What is salt?        

Sodium chloride or common salt is the chemical compound NaCl, composed of the elements sodium and chloride.  Salt occurs naturally in many parts of the world as the mineral  halite and as mixed evaporates in salt lakes.  Seawater  has lots of salt; it contains an average of 2.7% (by weight) NaCl, or 78 million metric tons per cubic kilometer, an inexhaustible supply (note: seawater also contains other dissolved solids; salt represents about 77% of the Total Dissolved Solids). Underground salt deposits are found in both bedded. sedimentary layers and domal deposits.  Deposits have been found to have encapsulated ancient microorganisms including bacteria.  Some salt is on the surface, the dried-up residue of ancient seas like the famed Bonneville Salt Flats in Utah.  Salt even arrives on earth from outer space in meteors and its presence on the planet Mars makes scientists think life may exist there (in fact, scientists speculate that salt-loving bacteria live in underground water on Mars -- as they have survived in suspended animation for 250 million years in Texas).    Conversely, surface salt depositions and man-made saltworks can be seen from space.  In ocean coastal areas, saltwater can "intrude" on underground freshwater supplies, complicating the lives of those who provide our drinking water supplies.

Sodium chloride crystals are cubic in form. Table salt consists of tiny cubes t bound together. 

It varies in color from colorless, when pure, to white, gray or brownish, typical of rock salt (halite). Chemically, it is 60.663% elemental chlorine (Cl) and 39.337% sodium (Na). The atomic weight of elemental chlorine is 35.4527 and that of sodium is 22.989768.    Properties of salt are collected in the U.S. Occupational Safety and Health Administration (OSHA) Chemical Sampling Information database.  More extensive information is available on Material Data Safety Sheets  (MSDSs) or can be found on the following table.

Properties of Pure Sodium Chloride

Sodium chloride is sold in several different particle sizes (gradation) and forms, depending on the intended end use. Discrete crystals can be seen in rock salt used for deicing. Fine granules are typical of table salt and even finer popcorn salt. Kosher salt, pickling salt and ice cream salt are slightly coarser. Small compressed pellets are used in water softeners and large salt blocks are used as salt licks for livestock. When viewed under strong magnification, all sodium chloride is crystalline. Very large cubic crystals, of two, three or more inches in size, can be seen in some salt mines. They are transparent and cleave into perfect cubes when struck with a hard object.

Purity of rock salt produced in North America varies depending on the type of salt (evaporated, rock, solar) and on the source. Rock salt typically ranges between 95% and 99% NaCl, and mechanically evaporated salt and solar salt normally exceed 99% NaCl. Evaporated salt made with purified brine has the highest purity, in some cases 99.99% NaCl. Voluntary standards, such as those developed by the American Society for Testing and Materials (ASTM), the American Water Works Association (AWWA) assure appropriate quality for the intended use. Mandatory specifications for food grade, drug/medical and analytical use include Food Chemicals Codex, U.S. Pharmacopoeia, and Reagent Grade Chemicals.  Special devices, refractometers, are used to measure salinity.

Common salt or sodium chloride is considered by the U.S. Food and Drug Administration as safe for its intended use. This GRAS (generally recognized as safe) classification, and the universal use of sodium chloride since antiquity, affirms its safety. The Merck Index refers to sodium chloride as "(n)ot generally considered poisonous." Many substances in everyday use can be toxic in high concentrations, even water. reports Toxic levels of sodium chloride are reported as:

Oral toxicity (The Registry of Toxic Effects of Chemical Substances, 1986):

Human; TDLo: 12,357 mg/kg/23 D-C

Mouse; LD50: 4,000 mg/kg

Rat; LD50: 3,000 mg/kg

Rabbit; LDLo: 8,000 mg/kg

Acute aquatic toxicity (U.S. EPA, Ambient Water Quality Criteria for Chloride, 1988):

Rana Breviceps (frog); No observed effect concentration (NOEC): 400 mg/L.
Daphnia pulex 48-hour LC50 or EC50: 1,470 mg/L
Daphnia magna (water flea); 48 hour EC50: 3,310 mg/L
Myriophyllum spicatum (water milfoil); Phytotoxicity (EC50 for growth): 5,962 mg/L
Pimephales promealas (fathead minnow); 69-hour LC50: 7,650 mg/L
Lepomis macrochirus (Bluegill) LC50 or EC50: 7,846 mg/L
Anguilla rostrata (American eel) 48-hour LC50 or EC 50: 13,085 mg/L

EPA says that the chlorides of calcium, magnesium and potassium are generally more toxic to fresh water species than sodium chloride.  Some Antarctic species depend on salt to protect them against the cold.

Sodium and chloride occur naturally in soils and waters, and are added by residential, commercial and industrial activity. Aquatic organisms and vegetation, including crops and roadside grasses, shrubs and trees, tolerate various concentrations of sodium and chloride. The following classification is used by the U.S. Department of Agriculture to indicate the degree of hazard of saline soils to food crops. It is based on conductivity and salinity hazard. (Conductivity can be converted to approximate mg/L dissolved solids).

USDA Salinity hazard ratings:

Low: 70 - 175 mg/l
Medium: 176 - 525 mg/l
High: 526 - 1,575 mg/l
Very high: more than 1,575 mg/l