Mill scale (ferrous metal)

Endpoint summary

Description of key information

The REACH Regulations stipulate the need to assess the bioaccumulation potential for aquatic organisms in order to be used for hazard classification and PBT assessment as well as to evaluate the potential risk for secondary poisoning. Secondary poisoning through the terrestrial food chain is not explicitly referred to in REACH. The scope of the terrestrial effects assessment under the adopted REACH regulation is restricted to soil organisms in a narrow sense, i.e, on non-vertebrate organisms living the majority of their lifetime within the soil and being exposed to substances via the soil pathway in line with the previous practice in the environmental risk assessment of new and existing substances in the EU. However, it can be assumed that if there is a concern this pathway will also have to be investigated.

 

In general, metals do not biomagnify unless they are present as, or have the potential to be, in an organic form, e.g. methylmercury is a classic example. Most inorganic metal compounds are not expected to biomagnify. The weight of evidence for iron support this observation and allow waiving the need for additional testing for secondary poisoning for the purpose of fulfilling the REACH obligations.

 

Iron is an essential trace element and is well regulated in all living organisms. It is a key micronutrient for phytoplankton growth where it is used for chlorophyll formation etc. Aquatic invertebrates relying on haemoglobin as respiratory pigment have typically higher iron levels than invertebrates such as gastropods, crustacea and bivalves, relying on phaetocyanin as respiratory pigment (e.g. Timmermans and Waler, 1989).

Differences in iron uptake rates are related to essential needs, varying with the species, size, life stage, seasons, the proportion of organic and inorganic components of the diet, the amount ingested and the conditions of the digestive tract, etc. The iron contents of fish, for example, are very low compared to those of mammals (Van Dijk et al., 1975). Iron homeostatic mechanisms are applicable across species with specific processes being active depending on the species and their life stages. The available evidence shows the absence of iron biomagnification across the tropic chain both in the aquatic and terrestrial food chains. The existing information suggests that iron does not biomagnify, but rather that it tends to exhibit biodilution. Differences in sensitivity among species are not related to the level in the trophic chain but to the capability of internal homeostasis and detoxification.

More information can be found in the attached document “White Paper on waiving for Secondary Poisoning for Al & Fe Compounds_2010”

Additional information

 

Biota possess various mechanisms that allow the modulation of accumulation and its potential toxic impact. They tend to regulate internal concentrations of metals through (1) active regulation, (2) storage, or (3) a combination of active regulation and storage over a wide range of environmental exposure conditions. Although these homeostatic control mechanisms have evolved largely for essential metals, it should be noted that non- essential metals are also often regulated to varying degrees because the mechanisms for regulating essential metals are not entirely metal-specific. Some species (mostly plants) could also be adapted to a natural enriched environment and as such accumulate high levels of metals. Most often these phenomena are very local and not an overall concern for secondary poisoning and biomagnification for inorganic metal species.

 

Bioaccumulation and biomagnification factors are typically used for estimating these processes. However, for many metals the BCF/BAF is not independent of exposure concentration for many metals (Brix and Deforest, 2000 and Mc Geer et al., 2003). Rather it is inversely related (i.e., decreasing BCF/BAFs with increasing exposure concentration) to exposure concentration. The observed inverse relationship has been explained by homeostatic regulations of internal tissue concentrations: at low metal concentrations organisms are actively accumulating metals in order to meet their metabolic requirements, while at high ambient metal concentration organisms are able to excrete excess metals or limit uptake. From the above it is clear that any conclusion based on the application of classical concepts (e.g., use of bioconcentration factors; BCF -biomagnification factors; BMF) to assess metal hazards in the same way as they are applied to organic substances should be treated with caution.

 

Iron is an essential trace element for organisms including micro-organisms, plants and animals. Iron plays an important role in biological processes, and iron homeostasis is under strict control. Many organisms actively regulate uptake of iron into cells. It is an important factor in oxygen transport in the blood, oxidative metabolism, electron transport, nitrogen fixation and other biological processes in cells. Iron is a constituent of haemoglobin, enzyme systems and chlorophyll molecules all essential to life. Iron can also be bound into various chelate complexes or proteins in biological material (Merck Index, 2001; Neilands, 1972; WHO, 1983). Intracellular iron homeostasis is achieved by coordinated regulation of iron storage by protein, ferritin and iron uptake protein. Besides these active regulation mechanisms, some groups of organisms have developed additional internal regulation mechanism (molecular binding and sequestration) as a strategy to cope against iron excess (Ganz and Nemeth, 2006). Vertebrate dietary iron exposure studies (mammals) further demonstrated that intestinal adsorption/biliary excretion of iron is regulated with varying dietary intakes (Stewart et al. 1950; Hahn et al. 1943).

 

Iron deficiency has been observed in intensive cultures of fish, crops and farm animals, but the most striking examples of iron deficiency come from agricultural farming practices. Studies with experimental animals have shown that iron deficiency may result in an increased absorption of toxic heavy metals and anaemia. In contrast to the widespread occurrence of iron deficiency, iron overload is a rare condition that only occurs in a number of special situations. These are special dietary situations or certain disease states, which cause a breakdown of the normal control of iron absorption. The end result is an excessive body store of iron.

 

The up/down regulation of iron uptake and excretion rates as well as the internal detoxification mechanisms (ferritin protein binding, iron granules) in fish and invertebrates can explain the observed absence of iron toxicity induced by accumulation of iron from dietary exposure (Uysal et al, 2008 ; Fleming and Joshi, 1991). The bioavailability of detoxified stored metals is equally important in the evaluation of a potential for secondary poisoning. If the stored metal-rich compounds can be broken down in the digestive tract then the metal can potentially be transferred across the food chain. Existing information suggests that iron does not biomagnify, but rather that it tends to exhibit biodilution (i.e., iron concentrations decreasing with increasing trophic level) (Quinn et al, 2003; Winterbourn et al., 2000; Ciesielski et al, 2006).