The three main routes of exposure into the body are:
A fourth route of entry [injection] is possible, but seldom a problem except in the medical profession and/or related fields.
This is the primary route of entry into the body. Once the substance is into the respiratory system it may:
- Harm the tissues of the respiratory tract, or lungs. May cause serious scarring [i.e. asbestos, ammonia, acid gasses, silicosis]; and/or
- Dissolve in the blood and be transported throughout the body. The dissolved material may affect one organ in the body more profoundly than another. These vital bodily parts are often referred to as "target organs".
Generally the effects of ingested material will be similar to that when it has been inhaled with the exception of the harm to the respiratory system. Never eat in an area where you are using toxic materials. Wash your hands thoroughly before eating, even if you feel that you have not been exposed to any substance which could cause you harm. This is generally considered the least common route of entry.
Skin is an effective barrier to many hazardous materials, however numerous substances are readily absorbed into the blood stream through the skin. Where the skin is broken from punctures, sores or rash chemicals and biological agents may enter the body more easily.
Carbon Tetrachloride, a substance formerly used as fire suppressant, spot remover and general solvent for many years, prior to the discovery of its tendency to cause cancer, was an example of one such agent. Once chemicals have entered the body by absorption, their effects on internal organs will be similar to those experienced when inhaled, or ingested.
Lethal dose and lethal concentration
The abbreviation LD50 stands for the term "Lethal Dose 50". It is defined as "a single dose of a substance that, when administered by a defined route in an animal assay, is expected to cause the death of 50 percent of a defined animal population. The LD50 dose is usually expressed as milligrams per kilogram of animal body weight [mg/kg]." It is generally considered a relative measure of toxicity and only an indicator. It does not indicate a safe dose.
The abbreviation LC50 stands for the term "Lethal Concentration 50". It is defined as "the concentration of a substance in air that, when administered by means of inhalation over a specified time in an animal assay is expected to cause the death of 50 percent of a defined animal population. Under the W.H.M.I.S. requirements, the LC50 criteria are based on 4-hour exposures." Other testing protocols use a 96 hour period, therefore there can be significant difference in the impacts of exposures depending on the time frame used in determining the LC50 value. The LC50 values is expressed by volume as parts of substance per million parts of air [ppm] for gases and vapours, or by weight as milligrams of material per litre or cubic metre of air [mg/L or mg/m3] for dusts, mists and fumes.
The hazard or potential harm of a chemical is always proportionate to exposure!
Therefore, a substance of relatively low toxicity can present a serious hazard to an unprotected or uninformed worker.
Occupational or industrial hygiene
The terms "Occupational Hygiene" and "Industrial Hygiene" are synonymous, although Occupational Hygiene is the preferred expression in Canada.
Industrial Hygiene has been defined by the American Industrial Hygiene Association as: "the science and art devoted to the recognition, evaluation and control of those environmental factors and stresses arising in or from the workplace, which may cause sickness, impaired health and well being, or significant discomfort and inefficiency among workers along with citizens of the community."
In terms of protection strategies, the standard principles are:
Once a potential hazard has been recognized, then its possible impact [risk] can be evaluated and effective control measures for the protection of personnel developed and established. The following are some possible options or examples for hazard control.
- Material isolation
- Adequate ventilation
Personal protective equipment
- Boots; gloves; coveralls; aprons; lab coats; face shields; safety glasses; barrier creams;
- personal hygiene;
- Filtration: particulate masks, cartridges and canister gas masks;
- Oxygen content with this form of protection cannot drop below 19.5%
- Supplied air: Compressed ambient air, airline respirator, airline respirator with escape bottle or Self Contained Breathing Apparatus [SCBA].
- Dedicated clothing
- Personal hygiene
- Isolating work areas from eating areas and/or food and drink storage
- Substitution of less hazardous chemicals
- Minimize quantities — stock only enough to accomplish the job.
Limits for employee exposure to toxic substances have been set by toxicologists, people who study the effects of substances on health, or by regulatory agencies. These limits are expressed in a number of ways, but the majority are established as either milligrams per cubic meter [mg/m3] of air for materials such as dust, fumes, mists or aerosols, and/or parts per million [ppm] for gases, vapours or smoke.
The W.C.B. uses the term "permissible concentration" to indicate exposure limits. These are usually based on either an 8 hour or 15 minute exposure period. In addition, there may be an established "ceiling" limit which cannot be exceeded. These limits can be found in their Industrial Health and Safety Regulations.