Silver chromate is a chemical compound of silver and hexavalent chromium. It is used in photography. Hexavalent chromium refers to chemical compounds that contain the element chromium in the +6 oxidation state. Chromium(VI) is more toxic than other oxidation states of the chromium atom because of its greater ability to enter cells and higher redox potential. Silver is a metallic element with the chemical symbol Ag and atomic number 47. It occurs naturally in its pure, free form, as an alloy with gold and other metals, and in minerals such as argentite and chlorargyrite. (14, 15, 10, 11)
belongs to the class of inorganic compounds known as transition metal oxides. These are inorganic compounds containing an oxygen atom of an oxidation state of -2, in which the heaviest atom bonded to the oxygen is a transition metal.
Hexavalent chromium's carcinogenic effects are caused by its metabolites, pentavalent and trivalent chromium. The DNA damage may be caused by hydroxyl radicals produced during reoxidation of pentavalent chromium by hydrogen peroxide molecules present in the cell. Trivalent chromium may also form complexes with peptides, proteins, and DNA, resulting in DNA-protein crosslinks, DNA strand breaks, DNA-DNA interstrand crosslinks, chromium-DNA adducts, chromosomal aberrations and alterations in cellular signaling pathways. It has been shown to induce carcinogenesis by overstimulating cellular regulatory pathways and increasing peroxide levels by activating certain mitogen-activated protein kinases. It can also cause transcriptional repression by cross-linking histone deacetylase 1-DNA methyltransferase 1 complexes to CYP1A1 promoter chromatin, inhibiting histone modification. Chromium may increase its own toxicity by modifying metal regulatory transcription factor 1, causing the inhibition of zinc-induced metallothionein transcription. Metallic silver is oxidized and may deposit in the tissues, causing arygria. The silver ion is known to inhibit glutathione peroxidase and NA+,K+-ATPase activity, disrupting selenium-catalyzed sulfhydryl oxidation-reduction reactions and intracellular ion concentrations, respectively. Silver nanoparticles are believed to disrupt the mitochondrial respiratory chain, causing oxidative stress, reduced ATP synthesis, and DNA damage. (14, 6, 7, 8, 9, 1, 10, 2, 3, 4)
Metabolism
Chromium is absorbed from oral, inhalation, or dermal exposure and distributes to nearly all tissues, with the highest concentrations found in kidney and liver. Bone is also a major storage site and may contribute to long-term retention. Hexavalent chromium's similarity to sulfate and chromate allow it to be transported into cells via sulfate transport mechanisms. Inside the cell, hexavalent chromium is reduced first to pentavalent chromium, then to trivalent chromium by many substances including ascorbate, glutathione, and nicotinamide adenine dinucleotide. Chromium is almost entirely excreted with the urine. Silver compounds can also be absorbed orally and dermally. It distributes throughout the body in the blood, particularily to the liver. Insoluble silver salts are transformed into soluble silver sulfide albuminates, bind to amino or carboxyl groups in RNA, DNA, and proteins, or are reduced to metallic silver by ascorbic acid or catecholamines. Metallic silver is oxidized and may deposit in the tissues, causing arygria. Silver is eliminated primarily in the faeces. (14, 1, 10)
Toxicity Values
Not Available
Lethal Dose
1 to 3 grams for an adult human (hexavalent chromium). (5)
Hexavalent chromium is a known carcinogen. Chronic inhalation especially has been linked to lung cancer. Hexavalent chromium has also been know to cause reproductive and developmental defects. Exposure to high levels of silver for a long period of time may result in a condition called arygria, a blue-gray discoloration of the skin and other body tissues. Argyria is a permanent effect but does not appear to be harmful to health. While silver itself is not toxic, most silver salts are, and may damage the liver, kidney, and central nervous system, as well as be carcinogenic. (14, 15, 16, 1)
Symptoms
Breathing hexavalent chromium can cause irritation to the lining of the nose, nose ulcers, runny nose, and breathing problems, such as asthma, cough, shortness of breath, or wheezing. Ingestion of hexavalent chromium causes irritation and ulcers in the stomach and small intestine, as well as anemia. Skin contact can cause skin ulcers. Exposure to high levels of silver for a long period of time may result in a condition called arygria, a blue-gray discoloration of the skin and other body tissues. Argyria is a permanent effect but does not appear to be harmful to health. Exposure to high levels of silver in the air has resulted in breathing problems, lung and throat irritation, and stomach pains. Skin contact with silver can cause mild allergic reactions such as rash, swelling, and inflammation in some people. (14, 10)
Treatment
There is no know antidote for chromium poisoning. Exposure is usually handled with symptomatic treatment. (10)
