Calcium occurrence

The main role of the human thyroid gland is production of thyroid hormones (iodinated amino acids), essential for adequate growth, development, and energy metaboHsm (1 6). Thyroid underfunction is an occurrence that can be treated successfully with thyroid preparations. In addition, the thyroid secretes calcitonin (also known as thyrocalcitonin), a polypeptide that lowers excessively high calcium blood levels. Thyroid hyperfunction, another important clinical entity, can be corrected by treatment with a variety of substances known as antithyroid dmgs. 

Occurrence. Carbon monoxide is a product of incomplete combustion and is not likely to result where a flame bums in an abundant air supply, yet may result when a flame touches a cooler surface than the ignition temperature of the gas. Gas or coal heaters in the home and gas space heaters in industry have been frequent sources of carbon monoxide poisoning when not provided with effective vents. Gas heaters, though properly adjusted when installed, may become hazardous sources of carbon monoxide if maintained improperly. Automobile exhaust gas is perhaps the most familiar source of carbon monoxide exposure. The manufacture and use of synthesis gas, calcium carbide manufacture, distillation of coal or wood, combustion operations, heat treatment of metals, fire fighting, mining, and cigarette smoking represent additional sources of carbon monoxide exposure (105—107). 

Occurrence. Brines are the main commercial source of calcium chloride [10043-52 ]. Some brines of Michigan, Ohio, West Virginia, Utah, and California contain over 4% calcium (27). Michigan is the leading state ia natural calcium chloride production with California a distant second. 

The findings recently reported can be relevant to understand the functional role of transmitter release from astrocytes in brain function. Interestingly, a recent report shows for the first time the occurrence in vivo of fast astrocytic calcium events in response to neuronal activity, peaking with millisecond time-scale from sensory stimulation (Winship et al. 2007). 

The often smooth top surface to the calcrete suggests that the calcrete formed by the precipitation of Ca (as carbonate) from sub-surface laterally and vertically flowing vadose water sourced from rainfall, which penetrates easily through the overlying sand. The sub-surface has been plugged with calcrete preventing vertical downwards penetration of vadose water. Instead, vadose water would move laterally until it evaporates or is removed by transpiration. Precipitation of calcium carbonate at the surface of the calcrete would occur, thus contributing to the laminated occurrence of the calcrete. 

Macrolide antibiotics (clarithromycin, dehydroerythromycin, etc.) and sulfonamides (sulfamethoxazole, sulfadimethoxine, sulfamethazine, and sulfathi-azole) are the most prevalent antibiotics found in the environment with levels around a few micrograms per liter, whereas fluoroquinolones, tetracyclines, and penicillins have been detected in fewer cases and usually at low concentrations (nanograms per liter) [3,20,23,72]. This result is not surprising, since penicillins are easily hydrolyzed and tetracyclines readily precipitate with cations such as calcium and are accumulated in sewage sludge or sediments. Several reviews have reported the environmental occurrence of different antibiotics in aquatic and soil compartments. Some of these data are detailed in Table 1. 

Occurrence. Carbon is distributed very widely in nature as calcium carbonate (limestone). Coal, petroleum and natural gas are chiefly hydrocarbons. Carbon is found as carbon dioxide in the atmosphere of the earth and dissolved in all natural waters. The atmosphere of Mars contains 96% C02. It is plentiful in the sun, stars, comets and the atmospheres of most planets. 

These studies revealed the occurrence of three distinct fluids i) a NaCI-rich brine with about 10-19 wt.% NaCI, 4-11 wt.% CaCb, and 0-6 wt.% MgCl2, ii) a calcium-rich brine with about 1-8 wt.% NaCI, 14-23 wt.% CaCb and almost no detectable MgCb ill) a low-salinity fluid with about 3-13 wt.% NaCI, 1-13 wt.% CaCb, and 0-0.6 wt.% MgCl2. 

Occasionally, the phosphate slime is difficult to settle in the lagoons because of its true colloidal nature, and the use of calcium sulfate or other electrolytes can promote coagulation, agglomeration, and settling of the particles. Usually an addition of calcium sulfate is unnecessary, because it is present in the wastewater from the sand-flotation process. Generally, it has been shown [33] that the clear effluent from the phosphate mining and beneficiation operation is not deleterious to fish life, but the occurrence of a dam break may result in adverse effects [19]. 

The occurrence and timing of effects on intracellular ionized calcium concentration, lysosomal mass, oxidative stress or plasma membrane permeability frequently provide additional information indicative of mechanism of toxicity (Table 14.5). 

---