3-Link]. Inorganic mercury (IHg) compounds are salts of Hg₂²⁺ and Hg²⁺, which are used in several industrial processes and can be found in batteries, fungicides, antiseptics or disinfectants (EFSA, 2008).

Organic mercury compounds have at least one carbon atom covalently bound to the mercury atom. Methylmercury (MeHg) is by far the most common form in the food chain (EFSA, 2008). Other organic mercury compounds like phenylmercury, thiomersal and merbromin (also known as Mercurochrome) have been used as fungicides and in pharmaceutical products (EFSA, 2008).

Elemental mercury and major mercury ions/species in environmental and biological samples (adapted from Kuban et al. (2007))[4-Link].

			CAS number
Elemental mercury		Hg 0	92786-62-4
Inorganic mercury ions	Mercurous ion	Hg 2 2+	Not available
	Mercuric ion	Hg 2+	7439-97-6
Organic mercury ions/species	Methylmercury	CH 3Hg +	22967-92-6
	Dimethylmercury	(CH 3) 2Hg	593-74-8
	Ethylmercury	CH 3CH 2Hg +	627-44-1
	Phenylmercury	C 6H 5Hg +	23172-37-4

The largest source of mercury exposure for most people in developed countries is inhalation of mercury vapour due to the continuous release of elemental mercury from dental amalgam. Exposure to methylmercury mostly occurs via the diet. Methylmercury collects and concentrates especially in the aquatic food chain, making populations with a high intake of fish and seafood particularly vulnerable (European Commission, 2005; Richardson et al., 2011)[5-Link,6-Link].

A Joint FAO/WHO Expert Committee on Food Additives (JECFA) defined provisional tolerable weekly intakes for methylmercury of 1.6 µg/kg body weight (b.w.) and of 4 µg/kg b.w. for inorganic mercury [7-Link]. In line with JECFA, an EFSA Panel on Contaminants in the Food Chain (CONTAM) established a tolerable weekly intake (TWI) for inorganic mercury of 4 µg/kg b.w., expressed as mercury [2-Link]. For methylmercury, new developments in epidemiological studies have indicated that n-3 long-chain polyunsaturated fatty acids in fish may counteract negative effects from methylmercury exposure. Together with the information that beneficial nutrients in fish may have confounded previous adverse outcomes in previous studies, the JECFA Panel established a TWI for methylmercury of 1.3 µg/kg b.w., expressed as mercury.

The mean dietary exposure across age groups does not exceed the TWI for methylmercury, with the exception of toddlers and other children in some surveys [2-Link]. The 95th percentile dietary exposure is close to or above the TWI for all age groups. High fish consumers, which might include pregnant women, may exceed the TWI by up to approximately six-fold. Unborn children constitute the most vulnerable group. Biomonitoring data from blood and hair indicate that methylmercury exposure is generally below the TWI in Europe, but higher levels are also observed. Exposure to methylmercury above the TWI is of concern. If measures to reduce methylmercury exposure are considered, the potential beneficial effects of fish consumption should also be taken into account.

Dietary inorganic mercury exposure in Europe does not exceed the TWI, but inhalation exposure of elemental mercury from dental amalgam is likely to increase the internal inorganic mercury exposure; thus the TWI might be exceeded [2-Link].