The halogens are a group of five elements having similar chemical properties. These are (in order of atomic mass and atomic number) fluorine 9, chlorine 19, bromine 35 iodine 53. Two others, astatine 85 and ltennessine117, are known to exist but are radioactive and ltennessine 117 is a synthetic element. Compounds of Aastatine 85 have only been made in nanogram quantities. They have no significant use except in research.  

The name halogen translates to "salt former." These are the elements that react to form compounds known as salts. Sodium chloride (NaCl) or table salt is a classic example. Potassium bromide (KBr) is another.

The halogens are characterized by having seven electrons in the outer shell of their atoms, thus tending to react by accepting one additional electron to achieving stability, albeit with one negative charge. Because only one electron shift is involved they are essentially electrovalent in nature. Therefore the resulting compounds tend to be water soluble and conduct an electric current, i.e. they are electrolytes.

In an aqueous solution of these salts dissociation is virtually total due to the polar nature of the water molecule. Instead of molecules of sodium chloride we have a population of sodium ions (Na+) and chloride ions (Cl-). Since these ions are generated in equal numbers the total charge associated with the solution is zero or neutral. Because of their strong attraction for electrons, halogen atoms will, when nothing else is available, unite with each other to form diatomic molecules. Thus we see these elements denoted in equations as X2 where “X” stands for any halogen atom.     

The halogens in their elemental state share another property — toxicity. Until recently, fluorine was a laboratory curiosity with little or no commercial use. With the advent of lead free gasoline however, this element, normally encountered as a gas, became an article of commerce and is now shipped in tank car quantities for use in the manufacture of this fuel. Fluorine is also used indirectly in the production of low friction plastics such as teflon and in halons such as freon. It is also used in the production of uranium isotopes by the gas diffusion process.

Fluorochlorohydrocarbons are also used extensively in air conditioning and in refrigeration. Fluorides are often added to toothpastes and, somewhat controversially, to municipal water supplies for the prevention of tooth decay. Compounds of fluorine sometimes occur naturally in some ground water sources. Elemental fluorine is extremely toxic and even very small concentrations in air can be lethal.  

Chlorine is usually encountered in the gaseous state but it is transported and stored under pressure as a liquid. It was the first chemical weapon and is still carried in the United States Armamentarium under the name “Bertholite.” However, it is also essential to life as the major negative electrolytic ion in the blood stream.

Chlorine, in its combined states, is all around us. As salt (NaCl) it preserves and flavors our food. As polyvinyl chloride it carries our water and other fluids as well as gasses, as chloramphenicol it fights infections, as “Clorox” it bleaches our laundry and serves as a general household disinfectant. As a hypochlorite the element is the most common agent used for disinfecting swimming pools or other aquatic devices.

Bromine exists as a low-boiling brown liquid with a high vapor pressure. Thus it must be handled as both a gas and a liquid. Bromine, in its combined forms, is a common element. It is used in such products as agricultural chemicals, dyestuffs, insecticides, pharmaceuticals and chemical intermediates as well as a bleaching agent for flour.

Some uses are being phased out for environmental reasons, but new ones continue to be found. Bromine compounds can be used as flame retardants. When added to furniture foam, plastic casings for electronics and textiles it makes them less flammable.

However, the use of bromine as a flame retardant has been phased out in the U.S. because of toxicity concerns. It may still be found in older applications that have not been updated and, occasionally, in some imports.

Organobromides are utilized in halon fire extinguishers for fighting fires in places like museums, airplanes, computer installations and tanks. Silver bromide is a chemical used in film photography. Before leaded fuels were phased out, bromine was used to prepare 1,2-di-bromoethane, which was used as an anti-knock agent in gasoline. It has also been utilized, in the form of methyl bromide, as a soil fumigant.

Elemental bromine is often used as an alternative to chlorine in disinfecting in swimming pools and other such aquatic facilities. Historically, bromides were administered as sedatives and to relieve headaches (remember Bromo-Seltzer?) This has been discontinued due to adverse side effects.

Iodine is a gray solid which will sublime into a violet vapor upon heating. Both the vapor and the solid are considered toxic. It is also soluble in organic liquids. Those of a certain age will remember big brown bottle of tincture of iodine, marked with the traditional skull and crossed bones, that stood ready in the family medicine cabinet for any cuts or scrapes that might be suffered.

The sting in the application of this remedy is also remembered. Tincture of iodine is a solution of elemental iodine in ethyl alcohol (ethanol, C2H5OH). This solvent contains oxygen in its molecular formula. If iodine is dissolved in carbon tetrachloride (CCl4), an organic liquid that does not contain oxygen in its molecular structure, the resulting solution is violet in color. This gives us a test for oxygen in organic liquids.

Adding iodine and getting a brown solution means an oxygen containing liquid. If violet, the liquid being tested has no oxygen in its formula. This differentiates between oxygen containing substances such as aldehydes, ketones, or alcohols and non oxygen containing substances such as hydrocarbons, carbon disulphide (CS2) or freons.

Iodine, too, has its physiological function. In the early 20th century it was discovered that a minute trace of iodine (usually as potassium iodide, KI) added to table salt (NaCl) prevented a common type of goiter. Iodine compounds are used as radiocontrast agents to visualize soft tissue structures during radiological examinations.

As a component of preparations such as “Betadine” it is a common disinfectant for the skin prior to minor surgery or the suturing of wounds. Iodoform (CHI3) has, in the past, been utilized as an antiseptic for wounds but, in recent years, largely been supplanted by superior and less odoriferous agents.

The halogens, as we have shown, are capable of significant biological activity. Indeed they are essential to the wellbeing of humans. Halogens also find multitudinous uses in the industrial world with the first four (F,Cl,Br, and I) being major articles of commerce. However, these elements are also highly toxic. Why with such widespread use and potential for exposure is halogen poisoning relatively rare? The answer lies in both the chemical and the physical properties of the elements involved. Several of these serve to protect humans from this toxicity.

The first of these is odor. It is doubtful that anyone was ever accidentally poisoned by any of the halogens; the stench is overpowering and so irritating that no one could remain in the vicinity. Unless victims are trapped like the soldiers at the Battle of Ypres in World War I these gasses are “self evacuating” when released.

The second contributing factor to the relatively low incidence of poisoning is that of state. The average consumer has very few, if any, contacts with elemental halogens. Exposure is restricted to compounds rather than the elements themselves. Thus we deal with non-stick cook ware but not with elemental fluorine. There is salt on our table or in our water softener but not elemental chlorine. We may use Betadine but most laymen have never seen elemental iodine.

When halogens in their elemental state are used to manufacture consumer products the procedure is carried out under the supervision trained operators using specialized equipment to control the process and prevent an accidental discharge. What the consumer sees, in most cases, is a reaction product that, when used in accordance with instruction, is a relatively safe compared to the original free element.

But there are two glaring exceptions to the previous statement. These are private water systems and swimming pools (including spas, hot tubs and hydrotherapy devices).

Private domestic water systems, if they are equipped for disinfection at all, are usually fitted with a metering device (‘chlorinater”) which automatically adds the disinfecting agent, either chlorine gas or calcium hypochlorite, as needed based on water flow. In many cases these systems are installed and maintained by a trained technician, but in others, particularly in remote locations, the system is operated and maintained by the property owner.

Unless that person is well trained and diligent in the performance of routine maintenance there is a potential for problems. Systems using calcium hypochlorite (aka HTH) or “dry chlorine” are usually preferred for small scale operations. High pressure cylinders are not required and there is z reduced possibility of a leak or an accidental release. Response to a spill of loose calcium hypochlorite or a compressed tablet is much easier than what is required for a chlorine gas leak.

However, these systems are not without hazards. Calcium hypochlorite reacts violently with certain common organic liquids such as glycerin, brake fluid, and antifreeze, among others. A three-inch “dry chlorine” tablet dropped into a cup full of these fluids will provide a pyrotechnic display that can cause a fire. If calcium hypochlorite becomes damp it will decompose and release chlorine gas just as if it were added to water.

Whether for water purification or aquatic sanitation, systems that employ elemental chlorine do have certain advantages. They inject the gas at a slow rate directly into the water passing through the filtering system rather than simply dumping it directly into the tank or pool as is all too often done with calcium hypochlorite. Also there is no danger of a clog when the disinfecting agent is a gas, thus eliminating the chance of an unnoticed interruption of disinfectant.

Little chance exists of a dangerously high concentration of disinfectant as often happens with the “one shot” method where several tablets or dry granules are mixed with a small amount of water first, then dumped into the pool or reservoir. If this “one shot” approach is used it should be done when the facility will be unoccupied for some time, giving the disinfectant a chance to become better dispersed. It is much better to add the disinfectant slowly through the recirculating filter pump, insuring more rapid, uniform and thorough mixing.

Swimming pools and other aquatic installations are similar to water systems in that they usually come equipped with some sort of automatic device to dispense disinfectant, especially in the case of large public installations. However, these systems differ from water supply systems in two very important aspects. One, the water is recirculated rather than being passed through and two, there is a good chance that the pool or facility will be occupied when disinfectant is added.

The dissolution of a solid such as sugar or salt in water increases with the temperature. However, the solubility of gasses in a liquid decreases as the temperature rises. Using heated water inadvertently decreases the amount of chlorine dissolved. Also, aquatic facilities have a large exposed surface areas that favors gaseous interchange between air and water.

In a closed system, such as a chemical reactor containing a chlorine gas, the amount of chlorine above the water will, eventually, reach equilibrium with the chlorine in the water, according to Dalton’s Law of Effusions.

However, our swimming pools are not closed systems. The chlorine above the water is constantly depleted as it disperses into the free air. Migration of chlorine to the atmosphere will continue until all of the chlorine above and below the water is removed. More chlorine must constantly be added to the pool to replace that lost into the atmosphere. Just how much disinfectant do we need to add?

Determining this is usually done with a small hand held colorometric test kit. A test reagent is added to an aliquot of pool water and the resulting color compared with a supplied standard to determine the level of disinfectant. If necessary, chlorine or whatever disinfectant is being used is added to the pool until matching colors indicate that the proper level of disinfectant.

Enter elemental bromine. Bromine imparts a very bad “medicine” taste to water and is not utilized for water purification. But bromine has gained popularity as a disinfectant for swimming pools. It also finds wide use as a disinfectant in cooling towers because it does not “burn off,” i.e., dissipate into the atmosphere, as fast as chlorine. Since bromine is not as stable with respect to sunlight as chlorine most outdoor pools and spas continue to employ chlorine. However, bromine is gaining popularity with indoor pools, spas and hydrotherapy facilities thanks mainly to the reduced odor it generates.

Since bromine does not effuse as rapidly as chlorine, it does not have to be replenished as often. Because bromine is usually a liquid at room temperature it can be added directly to the pool water without need of elaborate application equipment. The advantage is in the simplicity of application. Complicated and expensive “chlorinators” or the delivery of large gas cylinders is not required. The risk of a release due to an equipment failure is minimized.

The risks are partly due to the ease of application. Bromine is supplied in sealed glass ampules containing various amounts. Once the amount of bromine needed is calculated, the operator opens the vials and adds their contents directly to the water.

If done properly there is little risk but there are definite precautions that must be taken. Use a bromine ampule at the site of application, not in the back room. Only one ampule should be opened at a time in order to minimize release into the atmosphere. Once an ampule is opened it should be used immediately. Do not attempt to save unused bromine. Any excess should be added to a relatively large amount of water, then disposed of.

Bromine must be completely dispersed in the pool water. If possible it should be added through the recirculating filter system or, alternatively, as near to the incoming water source. Agitation will hasten the mixing. The liquid should be added slowly enough to ensure that it is immediately incorporated with the largest possible volume of water. Even though using bromine in an occupied area is not advised it is sometimes done.

Due to its higher molecular weight bromine will disperse more slowly than chlorine. Great care should be used so that no occupant of the facility comes in contact with the bromine or with an area of high concentration. Creating a small area of concentrated bromine can be hazardous. Failure to observe this precaution can lead to bromine poisoning.

Bromine poisoning has numerous manifestations depending upon the type of exposure and its duration. While rare, bromine poisoning is but possible by consuming contaminated food or being directly exposed to bromine gas. Dizziness and cough are symptoms of poisoning from inhaled bromine, while a burning sensation on the skin is a symptom of direct exposure. If exposed to elemental bromine, leave the contaminated area immediately, remove all clothing and bathe using plenty of soap.

People exposed to bromine for long periods may have long-term health problems. Bromide is a common endocrine disruptor. Because bromide is also a halide, it competes for the same receptors that are used in the thyroid gland (among other places) to capture iodine. This inhibits thyroid hormone production.

Although bromine can be used in swimming pools, it is more often found in spas and/or hydrotherapy installations. It does not break down as easily in hot water as chlorine, so it is favored where the water is very warm. Because it is more stable in water than chlorine when kept out of direct sunlight bromine is not as easy to wash off.

Showering and rinsing after swimming does not guarantee preventing side effects. Showering should be soapy, thorough and be done immediately after leaving the water. Life guards, therapists and other facility personnel need to be especially careful since repeated exposure is likely. Such personnel should wash frequently after each exposure.

The most commonly encountered cases of bromine poisoning associated with bathing facilities is from the use of bromine based disinfectants. Three brominating compounds are currently available for recreational hot tub, spa, therapy and swimming pool sanitation — elemental bromine, sodium bromide/monopotassium persulfate and bromochlorodimethylhydantoin.

Of these, elemental bromine is perhaps the most hazardous in that it is more likely to cause skin damage. The other two are usually in tablet form and dissolve in water slowly. While fine for a swimming pool, hot tubs and therapeutic baths are often emptied after each use, needing a disinfectant that can be used in small amounts and become effective immediately. Elemental bromine seems to meet these requirements. It does not have the strong odor associated with chlorine.

Bromine is not a cumulative like heavy metals but the damage that it can do is long lasting. While an effective disinfectant it is hazardous and must be used by operators who can be trusted to follow the proper protocols.

Sensitivity to bromine appears to vary between individuals and any adverse symptoms that become manifest should serve as a warning of a possible allergic condition or hypersensitivity. The appearance of any such symptoms should prompt the individual to stop exposure immediately and seek medical attention. Chlorine allergy, while not common, is fairly well known. Those allergic to chlorine may find themselves victims of bromine allergy.

While some swimmers experience red, irritated skin, dermatitis, red patches, rashes and blisters can happen in more serious cases. Skin rashes and irritations occur in swimmers once the level of contaminants in the pool rises. The contaminants react with bromine and produce byproducts that are irritating  and very similar to chloramines.

In short, persons utilizing aquatic facilities or undergoing hydrotherapy should exercise caution when exposing themselves to disinfected water. Pool operators, residential or commercial, should also exercise caution when working with disinfectants, algicides or other chemicals such as “shock treatments” to their facilities. These agents are hazardous and must be handled with care and according to the accepted protocols for their safe use.

Editor's note: The column was originally intended to run with a package of articles on the dangers of bromine poisoning that ran in the Spring 2017 issue of Industrial Fire World.