Barium sulfate

Endpoint summary

Additional information

Oral absorption: It exists an evaluation of toxicokinetic information performed by a renown scientific body (ATSDR, 2007) “The absorption of barium from the gastrointestinal tract is compound dependent. Barium sulfate is extremely insoluble and very little, if any, ingested barium sulfate is absorbed.

Dermal absorption: In the absence of measured data on dermal absorption, current guidance suggests the assignment of either 10% or 100% default dermal absorption rates. In contrast, the currently available scientific evidence on dermal absorption of metals (predominantly based on the experience from previous EU risk assessments) yields substantially lower figures, which can be summarised briefly as follows:   Measured dermal absorption values for metals or metal compounds in studies corresponding to the most recent OECD test guidelines are typically 1 % or even less. Therefore, the use of a 10 % default absorption factor is not scientifically supported for metals. This is corroborated by conclusions from previous EU risk assessments (Ni, Cd, Zn), which have derived dermal absorption rates of 2 % or far less (but with considerable methodical deviations from existing OECD methods) from liquid media.   However, considering that under industrial circumstances many applications involve handling of dry powders, substances and materials, and since dissolution is a key prerequisite for any percutaneous absorption, a factor 10 lower default absorption factor may be assigned to such “dry” scenarios where handling of the product does not entail use of aqueous or other liquid media. This approach was taken in the in the EU RA on zinc. A reasoning for this is described in detail elsewhere (Cherrie and Robertson, 1995), based on the argument that dermal uptake is dependent on the concentration of the material on the skin surface rather than it’s mass.   The following default dermal absorption factors for metal cations are therefore proposed (reflective of full-shift exposure, i.e. 8 hours): From exposure to liquid/wet media: 1.0 % From dry (dust) exposure:  0.1 %   This approach is consistent with the methodology proposed in HERAG guidance for metals (HERAG fact sheet - assessment of occupational dermal exposure and dermal absorption for metals and inorganic metal compounds; EBRC Consulting GmbH / Hannover /Germany; August 2007).

Inhalation absorption (for details on calculation please refer to the attachment)    

	absorption factors* via inhalation [%]
Test item
Barium sulfate (most representative form)	4.9
Barium sulfate (fine powder)	8.2

*: rounded values

The fate and uptake of deposited particles depends on the clearance mechanisms present in the different parts of the airway. In the head region, most material will be cleared rapidly, either by expulsion or by translocation to the gastrointestinal tract. A small fraction will be subjected to more prolonged retention, which can result in direct local absorption. More or less the same is true for the tracheobronchial region, where the largest part of the deposited material will be cleared to the pharynx (mainly by mucociliary clearance) followed by clearance to the gastrointestinal tract, and only a small fraction will be retained (ICRP, 1994). Once translocated to the gastrointestinal tract, the uptake will be in accordance with oral uptake kinetics.   In consequence, the material deposited in the head and tracheobronchial regions would be translocated to the gastrointestinal tract without any relevant dissolution in view of the low water solubility of barium sulfate, where it would be subject to gastrointestinal uptake at a ratio of 20%. The material that is deposited in the pulmonary region may be assumed by default to be absorbed to 100%. This absorption value is chosen in the absence of relevant scientific data regarding alveolar absorption although knowing that this is a conservative choice. Thus, the following predicted inhalation absorption factors can be derived for Barium sulfate. For further information on particle size see section 4.5.

 

Distribution

Following ingestion, in humans barium is predominantly found in bone: approximately 90% of the barium inthe body was detected in the bone. Approximately 1–2% of the total body burden was found in muscle, adipose,skin, and connective tissue. This information is supported by a number of studies. Significant increases in the levels of barium in bone were found in rats administered barium chloride in the diet or barium as a component of Brazil nuts for 29 days, although this study did not examine other tissues. A study in which rats were exposed to barium chloride and barium carbonate in drinking water found the following non-skeletal distribution (skeletal tissue was not examined in the study) 24 hours after ingestion: heart > eye > skeletal muscle > kidney > blood > liver (ATSDR, 2007).

Elimination

A study of two humans ingesting a normal diet found that fecal excretion of barium was 2–3 times higher than urinary excretion over a 30-day period. A 29-day rat study also demonstrated that the faeces was the primary route of excretion following exposure to barium chloride in the diet or barium from brazil nuts. A study in rats found that biliary excretion did not significantly contribute to the total amount of barium excreted in the faeces, suggesting that other physiological routes were responsible for faecal barium. A study of rabbits administered an intravenous injection of radiolabelled barium also found that barium was primarily excreted in the faeces. After the first day, faecal excretion was approximately twice as high as urinary excretion. The barium was primarily excreted in the first 5 days after exposure; after 9 days, approximately 50% of the dose was excreted (ATSDR, 2007).

Metabolism

Barium is not metabolised in the body, but it may be transported or incorporated into complexes or tissues (ATSDR, 2007).