Fluorine, chlorine, bromine, iodine and the more recently discovered radioactive element astatine, make up the family of elements known as the halogens. Except for astatine, the physical and chemical properties of these elements have been exhaustively studied. They occupy group VII in the periodic table, and they display an almost perfect gradation in physical properties.
The family relationship of the halogens is illustrated also by the similarity in the chemical properties of the elements, a similarity which is associated with the arrangement of seven electrons in the outer shell of the atomic structure of each of the elements in the group. All the members form compounds with hydrogen and the readiness with which union occurs decreases as the atomic weight increases. In like manner, the heats of formation of the various salts decrease with the increasing atomic weights of the halogens. The properties of the halogen acids and their salts show as striking a relationship; the similarity is apparent in organic halogen compounds, but, as the compound becomes chemically more complex, the characteristics and influences of other components of the molecule may mask or modify the gradation of properties.
Halogens are used in the chemical, water and sanitation, plastics, pharmaceutical, pulp and paper, textile, military and oil industries. Bromine, chlorine, fluorine and iodine are chemical intermediates, bleaching agents and disinfectants. Both bromine and chlorine are used in the textile industry for bleaching and shrink-proofing wool. Bromine is also used in gold mining extraction processes and in oil- and gas-well drilling. It is a fire retardant in the plastics industry and an intermediate in the manufacture of hydraulic fluids, refrigerating and dehumidifying agents, and hair-waving preparations. Bromine is also a component of military gas and fire-extinguishing fluids.
Chlorine is used as a disinfectant for refuse and in the purification and treatment of drinking water and swimming pools. It is a bleaching agent in laundries and in the pulp and paper industry. Chlorine is used in the manufacture of special batteries and chlorinated hydrocarbons, and in the processing of meat, vegetables, fish and fruit. In addition, it acts as a flame retardant. Chlorine dioxide is utilized in the water and sanitation and swimming pool industries for water purification, taste and odour control. It is a bleaching agent in the food, leather, textile, and pulp and paper industries, as well as an oxidizing agent, bactericide and antiseptic. It is used in cleaning and detaining leather and in bleaching cellulose, oils and beeswax. Nitrogen trichloride was formerly used as a bleach and “improver” for flour. Iodine is also a disinfectant in the water and sanitation industry and acts as a chemical intermediate for inorganic iodides, potassium iodide, and organic iodine compounds.
Fluorine, fluorine monoxide, bromine pentafluoride and chlorine trifluoride are oxidizers for rocket fuel systems. Fluorineis also used in the conversion of uranium tetrafluoride to uranium hexafluoride, and chlorine trifluoride is used in nuclear reactor fuel and for cutting oil-well tubes.
Calcium fluoride, found in the mineral fluorspar, is the primary source of fluorine and its compounds. It is used in ferrous metallurgy as a flux to increase fluidity of the slag. Calcium fluoride is also found in the optical, glass and electronics industries.
Hydrogen bromide and its aqueous solutions are useful for manufacturing organic and inorganic bromides and as reducing agents and catalysts. They are also used in the alkylation of aromatic compounds. Potassium bromide is used to manufacture photographic papers and plates. Large quantities of phosgene gas are required for numerous industrial syntheses, including the manufacture of dye-stuffs. Phosgene is also used in military gas and in pharmaceuticals. Phosgene is found in insecticides and fumigants.
The similarity which these elements exhibit in chemical properties is apparent in the physiological effects associated with the group. The gases (fluorine and chlorine) and the vapours of bromine and iodine are irritants of the respiratory system; inhalation of relatively low concentrations of these gases and vapours gives an unpleasant, pungent sensation, which is followed by a feeling of suffocation, coughing and a sensation of constriction in the chest. The damage to the lung tissue which is associated with these conditions may cause the lungs to become overloaded with fluid, resulting in a condition of pulmonary oedema which may well prove fatal.
Fluorine and its compounds
The majority of fluorine and its compounds is obtained directly or indirectly from calcium fluoride (fluorspar) and phosphate rock (fluorapatite), or chemicals derived from them. The fluoride in phosphate rock limits the usefulness of this ore and, therefore, the fluoride must be removed almost completely in the preparation of elemental phosphorus or food-grade calcium phosphate, and partially in the conversion of fluorapatite to fertilizer. These fluorides are recovered in some cases as aqueous acid or as calcium or sodium salts of the liberated fluoride (probably a mixture of hydrogen fluoride and silicon tetrafluoride), or released to the atmosphere.
Fire and explosion hazards
Many of the fluorine compounds present a fire and explosion hazard. Fluorine reacts with nearly all materials, including metal containers and piping if the passivating film is broken. The reaction with metals can produce hydrogen gas. Absolute cleanliness is required in conveying systems to prevent localized reactions and subsequent fire hazards. Special lubricant-free valves are used to prevent reactions with lubricants. Oxygen difluoride is explosive in gaseous mixtures with water, hydrogen sulphide or hydrocarbons. When heated, many fluorine compounds produce poisonous gases and corrosive fluoride fumes.
Hydrofluoric acid. Skin contact with anhydrous hydrofluoric acid produces severe burns that are felt immediately. Concentrated aqueous solutions of hydrofluoric acid also cause an early sensation of pain but dilute solutions may give no warning of injury. External contact with liquid or vapour causes severe irritation of eyes and eyelids that may result in prolonged or permanent visual defects or total destruction of eyes. Fatalities have been reported from skin exposure to as little as 2.5% of total body surface.
Quick treatment is essential and should include washing copiously with water on the way to the hospital, then soaking in an iced solution of 25% magnesium sulphate if possible. Standard treatment for mild to moderate burns involves the application of a calcium gluconate gel; more severe burns may require injection in and around the affected area with 10% calcium gluconate or magnesium sulphate solution. Sometimes local anaesthesia may be needed for pain.
Inhalation of concentrated hydrofluoric acid mists or anhydrous hydrogen fluoride may cause severe respiratory irritation, and as little as 5 minutes’ exposure is usually fatal within 2 to 10 hours from haemorrhagic pulmonary oedema. Inhalation may also be involved in skin exposures.
Fluorine and other fluorinated gases. Elemental fluorine, chlorine trifluoride and oxygen difluoride are strong oxidizers and may be highly destructive. At very high concentrations, these gases may have an extremely corrosive effect on animal tissue. However, nitrogen trifluoride is strikingly less irritating. Gaseous fluorine in contact with water forms hydrofluoric acid, which will produce severe skin burns and ulceration.
Acute exposure to fluorine at 10 ppm causes slight skin, eye and nasal irritation; exposure above 25 ppm is intolerable, although repeated exposures may cause acclimatization. High exposures may cause delayed pulmonary oedema, haemorrhage and kidney damage, and possibly be fatal. Oxygen difluoride has similar effects.
In an acute rat inhalation study with chlorine trifluoride, 800 ppm for 15 minutes and 400 ppm for 25 minutes were fatal. The acute toxicity is comparable to that of hydrogen fluoride. In a long-term study in two species, 1.17 ppm caused respiratory and eye irritation, and in some animals, death.
In long-term repeated inhalation animal studies with fluorine, toxic effects on the lungs, liver and testicles were observed at 16 ppm, and irritation of mucous membranes and lungs observed at 2 ppm. Fluorine at 1 ppm was tolerated. In a subsequent multi-species study, no effects were observed from 60-minute exposures at concentrations up to 40 ppm.
There are sparse data available on industrial exposure of workers to fluorine. There is even less experience of long-term exposure to chlorine trifluoride and oxygen difluoride.
Ingestion of quantities of soluble fluorides in the range of 5 to 10 grams is almost certainly fatal to human adults. Human fatalities have been reported in connection with the ingestion of hydrogen fluoride, sodium fluoride and fluosilicates. Non-fatal illnesses have been reported due to ingesting these and other fluorides, including the sparingly soluble salt, cryolite (sodium aluminium fluoride).
In industry, fluoride-bearing dusts play a part in a considerable proportion of cases of actual or potential fluoride exposure, and dust ingestion may be a significant factor. Occupational fluoride exposure may be largely due to gaseous fluorides, but, even in these cases, ingestion can rarely be ruled out completely, either because of contamination of food or beverages consumed in the workplace or because of fluorides coughed up and swallowed. In exposure to a mixture of gaseous and particulate fluorides, both inhalation and ingestion may be significant factors in fluoride absorption.
Fluorosis or chronic fluorine intoxication has been widely reported to produce fluoride deposition in skeletal tissues of both animals and humans. The symptoms included increased radiographic bone opacity, formation of blunt excrescences on the ribs, and calcification of intervertebral ligaments. Dental mottling is also found in cases of fluorosis. The exact relationship between fluoride levels in urine and the concurrent rates of osseous fluoride deposition is not fully understood. However, provided urinary fluoride levels in workers are consistently no higher than 4 ppm, there appears to be little need for concern; at a urinary fluoride level of 6 ppm more elaborate monitoring and/or controls should be considered; at a level of 8 ppm and above, it is to be expected that skeletal deposition of fluoride will, if exposure is allowed to continue for many years, lead to increased osseous radio-opacity.
The fluoborates are unique in that absorbed fluoborate ion is excreted almost completely in the urine. This implies that there is little or no dissociation of fluoride from the fluoborate ion, and hence virtually no skeletal deposition of that fluoride would be expected.
In one study of cryolite workers, about half complained of lack of appetite, and shortness of breath; a smaller proportion mentioned constipation, localized pain in the region of the liver, and other symptoms. A slight degree of fluorosis was found in cryolite workers exposed for 2 to 2.5 years; more definite signs were found in those exposed nearly 5 years, and signs of moderate fluorosis appeared in those with more than 11 years of exposure.
Fluoride levels have been associated with occupational asthma among workers in aluminium reduction potrooms.
Calcium fluoride. The hazards of fluorspar are due primarily to the harmful effects of the fluorine content, and chronic effects include diseases of teeth, bones and other organs. Pulmonary lesions have been reported among persons inhaling dust containing 92 to 96% calcium fluoride and 3.5% silica. It was concluded that calcium fluoride intensifies the fibrogenic action of silica in the lungs. Cases of bronchitis and silicosis have been reported amongst fluorspar miners.
Industrial plants using quantities of fluorine compounds, such as iron and steelworks, aluminium smelters, superphosphate factories and so on, may emit fluorine-containing gases, smokes or dusts into the atmosphere. Cases of environmental damage have been reported in animals grazing on contaminated grass, including fluorosis with dental mottling, bone deposition and wasting; etching of window glass in neighbouring houses has also occurred.
Bromine and its compounds
Bromine is widely distributed in nature in the form of inorganic compounds such as minerals, in seawater and in salt lakes. Small amounts of bromine are also contained in animal and vegetable tissues. It is obtained from salt lakes or boreholes, from seawater and from the mother liquor remaining after the treatment of potassium salts (sylnite, carnallite).
Bromine is a highly corrosive liquid, the vapours of which are extremely irritating to the eyes, skin and mucous membranes. On prolonged contact with tissue, bromine may cause deep burns which are long in healing and subject to ulceration; bromine is also toxic by ingestion, inhalation and skin absorption.
A bromine concentration of 0.5 mg/m3 should not be exceeded in case of prolonged exposure; in a bromine concentration of 3 to 4 mg/m3, work without a respirator is impossible. A concentration of 11 to 23 mg/m3 produces severe choking, and it is widely considered that 30 to 60 mg/m3 is extremely dangerous for humans and that 200 mg/m3would prove fatal in a very short time.
Bromine has cumulative properties, being deposited in the tissues as bromides and displacing other halogens (iodine and chlorine). Long-term effects include disorders of the nervous system.
Persons exposed regularly to concentrations three to six times higher than the exposure limit for 1 year complain of headache, pain in the region of the heart, increasing irritability, loss of appetite, joint pains and dyspepsia. During the fifth or sixth year of work there may be loss of corneal reflexes, pharyngitis, vegetative disorders and thyroid hyperplasia accompanied by thyroid dysfunction. Cardiovascular disorders also occur in the form of myocardial degeneration and hypotension; functional and secretory disorders of the digestive tract may also occur. Signs of inhibition of leucopoiesis and leucocytosis are seen in the blood. The blood concentration of bromine varies between 0.15 mg/100 cm3 to 1.5 mg/100 cm3 independently of the degree of intoxication.
Hydrogen bromide gas is detectable without irritation at 2 ppm. Hydrobromic acid, its 47% solution in water, is a corrosive, faintly yellow liquid with a pungent smell, which darkens on exposure to air and light.
The toxic action of hydrobromic acid is two to three times weaker than that of bromine, but more acutely toxic than hydrogen chloride. Both the gaseous and aqueous forms irritate the mucous membranes of the upper respiratory tract at 5 ppm. Chronic poisoning is characterized by upper respiratory inflammation and digestive problems, slight reflex modifications and diminished erythrocyte counts. Olfactory sensitivity may be reduced. Contact with the skin or mucous membranes may cause burns.
Bromic acid and hypobromous acid. The oxygenated acids of bromine are found only in solutions or as salts. Their action on the body is similar to that of hydrobromic acid.
Ferroso-ferric bromide. Ferroso-ferric bromides are solid substances used in the chemical and pharmaceutical industries and in the manufacture of photographic products. They are produced by passing a mixture of bromine and steam over iron filings. The resultant hot, syrupy brome salt is tipped into iron containers, where is solidifies. Wet bromine (that is, bromine containing more than about 20 ppm of water) is corrosive to most metals, and elemental bromine has to be transported dry in hermetically sealed monel, nickel or lead containers. To overcome the corrosion problem, bromine is frequently transported in the form of ferroso-ferric salt.
Bromophosgene. This is a decomposition product of bromochloromethane and is encountered in the production of gentian violet. It results from the combination of carbon monoxide with bromine in the presence of anhydrous ammonium chloride.
The toxic action of bromophosgene is similar to that of phosgene (see Phosgene in this article).
Cyanogen bromide. Cyanogen bromide is a solid used for gold extraction and as a pesticide. It reacts with water to produce hydrocyanic acid and hydrogen bromide. Its toxic action resembles that of hydrocyanic acid, and it probably has similar toxicity.
Cyanogen bromide also has a pronounced irritant effect, and high concentrations may cause pulmonary oedema and lung haemorrhages. Twenty ppm for 1 minute and 8 ppm for 10 minutes is intolerable. In mice and cats, 70 ppm causes paralysis in 3 minutes, and 230 ppm is fatal.
Chlorine and its inorganic compounds
Chlorine compounds are widely found in nature, comprising about 2% of the earth’s surface materials, especially in the form of sodium chloride in sea water and in natural deposits as carnallite and sylvite.
Chlorine gas is primarily a respiratory irritant. In sufficient concentration, the gas irritates the mucous membranes, the respiratory tract and the eyes. In extreme cases difficulty in breathing may increase to the point where death can occur from respiratory collapse or lung failure. The characteristic, penetrating odour of chlorine gas usually gives warning of its presence in the air. Also, at high concentrations, it is visible as a greenish-yellow gas. Liquid chlorine in contact with skin or eyes will cause chemical burns and/or frostbite.
The effects of chlorine may become more severe for up to 36 hours after exposure. Close observation of exposed individuals should be a part of the medical response programme.
Chronic exposure. Most studies indicate no significant connection between adverse health effects and chronic exposure to low concentrations of chlorine. A 1983 Finnish study did show an increase in chronic coughs and a tendency for hypersecretion of mucous among workers. However, these workers showed no abnormal pulmonary function in tests or chest x rays.
A 1993 Chemical Industry Institute of Toxicology study on the chronic inhalation of chlorine exposed rats and mice to chlorine gas at 0.4, 1.0 or 2.5 ppm for up to 6 hours a day and 3 to 5 days/week for up to 2 years. There was no evidence of cancer. Exposure to chlorine at all levels produced nasal lesions. Because rodents are obligatory nasal breathers, how these results should be interpreted for humans is not clear.
Chlorine concentrations considerably higher than current threshold values may occur without being immediately noticeable; people rapidly lose their ability to detect the odour of chlorine in small concentrations. It has been observed that prolonged exposure to atmospheric chlorine concentrations of 5 ppm results in disease of the bronchi and a predisposition to tuberculosis, while lung studies have indicated that concentrations of 0.8 to 1.0 ppm cause permanent, although moderate, reduction in pulmonary function. Acne is not unusual in persons exposed for long periods of time to low concentrations of chlorine, and is commonly known as “chloracne”. Tooth enamel damage may also occur.
In all, there are five oxides of chlorine. They are dichlorine monoxide, chlorine monoxide, chlorine dioxide, chlorine hexoxide and chlorine heptoxide; they have mainly the same effect on the human organism and require the same safety measures as chlorine. The one most used in industry is chlorine dioxide. Chlorine dioxide is a respiratory and eye irritant similar to chlorine but more severe in degree. Acute exposures by inhalation cause bronchitis and pulmonary oedema, the symptoms observed in affected workers being coughing, wheezing, respiratory distress, nasal discharge, and eye and throat irritation.
Nitrogen trichloride is a powerful irritant to the skin and mucous membranes of the eyes and respiratory tract. The vapours are as corrosive as chlorine. It is highly toxic when ingested.
The mean lethal concentration (LC50) of nitrogen trichloride in rats is 12 ppm according to one study involving exposing the rats at concentrations from 0 to 157 ppm for 1 hour. Dogs fed on flour bleached with nitrogen trichloride rapidly develop ataxia and epileptiform convulsions. Histological examination of experimental animals has shown cerebral cortex necrosis and Purkinje cell disorders in the cerebellum. The red cell nucleus may also be affected.
Nitrogen trichloride may explode as the result of an impact, exposure to heat, supersonic waves, and even spontaneously. The presence of certain impurities may increase the explosion hazard. It will also explode on contact with traces of certain organic compounds—in particular, turpentine. Decomposition results in highly toxic chlorinated decomposition products.
Phosgene. Commercially, phosgene (COCl2) is manufactured by the reaction between chlorine and carbon monoxide. Phosgene is also formed as an undesirable by-product when certain chlorinated hydrocarbons (especially dichloromethane, carbon tetrachloride, chloroform, trichloroethylene, perchloroethylene and hexachloroethane) come into contact with an open flame or hot metal, as in welding. The decomposition of chlorinated hydrocarbons in closed rooms can result in the accumulation of harmful concentrations of phosgene, as for example from the use of carbon tetrachloride as a fire-extinguishing material, or tetrachloroethylene as a lubricant in the machining of high-grade steel.
Anhydrous phosgene is not corrosive to metals, but in the presence of water it reacts to from hydrochloric acid, which is corrosive.
Phosgene is one of the most poisonous gases used in industry. The inhalation of 50 ppm for a short time is fatal to test animals. For humans, prolonged inhalation of 2 to 5 ppm is dangerous. An additional hazardous property of phosgene is the lack of all warning symptoms during its inhalation, which may merely cause light irritation of the mucous membranes of the respiratory tract and eye at concentrations of 4 to 10 ppm. Exposure to 1 ppm for extended periods can cause delayed pulmonary oedema.
Light cases of poisoning are followed by temporary bronchitis. In serious cases, delayed pulmonary oedema can occur. This can occur after a latent period of several hours, usually 5 to 8, but seldom more than 12. In most cases, the patient remains conscious until the end; death is caused by asphyxiation or heart failure. If the patient survives the first 2 to 3 days, the prognosis is generally favourable. High concentrations of phosgene cause immediate acid damage to the lung and rapidly cause death by suffocation and termination of circulation through the lungs.
Free chlorine destroys vegetation and, as it may occur in concentrations causing such damage under unfavourable climatic conditions, its release into the surrounding atmosphere should be prohibited. If it is not possible to utilize the liberated chlorine for the production of hydrochloric acid or the like, every precaution must be taken to bind the chlorine, for instance by means of a lime scrubber. Special technical safety measures with automatic warning systems should be installed, in the factories and in the surroundings, wherever there is a risk that appreciable quantities of chlorine may escape to the surrounding atmosphere.
From the point of view of environmental pollution, particular attention should be paid to cylinders or other vessels used for the transport of chlorine or its compounds, to measures for the control of possible hazards, and to steps to be taken in case of emergency.
Iodine and its compounds
Iodine does not occur free in nature, but iodides and/or iodates are found as trace impurities in deposits of other salts. Chilean saltpetre deposits contain enough iodate (about 0.2% sodium iodate) to make its commercial exploitation feasible. Similarly, some naturally occurring brines, especially in the United States, contain recoverable quantities of iodide. Iodide in ocean water is concentrated by some seaweeds (kelp), the ash of which was formerly a commercially important source in France, the United Kingdom and Japan.
Iodine is a powerful oxidizing agent. An explosion may result if it contacts materials such as acetylene or ammonia.
Iodine vapour, even in low concentrations, is extremely irritating to the respiratory tract, eyes and, to a lesser extent, the skin. Concentrations as low as 0.1 ppm in the air may cause some eye irritation upon prolonged exposure. Concentrations higher than 0.1 ppm cause increasingly severe eye irritation along with irritation of the respiratory tract and, ultimately, pulmonary oedema. Other systemic injury from the inhalation of iodine vapour is unlikely unless the exposed person already has a thyroid disorder. Iodine is absorbed from the lungs, converted to iodide in the body, and then excreted, mainly in urine. Iodine in crystalline form or in strong solutions is a severe skin irritant; it is not easily removed from the skin and, after contact, tends to penetrate and cause continuing injury. Skin lesions caused by iodine resemble thermal burns except that iodine stains the burned areas brown. Ulcers that are slow to heal may develop because of iodine remaining fixed to the tissue.
The probable mean lethal oral dose of iodine is 2 to 3 g in adults, due to its corrosive action on the gastrointestinal system. In general, iodine-containing materials (both organic and inorganic) appear to be more toxic than analogous bromine- or chlorine-containing materials. In addition to “halogen-like” toxicity, iodine is concentrated in the thyroid gland (the basis for treating thyroid cancer with 131I), and therefore metabolic disturbances are likely to result from overexposure. Chronic absorption of iodine causes “iodism”, a disease characterized by tachycardia, tremor, weight loss, insomnia, diarrhoea, conjunctivitis, rhinitis and bronchitis. In addition, hypersensitivity to iodine may develop, characterized by skin rashes and possibly rhinitis and/or asthma.
Radioactivity. Iodine has an atomic number of 53 and an atomic weight ranging from 117 to 139. Its only stable isotope has a mass of 127 (126.9004); its radioactive isotopes have half-lives from a few seconds (atomic weights of 136 and higher) to millions of years (129I). In the reactions that characterize the fission process in a nuclear reactor, 131I is formed in abundance. This isotope has a half-life of 8.070 days; it emits beta and gamma radiation with principal energies of 0.606 MeV (max) and 0.36449 MeV, respectively.
Upon entering the body by any route, inorganic iodine (iodide) is concentrated in the thyroid gland. This, coupled with the abundant formation of 131I in nuclear fission, makes it one of the most hazardous materials that can be released from a nuclear reactor either deliberately or by accident.