Basic concept in hazardous chemicals: corrosives

ACCIDENTAL CONTACT WITH corrosives is one of the potential dangers in a laboratory or industrial setting. In 1987, more than 60,000 victims of chemical burn injuries from corrosives in the United States sought professional medical care, with more than 3,000 deaths related to skin or gastrointestinal chemical injuries arising from these incidents.

Universally, the term corrosives refer to substances without any toxicological activity which produce severe tissue destrustion. Most common are the strong acids or alkalis, but any strong oxidising or reducing agent such as potassium permanganate and diborane respectively may also be included.

Most corrosives produce significant injuries through direct chemical reaction on living tissues rather than through heat damage. In most cases, the degree of tissue destruction depends on the concentration of the toxic agent and the duration of the contact.

When the skin is exposed to a corrosive, its keratinous covering is destroyed and its underlying dermal tissues are exposed to continuous necrotising action. The absorption of some corrosives through the skin may cause systemic toxicity: dichromate poisoning produces liver failure, acute tubular necrosis and result in hypercalcemia; picric acid and phosphorus burns may be followed by nephrotoxicity and absorption of phenol may be associated with central nervous system depression and hypotension.

While both alkalis and acids cause tissue destruction, acids produce a coagulation necrosis which results in a superficial burn.

Alkalis in contrast tend to produce a penetrating tissue destruction. Thus an ingested alkali more frequently causes oesophageal bums leading to perforation and stricture formation. Acid ingestion, in contrast, more frequently cause burns in the stomach, particularly in the pyloric region.

In industry, corrosives are widely used in the semiconductor industry as well as in the production of organochemicals and polymers. The physical states of these chemicals can be in the form of solids, liquids or gas with the gaseous state production their harmful effects on the respiratory tract while the solids and liquids on the skin.

With the hope of creating awareness and safety concern among workers in the industry, some of the commonly used corrosives to be noted include:

Hydrofluoric acid, one of the strongest acids, is used primarily in the semiconductor industry and is a primary component of rust removing agents. It is used in germicides, dyes, plastics and glass etching. Its toxicity has been attributed to the fluoride ion which permeates into the tissue as a result of the penetrating power of the acid. Even at low concentrations, the fluoride ion will cause necrosis of soft tissues and also cause decalcification and corrosion of the bone.

This destruction is accompanied by the formation in tissue of relatively insoluble calcium and magnesium fluoride which are the only insoluble salt complexes in the tissue that will inactivate the fluoride ion. All other salts are soluble and fully dissociable and will release fluoride to continue tissue injury.

Phenol is an aromatic acidic alcohol. This compound and its derivatives are highly reactive corrosive contact poisons that damage cells by denaturing and precipitating cellular proteins. The characteristic sweet and tarry odour of phenol usually signal its presence.

Phenol has a marked corrosive effect on any tissue. If it comes into contact with the eyes, it may even cause blindness. On contact with the skin, it usually causes a whitening of the exposed area but no pain. If the chemical is not removed promptly however, it may cause a severe burn and may lead to systemic poisoning.

Among the systemic poisoning effects noted are metabolic acidosis, red blood cell haemolysis, acute liver disease and inflammation of the renal nephrons. In cases of skin contact, irrigation of the skin surface with large volumes of water delivered at low pressure is preferred.

Based on experiments carried out on the effective methods of neutralising the effective methods of neutralising the effect of phenol, wiping the skin with a solution of polyethylene glycol is highly recommended. It was found that this solution reduces the morbidity and burn severity from phenol contamination.

Elemental phosphorus exists in two forms: the red granular non-abdsorbable form is non-toxic; the other, a yellow, waxy translucent solid (white phosphorus) ignites spontaneously in the air and must be preserved in water.

In industrial applications, white phosphorus is used in the manufacture of rat poisons, insecticides; fireworks and fertilisers. White phosphorus burns spontaneously in air and the vapour released is an irritant to the respiratory tract. When it ignites in air, it is oxidised to phosphorus pentoxide. With addition of water, this compound forms metaphosphoric and orthophosphoric acids.

The aerly signs of systemic intoxication by the compound are abdominal pain, jaundice and a garlic odour to the breath. In addition, the fumes can also cause severe eye irritation.

In cases of skin contact, the mechanism of injury appears to be from the heat of reaction rather than the liberation of further inorganic acids or cellular dehydration. This thermal injury often results in a painful partial of full thickness burn. If such cases occur, the best first aid measure is to wash the skin with a suspension of 5% sodium bicarbonate and 3% copper sulphate in 1% hydroxyethyl cellulose.

Sodium hydroxide is also known as caustic soda. It is used in the manufacture of rayon, mercerised cotton, soap, paper, aluminium, petroleum products, metal cleaning and oxide coating.

Sodium hydroxide is a severe irritant of the eyes, mucous membrane and skin. The effects of inhalation of sodium hydroxide is usually of secondary importance in industrial exposures. These effects vary from mild irritation of the nose to severe pneumonitis, depending upon the severity of the exposure. The greatest hazard is rapid tissue destruction of eyes and skin upon contact with the substance.

Chlorine forms hydrochloric acid (HCI) in the lungs, causing severe tissue damage which can be fatal. Its uses in the industry include metal fluxing, sterilisation of water supplies and swimming pools, synthesis of chlorinated organic chemicals and plastics, pulp and paper manufacturing, detinning and dezincing iron.

As with many other corrosive gases, the effects of exposure may not be noticed for a few days. In all cases medical attention should be sought immediately following exposure, not at the onset of symptoms.

Ahnydrous HCI (HCI gas) is extremely corrosive to almost everything, including stainless steel. Symptoms of exposure are similar to chlorine. The major effects from contact to this fume are usually limited to the upper respiratory tract. Exposure to the gas immediately causes cough, burning of the throat and a tedency to choke. Exposure of the skin to a high concentration of the gas or to a concentrated solution of the liquid will cause burns. Repeated or prolonged exposure to dilute solutions may cause inflammation of the skin.

Anhydrous ammonia (NH3) is a severely corrosive alkaline vapour with a pungent odour. Ammonia is a severe irritant of the eyes, respiratory tract and skin. Exposure and inhalation to a concentration range of 2,500-6,500 ppm causes severe corneal irritation, dyspnea, bronchospasm, chest pain and pulmonary oedema, which may be fatal. Liquid anhydrous ammonia in contact with the eyes may cause serious injury to the cornea and deeper structures and sometimes blindness. On the skin, it causes first and second degree burns which are often severe and if extensive, may be fatal.

Phosphine gas is a severe pulmonary irritant and an acute systemic poison. It is a colourless gas with a fishy odour. It is used in the making of insecticides for fumigation, phosphonium halides and as a doping agent in the semiconductor industry.

Overexposure can cause either sudden or delayed death due to lung destruction. Phosphine is toxic at levels near the odour threshold. The permissible exposure limit as set by the American Occupational Safety and Health body is 0.3 ppm averaged over an eight hour shift. It is slightly heavier than air. Phosphine is pyrophoric, i.e., it is spontaneously flammable in air. Exposures of 2,000 ppm for a few minutes are lethal; exposures of 7 ppm for several hours can be tolerated. The odour threshold for phosphine is approximately one ppm.

Diborane is a colourless gas lighter than air with a repulsive sweet odour. It is used in the manufacture of rocket propellants, reducing agents and rubber vulcanisers.

It acts as a pulmonary (lung) irritant. The permissible exposure limit is 0.1 ppm averaged over eight hours. Like phosphine, it is pyrophoric.

Fires involving diborane can produce other toxic fumes. The threshold of odour detection is approximately 3 ppm but is not a reliable indicator of danger. The repulsive odour of diborane is described as rotten eggs, sickly sweet, musty or just plain foul.

Arsine is the most toxic of all the hydride gases. In industry, arsine is used as a dopant in the semiconductor industry. Arsine is more toxic and more dangerous than either diborane or phosphine. It acts as a blood destroying agent causing death through massive irreparable damage to the liver and kidneys.

Among the signs and symptoms of poisoning include headache, malaise, weakness, dizziness, dyspnea, abdominal pain, nausea, vomiting and jaundice.

A characteristic of arsine poisoning is the appearance of an unusual bronze skin colour due to the pigmentation of the arsine onto the skin and mucous membranes.

In any exposure to corrosives, the first aid measures to be taken depend on the exposure site. The underlying principle in the handling of exposure to corrosives is that chemical burns continue to destroy tissue untul the causative agent is inactivated or removed. Therefore it is imperative to initiate treatment as soon as possible after contact.

Speed is most important in treating chemical burns. Since contact time is a critical determinant of the severity of injury, hydrotherapy of the skin exposed to a corrosive must be initiated immediately.

This is normally achieved by gentle irrigation with large volumes of water under low pressure for a long time This dilutes the toxic agent and washes it out of the skin.

An important exception to this is in the case of lithium and sodium metal burns. In such cases, water is contraindicated since it combines with the metals to produce sodium or lithium hydroxides which release hydrogen gas, often with explosive effect!.

Consequently, the emergency treatment of skin contact with these metals is to remove all large particles from the skin and immerse them in mineral oil to prevent combustion.

Besides the skin effect, ocular injuries with the possibility of blindness are among the most disastrous effects of corrosives. Thus immediate measures are very important in cases of ocular contact with corrosives.

Regardless of the nature of the chemical copious irrigation is most important. This can be done by having the eyes submerged into a container of tap water and instructing the victim to open and close the eyes continuously. In the absence of a container, the face and the eyes should be held beneath a water fountain and washed with running water.

Irrigation should be continued while the patient is being transported to the hospital. Of the known chemicals, alkali and anhydrous ammonia appear to cause the worst effect to the eyes. The alkali can move rapidly towards the inner part of the eye to cause further damage. On the other hand, acid burns are better tolerated by the eyes because this organ has marked acid buffering capacity. Acid can be rapidly neutralised by the tear film and thus the effect of acids is confined towards the outer part of the eye.

When corrosives are being used in any acitivity, protective gloves, goggles, suitable breathing apparatus, if required, and an apron must be worn to prevent contact with the chemicals. If staff are educated and protected against these hazardous products, chemical burn injuries could be eliminated.

The writer is a pharmacist and Head of Toxicology Laboratory at the National Poison Centre, Universiti Sains Malaysia.