Important & Useful HSE Terms Explained

The Threshold Limit Value (TLV) of a chemical substance is a level to which it is believed a worker can be exposed day after day for a working lifetime without adverse effects. TLV is a reserved term of the American Conference of Governmental Industrial Hygienists (ACGIH).

ACGIH (TLV-TWA) - The TWA concentration for a conventional 8 h work day and 40 h work week, to which it is believed that nearly all workers may be repeatedly exposed, day after day for lifetime without adverse effect.

ACGIH (TLV-STEL) indicates Short Term Exposure Limit. A 15 minutes exposure that should not be exceeded at any time during a work day.

Lower Explosive Limit (LEL): The lowest concentration (percentage) of a gas or a vapor in air capable of producing a flash of fire in presence of an ignition source (arc, flame, heat). The term is considered by many safety professionals to be the same as the lower flammable limit (LFL). At a concentration in air lower than the LEL, gas mixtures are "too lean" to burn.

Upper Explosive Limit (UEL): Highest concentration (percentage) of a gas or a vapor in air capable of producing a flash of fire in presence of an ignition source (arc, flame, heat). Concentrations higher than UFL or UEL are "too rich" to burn.


Specific Gravity - The term specific gravity, symbolized sp gr, refers to the ratio of the density of a solid or liquid to the density of water at 4 degrees Celsius. The term can also refer to the ratio of the density of a gas to the density of dry air at standard temperature and pressure, although this
specification is less often used. Specific gravity is a dimensionless quantity; that is, it is not expressed in units.


 
Boiling Point - The boiling point of a substance is the temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid and the liquid changes into a vapor.

Flash Point - The flash point of a volatile material is the lowest temperature at which vapors of the material will ignite, given an ignition source.

Auto Ignition Temperature - The auto ignition temperature or kindling point of a substance is the lowest temperature at which it spontaneously ignites in normal atmosphere without an external source of ignition, such as a flame or spark.


Parts Per Million or ppm means out of a million - Usually describes the concentration of something in water or soil. One ppm is equivalent to 1milligram of something per liter of water (mg/l) or 1 milligram of something per kilogram soil (mg/kg).