The measure of chemical toxic levels:
The American Conference of Governmental Industrial Hygienists (ACGIH) has set such levels, for human exposure in the industry. They are called Threshold Limit Values -TLV’s which were earlier known as Maximum Allowable Concentrations-MAC’s.
Four categories of Threshold Limit Values set by ACGIH are as follows:
TLV-TWA:
This is the maximum allowable concentration of the toxic gas in air in which majority of the workers may be able to work for a normal 8 hrs/day or 40 hrs/week without any adverse health risk.
Time-weighted average is calculated by taking various readings of the toxic gas at a constant interval. The average of this reading is known as TLV-TWA.
TLV is expressed as parts per million-ppm.
TLV-TWA Calculation:
TLV-TWA = C1T1+C2T2+……..CnTn
T1+T2……..Tn
TLV-STEL:
Maximum concentration of the toxic gas to which the worker can be exposed for a short period of 15 minutes without suffering irritation, chronic or irreversible tissue change or any other adverse health risk.
The STEL limit should be considered a maximum allowable concentration, or absolute ceiling, not to be exceeded at any time during a continuous 15 minutes period.
No more than four 15 minutes exposure periods per day are permitted with at least 60 minutes between each exposure period, provided that the daily TLV-TWA is not exceeded.
TLV-C:
Ceiling is the concentration that should never be exceeded, never even instantaneously.
TLV-Skin:
TLV-Skin indicates substances which can add to the overall exposure being absorbed through the skin including membranes and eyes as well as by other means.
The TLV’s for fumes, mists and some dusts are given in mg/m3.
IDLH: (Immediately dangerous to life or health)
IDLH value has been developed by NIOSH (National Institute for occupational safety and Health-US)
This is the maximum concentration of toxic gas which may cause death or immediate/delayed permanent health effect or prevent the escape from such an environment.
SL #
|
Name of the chemical
|
TLV-TWA (ppm)
|
TLV-STEL (ppm)
|
IDLH (ppm)
|
1.
|
Chlorine gas (Cl2)
|
0.5
|
1.0
|
10
|
2.
|
Sulphur-di-oxide (SO2)
|
2
|
5
|
100
|
3.
|
Hydrogen Sulphide (H2S)
|
10
|
15
|
100
|
4.
|
Ammonia (NH3)
|
25
|
35
|
300
|
5.
|
Carbon monoxide (CO)
|
50
|
400
|
--
|
Lethal Concentration Fifty (LC50):
A calculated concentration in air which when administered by the respiratory route is expected to kill 50% of a population of experimental animals during an exposure of four hours. Ambient concentration is expressed in mg/litre.
Lethal Dose Fifty (LD 50):
A calculated dose of a chemical substance which is expected to kill 50% of population of experimental animals exposed through a route other than respiration. Dose is expressed in mg/kg of body weight.
1. Physical Properties of chemicals
2. Freezing point:
Temperature at which the liquid form of a chemical will turn into solid form.
1. Melting point:
The melting point of a solid is the temperature at which a solid turns into liquid. Solids that have meting points close to ambient temperature may and in contact with other substances become solvents.
1. The boiling point of substance is the temperature at which a liquid turns into vapour.
The higher the vapour concentration in the air, the grater the need for respiratory protection.
2. Vapour density:
The vapour density of a gas or vapour determines whether it will be heavier or lighter than air.
The vapour density is the ratio of the density of a gas to that of air.
Vapour Density= Density of gas
Density of air
3. Density:
The Density of a material is its mass per unit volume. (Unit= gm/cc)
Density= Mass
Volume
4. Vapour Pressure:
Vapour pressure is the pressure exerted by a vapour directly above the liquid surfaces in a closed container (rate of evaporation); usually measured in millimeters of mercury (mm Hg).
The higher the vapour pressure, the more readily substances give off vapours.
5. Gravity:
The specific gravity of a liquid determines whether that liquid will be heavier or lighter than water. If the liquid is not soluble in water and has a specific gravity less than 1, the liquid will float on top of the water; if the liquid has a specific gravity greater than 1, the liquid will sink below the water.
The specific gravity is the ratio of the density of a liquid to that of water.
Sp. Gravity = Density of liquid
Density of water
6. Flammable properties of chemicals:
7. Flash point:
The flash point of a liquid is the temperature at which it gives off vapour in sufficient concentration to form an ignitable mixture with air. A liquid’s flash point is the primary property or characteristics used to determine its relative degree of flammability. Since it is the vapours of flammable liquids that burn, vapour generation is a primary factor in determining the liquid’s fire hazard.
1. Fire point:
Fire point is the lowest temperature at which combustion can occur.
1. Ignition (Auto ignition) Temperature:
The ignition temperature of a substance, whether solid, liquid or gas is the minimum temperature required to cause self-sustained combustion in the absence of any source of ignition.
1. Flammable limits:
For a vapour or gas to burn, the correct mixture of fuel and oxygen must present. If there is too much or too little vapour or oxygen, a fire will not occur even though the fuel may be at its flash point.
1. Lower Flammable Limit: (LEL/LFL)
The minimum concentration of vapour or gas in air by volume below which propagation of flame does not occur on contact with a source of ignition.
1. Upper Flammable Limit: (UEL/UFL)
The maximum concentration of vapour or gas in air by volume above which propagation of flame does not occur on contact with a source of ignition.
1. Flammable Range:
Any vapour concentration outside the LEL and UEL range, or flammable range, is not an ignitable mixture, as illustrated below:
7. Reactivity:
Reactivity is the susceptibility of a material to release energy either by itself or in combination with other material.
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