Zinc is one of the most common elements in the earth's crust. Zinc is found in the air, soil, and water and is present in all foods. In its pure elemental form, zinc is a bluish-white shiny metal. It also exists as a variety of salts and combines with other elements, such as chlorine, oxygen, and sulfur, to form zinc compounds. Most zinc ore found naturally in the environment is in the form of zinc sulfide. Zinc is an essential element for life when present in trace amounts. Too little zinc in the diet can lead to poor health, reproductive problems, and lowered ability to resist disease. Too much zinc can be harmful to health.
Zinc is one of the most commonly used metals in the world. Its major uses are for galvanizing steel, producing alloys, and for serving as an ingredient in rubber, ceramics, and paints. Zinc compounds are also used to preserve wood and in manufacturing and dyeing fabrics. Zinc chloride is the major ingredient in smoke from smoke bombs. Zinc compounds are used by the drug industry as ingredients in some common products, such as sun blocks, diaper rash ointments, deodorants, athlete's foot preparations, acne and poison ivy preparations, and anti-dandruff shampoos.
Most zinc enters the environment as the result of human activities, such as mining, purifying of zinc, lead, and cadmium ores, steel production, coal burning, and burning of wastes. Zinc may enter the environment in the discharge from galvanizing plants, as a leachate from galvanized structures and natural ores, and from municipal waste treatment plant discharges. Zinc compounds that may be found at hazardous waste sites include zinc chloride, zinc oxide, zinc sulfate, and zinc sulfide.
The level of zinc in soil increases mainly from disposal of zinc wastes from metal manufacturing industries and coal ash from electric utilities. In air, zinc is present mostly as fine dust particles which eventually settles over land and water. Rain and snow aid in removing zinc from air. Most of the zinc in bodies of water, such as lakes or rivers, settles on the bottom. However, a small amount may remain either dissolved in water or as fine suspended particles. The level of dissolved zinc in water may increase as the acidity of water increases. Most of the zinc in soil is bound to the soil and does not dissolve in water. However, depending on the characteristics of the soil, some zinc may reach groundwater. Zinc may be taken up by animals by drinking or eating soil and other animals containing zinc.
Zinc can enter the body through the digestive tract by ingestion of zinc-containing food or water. Zinc can also enter through inhalation of zinc dust or fumes from zinc-smelting or zinc-welding operations. The amount of zinc that passes directly through the skin is relatively small. Zinc is stored throughout the body. Zinc increases in blood and bone most rapidly after exposure. Zinc may stay in the bone for many days after exposure. Normally, zinc is excreted through urine and feces.
Average daily zinc intake through diet generally ranges from 7 to 16.3 mg. Food may contain levels of zinc ranging from approximately 2 ppm (e.g., leafy vegetables) to 29 ppm (meats, fish, poulty). Zinc is also present in most drinking water. Drinking water or other beverages may contain high levels of zinc if they are stored in metal containers or flow through zinc-coated pipes. Drinking water may also be contaminated by zinc from industrial sources or toxic waste sites. High-level exposure to zinc may also result from taking too many zinc dietary supplements. Fetuses and nursing children may be exposed to the zinc in the blood or milk of their mothers.
In general, levels of zinc in air are relatively low and fairly constant. Average levels of zinc in the air are less than 1 ug/m3, but range from 0.1 to 1.7 ug/m3 in areas near cities. Air near industrial areas may have higher levels of zinc. The average zinc concentration for a 1-year period was 5 ug/m3 in one area near an industrial source.
Many zinc salts are highly soluble in water. Zinc and its salts are highly persistent in water, with half-lives greater than 200 days. Powdered zinc is explosive and may burst into flames if stored in damp places. Poisonous gas is produced in fire.
| BENCHMARKS | |
|
Canadian Water Quality Guidelines | |
| Drinking water guideline | 5.0 mg/l |
| Protection of freshwater aquatic life | 0.03 mg/l |
| Irrigation water | 1.0 -5.0 mg/l depending on pH |
| Livestock watering | 50.0 mg/l |
|
Ontario Water Quality Objectives | |
| PWQO | 0.03 mg/l |
| Interim PWQO revised | 0.02 mg/l |
U.S.EPA | |
| Drinking water (taste) | 5 ppm |
| Releases of more than 1,000 (or in some cases 5,000) pounds of zinc or its compounds into the environment must be reported. | |
OSHA | |
| Maximum concentration limit
workplace air |
1 mg/m³ zinc chloride fumes
5 mg/m³ for zinc oxide fumes 8-hour workday/40-hour workweek |
NIOSH | |
| Maximum concentration limit
workplace air |
1 mg/m³ zinc chloride fumes 5 mg/m³ for zinc oxide fumes 10-hour workday/40-hour workweek |
|
Recommended dietary allowance (RDA) for zinc | |
| Men | 15 mg/day |
| Women | 12 mg/day |
| Children | 10 mg/day |
| Infants | 5 mg/day |
LC50 Values | |
| Raibow trout | 0.64 - 3.0 mg/l depending on hardness |
| Daphnia magna | 0.35 mg/l |
| Fathead minnow | 0.64 - 3.0 mg/l depending on hardness |
HUMAN HEALTH
Large doses of zinc (10-15 times higher than the RDA) taken by mouth even for a short time may cause stomach cramps, nausea, and vomiting. Ingesting high levels of zinc for several months may cause anemia, damage the pancreas, and decrease levels of high-density lipoprotein (HDL) cholesterol.
Harmful health effects generally begin at levels from 10-15 times the RDA (in the 100 to 250 mg/day range).
Inhaling large amounts of zinc (as zinc dust or fumes from smelting or welding) can cause a specific short-term disease called metal fume fever. This is believed to be an immune response affecting the lungs and body temperature. Very little is known about the long-term effects of breathing zinc dust or fumes.
High exposure to zinc dust can cause cough with phlegm.
Skin irritation will probably occur in people exposed to some zinc compounds.
Metal particles can irritate the eyes.
Zinc has not been classified for human carcinogenicity.
MEDICAL TESTS
Zinc can be measured in blood or feces.
Zinc can also be measured in urine, saliva, and hair. The amount of zinc in hair is indicative of long-term exposure, but the relationship between levels in hair and the amount exposed to is not clear.
ANIMAL HEALTH
Eating food containing very large amounts of zinc (1,000 times higher than the FDA) for several months caused many health effects in rats, mice, and ferrets, including anemia and injury to the pancreas and kidney.
Rats that ate very large amounts of zinc became infertile. Female rats fed very large amounts of zinc after becoming pregnant gave birth to smaller pups.
Putting low levels of certain zinc compounds, such as zinc acetate and zinc chloride, on the skin of rabbits, guinea pigs, and mice caused skin irritation.
The toxicity of zinc to aquatic life is related to water hardness, with increased toxicity occurring in softer waters. Zinc and its salts have high acute and chronic toxicity to aquatic life.
Some fish accumulate zinc in their bodies if they live in water containing zinc. The concentration of zinc found in fish tissues is expected to be considerably higher than the average concentration of zinc in the water from which the fish was taken.
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