Basic concepts in hazardous chemicals: Flammability

A CHEMICAL THAT CAUSES ILLNESS or death is often referred as a poison. This concept is rather misleading and has resulted in a great deal of confusion in the public mind about the nature of the toxic action of chemicals.

For a chemical to be considered a poison, the most important characteristic is the quantity of the chemical being taken into the body. All chemicals can be harmful if taken in large enough doses or by the wrong route of entry into the body. For example, sodium chloride if routinely ingested in excessive amounts can be fatal. On the other hand, a very small amount can be a fatal dose if injected into the bloodstream.

Another distinction that should be made is the difference between toxicity and hazard. The hazard presented by a chemical has two components : the inherent ability of a chemical to do harm by virtue of its explosiveness, flammability, corrosiveness, toxicity and so forth. The ease with which contact can be established between the chemical and the object of concern is another factor. These two components together describe the chance or probability that a chemical will do harm.

For example, an extremely toxic chemical such as strychnine when kept in a sealed vial can be handled freely by people with no chance of a poisoning. Its toxicity has not changed but it presents no hazard because no casual contact can be established with the chemical.

Conversely a chemical that is not highly toxic can be very hazardous when used in a manner that makes it readily available for accidental ingestion. For example boric acid is not very toxic in powder form but is very dangerous when used as a component in cockroach tablets. When supplied in this form and applied on the floor, children tend to pick them up and swallowed them.

Many chemicals can cause harm by virtue of their toxicity. In this case, the toxicity of a chemical refers to its ability to damage an organ system, such as the liver and the kidney or to disrupt a biochemical process such as the blood clotting mechanism or to disturb an enzyme system at some sites in the body removed from the site of contact.

In actual practice, hazards do not group themselves in neat categories, but usually occur in combination and/or sequence. The folowing are some characteristics of flammability, a common chemical hazard.

In the transportation of flammable materials, a typical symbol being used as a precautionary label is shown in the orange diamond-shaped box. To handle a flammable substance safely, its flammability characteristics, flash point, upper and lower limits of flammability and ignition requirements must be known. This information usually appears on each chemical fact sheet known as Material Safety Data Sheet (MSDS).

A summary of the importance of each parameter on flammability is listed below:

• The flash point for a liquid is the lowest temperature at which the liquid gives off enough vapour to form an ignitable mixture with air and produce a flame when a source of ignition is present. Many common solvents and chemicals have flash points that are lower than room temperature.
• The ignition (or autoignition) temperature of a substance - solid, liquid, or gas - is the minimum temperature required to initiate self-sustained combustion. Some ignition temperatures can be quite low. Autoignition or spontaneous combustion occurs when a substance reaches its ignition temperature without the application of external heat. This characteristic is particularly important to keep in mind in the storage and disposal of chemicals.
• Each flammable gas and liquid (as a vapor) has a limited range of flammable concentration in mixtures with air. The lower flammable limit (or lower explosive limit) is the minimum concentration below which a flame is not propagated when an ignition source is present - such a mixture would be too lean to burn. The upper flammable limit (or upper explosive limit) is the maximum concentration of vapour in air above which a flame is not propagated - such a mixture is too rich. The flammable range (or explosive range) lies in between the two limits.

In essence, a fire or combustion occurs when an oxidising agent (such as oxygen), combustible material (such as fuel) and an ignition source (such as an open flame) are added together. The understanding of the fire chemistry needs to be appreciated in any measure to control fire.

Fire is an exothermic (gives off heat) oxidative reaction that can involve solid, liquid or gaseous fuel. Flaming results from the vapourisation of fuels (fuels being heated to the point of giving off gases) and their ignition and subsequent oxidation. It is also important to recognise that not all fires exhibit flaming. Some solids may exhibit smoldering which is glowing combustion on the surface of the material. In addition, combustion that normally occurs in confinement with the subsequent generation of pressure will result in an explosion.

For example, flammable gases that are pre-mixed with oxygen upon ignition will expand and result in considerable pressure. If confined, they may explode the container. The combustible material must be heated to its ignition temperature before it will support combustion or the spread of flames. Burning will continue until the combustible material is consumed or the oxidant is depleted or below the necessary amount for combustion or the heat is removed or prevented from reaching the combustible materials not allowing for fuel vapourisation or the flame is chemically inhibited or cooled to stop the oxidation process.

A flammable material is any liquid, solid or gas that will ignite easily or burn rapidly. Flammable solids can be found in a number of different forms, each of which may exhibit different properties.

Example of flammable solid are dusts and powders (flour, cellulose, fine metals); spontaneously ignitable materials (white phosphorus, sodium hydride) and endothermic materials (such as fish meal). As for gases, they can ignite and burn in normal atmospheric concentrations of oxygen (that is room air).

Some examples of flammable gases are butane, methane, hydrogen and acetylene. Leakage of compressed or liquefied gases can produce a flammable or explosive atmosphere in a confined space. This is obviously true where the gases themselves are flammable and under high pressure but may also be true in the use of nonpressurised liquefied gases.

Even relatively safe liquefied gases such as liquid air or liquid nitrogen, if kept in open vessels for too long, will generate concentrations of liquid oxygen which can contribute to an explosion. Proper care with compressed gas cylinders and cryogenic set-ups is essential.

Flammable liquids do not burn; their vapors do. As it is unlikely that air can be excluded and unrealistic (given the constant possibility of a spill) to assume that the vopour concentration can be controlled, the primary safety principle for dealing with flammable liquids is strict control of ignition sources. Ignition sources include electrical equipment, open flames, static electricity, and, in some cases, hot surfaces.

Others working in the vicinity should be informed of the presence of flammable substances so that ignition sources can be eliminated. Obviously, it is very important to know which of those sources is capable of igniting a substance that is being used.

Remember that most flammable vapors are heavier than air, and will spread out horizontally for considerable distances until an ignition source is contacted. If possible, flammable liquids should be handled only in areas free of ignition sources. Heating should be limited to water and oil baths, heating mantles, and heating tapes. Static-generated sparks can be sudden ignition sources.

When transferring flammable liquids in metal equipment, take care that metal lines and vessels are bonded together and grounded to a common ground. Ventilation is very important. A fume hood should be used when flammable liquids are allowed to stand in open containers or are handled in any way.

Two important considerations must be made storing flammable materials; the chemical properties of the material and the proper container to control the potentially hazardous properties.

The container of choice must be leak-proof and must control if not prevent the release of flammable vapours. For portable containers, this usually means self-closing or sealed spouts.

Initial consideration and planning of the storage location should include the types of materials being stored, the chemical properties of the materials, the area in which storage will occur and the activities or potential activities in the area.