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Rare Earth Compounds Toxicity


Occurrence and Chemical Properties :
 
The term rare earths usually refers to the lanthanum series (atomic numbers 57 to 71) with yttrium (39), also called the lanthanons (39, 57 to 71).
They are found together in various combinations in many ores :
-monazite,
-xenotime,
-gadolinite,
-samarskite,
-fergusonite,
-apatite,
-euxenite,
-bastnaesite,
 
mainly in the following countries :
-Norway,
-Sweden,
-USA,
-Australia,
-India,
-Canada,
-Brazil.
 
They have similar molecular structures, this makes separation of them difficult.
All are relatively soft metals, malleable, with a bright silver luster, altough some oxidize readily in air.
The most common is cerium and it is more abundant than lead or copper.
 
Industrial Processes and Uses :
 
The rare earths are the subject of much research and new uses will be found.
 
Cerium is used in :
-optical glass,
-electrodes,
-printing,
-dyeing,
-ceramics,
-fireworks,
-metallurgical alloys,
-mildew-proofing (textile industry),
-tracer bullets,
-carbon arc rods,
-etc.
 
Praseodymium is used :
-to color glass,
-to make arc cores for lights in theater projection machines, searchlights, and other intense lighting situations.
-to make alloys like most rare earths.
 
Neodymium is a source of promethium.
It is used :
-to make purple glass,
-in doping glass lasers,
-in carbon arc rods.
 
Promethium is used in luminescent dials.
 
Samarium is used :
-in making infra-red absorbing glass,
-as a dopant for lasers,
-as a neutron absorber in nuclear reactors,
-as a constituent of television phosphors.
 
Terbium is used as a solid-state and laser dopant.
 
Erbium is used :
-in metallurgy,
-nuclear research,
-to color glass and porcelain.
 
Thullium has 16 important isotopes. Isotope 170 is used in small portable radiation units.
 
Ytterbium is another rare earth that is used in :
-lasers,
-alloys,
-and as an x-ray source for portable irradiation devices.
Ytterbium 169 emits gamma rays and is used in radiography of small castings and other small materials.
 
Yttrium is used :
-in optical glasses,
-in ceramics,
-in color TV tubes,
-in alloys to increase their strenght
 
Europium can be used to detect chrome in contaminated water
Uses in Medicine :
 
Many rare earths were and/or are used as diagnostic and therapeutic tools.
 
Rare Earth Analysis :
 
With the exception of cerium, atomic absorption spectrophotometry is the method of choice for rare earth quantitative determination.
 
Biologic effects :
 
I-Absorption :
The rare earth metals are not absorbed from the skin, are poorly absorbed from the gastrointestinal tract, and are slowly absorbed from the lungs or on injection.
On absorption, scandium and the rare earth metals tend to collect in the liver and the skeleton. There is a transition to more bone with the rare earths of higher atomic number. After deposition in the skeleton it may take years before removal is completed.
Starvation lessened the excretion.
 
II-Clinical Findings :
 
A-General :
Headache and nausea from dust and fumes from using cored light carbons containing lanthanum has been reported.
 
B-Lungs :
 
1-Acute exposure :
If in great enough quantity, the rare earth compounds can produce, on inhalation, acute chemical irritation pneumonitis, and bronchitis.
 
2-Chronic exposure :
More than 20 cases of occupational pneumoconiosis have been reported, mainly in photoengravers and projectionists, from chronic inhalation of rare earth oxide fumes from carbon arc lamps.
Cerium, lanthanum, and neodymium are the major oxides in the fumes; but samarium, terbium, ytterbium,and lutenium have also been removed from fibrotic pneumoconiosis nodules. In some of these cases abnormal levels of rare earths were demonstrated in the nails suggesting absorption from the lungs.
 
C-Skin :
Terbium chloride has been noted to be an irritant to the intact skin.
Gadolinium and samarium chlorides produced ulcers on abraded skin.
Lutetium and europium chlorides caused extensive scarring on abraded skin, and, along with dysprosium, holmium, and erbium, produced nodules from intradermal injections.
 
D-Eyes :
The chloride of each of the rare earths caused ocular irritation in the form of transient conjunctivitis. In addition, terbium chloride produced corneal damage.
The lanthanons can produce opacity to the abraded cornea.
 
Diagnostic Methods :
 
There are no signs or symptoms pathognomonic of toxicity from lanthanons.A history of exposure would be necessary in making the diagnosis.
 
A-Chest X-Ray :
 
1-Acute Exposure :
A chest x-ray after massive acute inhalation might show signs of oedema or pneumonitis.
 
2-Chronic Exposure :
Chronic inhalation can produce a diffuse nodular pattern on chest x-ray from the birotic nodules composed of rare earths. The pulmonary function test might show a restrictive pattern.
Energy dispersive x-ray fluorescence of bronchoalveolar lavage fluids can establish or confirm occupational exposure of workers to rare earths.
 
B-Urinary Excretion :
Excretion of the rare earth metals in the urine is a small part of the total excretion by the body but could be measured accurately. No standards of urinary level have been established, because it is not yet known if there is a correlation between the urinary level and either the exposure or the fecal excretion.
 
C-Fecal Excretion :
Ingestion estimates have usually been based on measurements of the rare earths in the feces.
Fecal excretion of lanthanons is measurable. The level of excretion varies greatly with the route of exposure and the solubility of the lanthanon.
No indices of toxicity based on fecal excretion have been established, but fecal levels do afford an inconvenient method of monitoring exposure.
 
 
Treatment :
 
Some lanthanons delay healing, cause irritation and ulcers on abraded skin, and form nodules intradermally. Extra care should be used in cleaning wounds contaminated with a lanthanon.
If lanthanon toxicity is suspected, the patient should be removed from further exposure.
There is no specific therapy, and treatment of any findings that are believed to be related to the lanthanon exposure, such as acute chemical pneumonitis, hematuria or leukopenia, are treated in the usual manner.
 
 
Exposure Limit :
 
The VEMP in Quebec for yttrium is 1mg/m3.
 
Exposure Control :
 
Incidental ingestion is considered harmless.
Skin contact should be avoided, especially with rare earths of the terbium and ytterbium groups.
Employees with skin cuts, abrasion, dermatitis, conjunctivitis, corneal injuries, or keratoconus need protection that will prevent contamination of the skin lesion or the eye by any rare earth.
Employees with lung disease, decreased pulmonary function, or x-ray evidence of fine lung opacities (pneumoconiosis) need special consideration to avoid rare earth dusts or fumes.
Consideration should be given to avoiding exposure to rare earths for employees with increased blood coagulation time or leukopenia.
Heated rare earths give off toxic fumes that should be controlled.
 
 
 
 
References :
 
1-Occupational Medicine, Carl Zenz, last edition.
2-Sax's Dangerous Properties of Industrial Materials, Lewis C., last edition.
3-Toxicologie Industrielle et Intoxications Professionnelles, Lauwerys R.R.,last edition.
4-INRS, Exposition aux poussières de terres rares, Peltier A., 1986
 
 

Out Bound Links




Edouard Bastarache M.D.
Occupational & Environmental Medicine
Author of "Substitutions for Raw Ceramic Materials"
Tracy, Québec, CANADA

edouardb@sorel-tracy.qc.ca
http://www.sorel-tracy.qc.ca/~edouardb/




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