Workplace exposure limits are an important tool for risk assessment, providing valuable data for occupational safety, and control of hazardous substances. An education of gas hazards helps to understand how workplace exposure limits are applied.
There are 3 types of gas hazards; Flammable (Combustible), Toxic and Asphyxiant.
Flammable Gas Hazards
Combustion is a fairly simple chemical reaction in which Oxygen is combined with another substance resulting in the release of energy, normally as heat. Flammable hazards E.g Methane, Pentane, Propane. Flammable gases do not have a personal exposure limit yet pose a risk of fire or explosion – a risk to either people, plant materials or property. Flammable gases are measured using flammable limits.
There is only a limited band of gas/air concentration which will produce a combustible mixture. This band is specific for each gas and vapour. It is bounded by an upper explosive level known as the UEL and the lower explosive level (LEL).
In most industrial plants in a normal scenario, there are no gas leaks in the surrounding area. Therefore the early warning gas detection system will only be required to detect levels from 0% of gas up to the LEL. By the time this concentration is reached, shut down procedures or site clearance should have been put into operation.
Toxic Gas Hazards
Toxic hazards present a risk of poisoning. E.g Hydrogen Sulphide, Chlorine, Carbon Monoxide.
Some gases are poisonous and can be dangerous to human health even at very low concentrations. The measurements most often used for the concentration of toxic gases are parts per million (ppm) and parts per billion (ppb).
More people die from toxic gas exposure than from explosions caused by the ignition of flammable gas. You should be aware that there are a large group of gases which are both toxic and combustible).
The main concern with toxic gas exposure is the effect on workers which can be inhaled, ingested or absorbed. Since adverse effects can often result from long term exposure. it is important not only to measure the concentration of gas but also the total time of exposure.
Air is made up of several different gases including Oxygen. Normal ambient air contains a concentration of 20.9%. When the Oxygen level falls below 19.5%, the air is considered to be Oxygen deficient. At 24%, the air is oxygen enriched.
Oxygen depletion can be caused by combustion, oxidation, bacterial action or chemical reaction. Asphyxiants threaten suffocation. This can be caused by oxygen deficiency where Oxygen is consumed or displaced by another gas (inert atmospheres)
Oxygen enrichment can cause items to spontaneously combust because increased Oxygen levels increase flammability.
Workplace Exposure Limits
The term ‘workplace exposure limits’ or occupational hazard monitoring is generally used to cover the area of industrial health monitoring. This is associated with the exposure of employees to hazardous conditions of gases and dust etc. In other words, the aim is to ensure that levels in the workplace are below the statutory limits.
Toxic gas instruments are worn by a worker as near to the breathing zone as possible to ensure a true representation of inhaled air.
The Short Term Exposure Limit (STEL). The concentration for which it is believed that most workers can be continuously exposed for a short period without suffering irritation. tissue damage or narcosis. STEL is a permissible 15 minute exposure period which should not be exceeded at any time during the workday. For some substances, a brief exposure is considered so critical that they are set only a STEL which should not be exceeded even for a shorter time.
The Long-Term Exposure Limit is an 8 hour time weighted exposure period. The Time Weighted Average (TWA) concentration for a conventional 8 hour working day and 40 hour work week, to which it is believed that nearly all workers may be repeatedly exposed without adverse effect.
Occupational exposure limits in the UK fall under the Control of Substances Hazardous to Health Regs (COSHH). The COSHH require the employer to ensure that employee’s exposure to substances hazardous to health is either prevented or controlled.
The list of exposure limits are stipulated in the EH40 Guidelines are legally enforceable by the UK Health and Safety Executive.
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