Gas Properties
Toxic Gas
Human beings are being exposed to toxic substances to an ever- increasing degree. The increasing use of gases and highly volatile materials represents a danger to life.
Many gases will cause sickness, paralysis or death even when present in the atmosphere in minute concentrations.
The effects of toxic gases can be caused by skin contact, ingestion or inhalation (most common). Continuous monitoring using gas detection devices ensures that defects in installations can be detected quickly and damage to health & property prevented.
The normal concentration of oxygen in air is 20.9%. However, it is possible for this level to be reduced during industrial operations, either because the oxygen is being consumed or absorbed or being replaced by other gases. Oxygen deficiency is a common cause of unexpected death and personnel need to be warned before the oxygen level becomes dangerously low. It is not often realised that the enrichment of oxygen in the atmosphere is also dangerous. Oxygen becomes toxic at high concentrations and the flammability of materials increases.
The two most common toxic gases encountered in industry are Carbon Monoxide (CO) and Hydrogen Sulphide (H2S). CO is a toxic, flammable, colourless & odourless gas, which is slightly lighter than air.
It is classified as a chemical asphyxiant and therefore, only small amounts of CO in the air will cause toxic reactions to occur. Excessive CO in the combustion products of oil, coal or gas boilers, furnaces or air heaters is a clear indication of incorrect or dangerous operation. Serious CO hazards are often present in situations such as vehicle maintenance workshops, multi-storey or underground carparks, road tunnels, confined spaces, coking plants, steelworks etc.
H2S is a colourless, very flammable gas, which in low concentrations has an offensive odour described as that of rotten eggs. H2S is considered a toxic gas that is extremely poisonous in very small quantities. Although it can be detected by the sense of smell in the lower ppb, sensory cells are anesthetised within 2 – 15 minutes exposure at levels > 5ppm, making it impossible to sense dangerous concentration
H2S is encountered in the chemical industry in the performance analyses, production of sulphur, sulphuric acid, plastics & dyes and with fertiliser & detergent raw materials. H2S is emitted in the petroleum industry in the production, transportation and processing of oil & natural gas. It is produced in the paper industry when making cellulose, in the rubber industry, coal powered fire stations, sewage treatment plants and waste disposal facilities.
Chlorine (Cl2) is a toxic, corrosive and oxidant liquefied gas. In appearance, it is greenish-yellow, a heavy gas with a suffocating odour. On its own it is non-combustible in air, but most combustible materials will burn in chlorine as they do in oxygen; flammable gases and vapours will form explosive mixtures with chlorine. It reacts explosively, or forms explosive compounds with many common chemicals, especially acetylene, turpentine, ether, ammonia gas, fuel gas, hydrocarbons and hydrogen.
Chlorine has a Threshold Limit Value (TLV – 8-hour exposure) or Time Weighted Average (TWA) of 0.5ppm and its toxic dose level is 15ppm (human inhalation). The recommended detection range is 0-5ppm. Typical applications in which chlorine is used are water treatment, bleaching (paper & textiles), plastics, refining of certain metals, oxidant and as a de-odouriser.
Sulphur Dioxide (SO2) is a toxic, corrosive, liquefied gas, which is produced as a by-product of many industrial processes. In appearance it is a colourless, inflammable gas with a strong suffocating odour. It has a TLV of 2ppm and a recommended detection range of 0-20ppm. The physiological effects of SO2 on industrial workers are comparatively well known with concentrations as low as 2ppm causing eye, nose, throat and upper respiratory tract irritation. This becomes significant at levels of 10ppm and over.
Typical applications are in the smelting, bleaching, preservative, power industry etc. SO2 is also a prime factor in the production of acid rain and the subsequent damage to the world’s environment.
