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Threshold limit value is used to describe the limit below which workers can be safely exposed to a substance on a daily basis. This limit is used in connection with concentrated airborne particles only, not liquids. Threshold limit value in the workplace is commonly based on eight- or seven-hour shifts, which are the normal number of hours in workday.
The threshold limit value is a system for the analysis of exposure to chemical substances and physical agents developed by the American Conference of Governmental Industrial Hygienists (ACGIH), originally called the National Conference of Governmental Industrial Hygienists. This organization developed the threshold limit values in the 1940s and 1950s through extensive research, and these values serve as a guide for industries in the protection of their workers. These values can be applied in industrial ventilation, biological exposure and chemical substances in the workplace.
Substances that are included in the study of threshold limit value include workplace fumes, gases, mists, sprays, vapor and dusts. All of these substances have their own units of measurement. Vapors and gases are measured in parts per million (PPM), and particulates such as mists, fumes, smoke and dusts are measured in milligrams per cubic meter. The purpose of establishing the threshold limit value is to safeguard the health of workers.
Formaldehyde, for example, is an industrial and commercial chemical that is quite common in many workplaces. It is a suspected human carcinogen, meaning that long-term exposure to it might lead to cancer. Short-term effects to low concentrations of formaldehyde include mild discomfort, such as a ticklish sensation at the back of the throat. Exposure to higher concentrations might cause lung inflammation, pulmonary edema or even death.
Particulates are a mixture of liquid droplets and solid particles in the air. Some of these particles are so small that they cannot be seen with the naked eye. Some of the sources of particulate matter include industries and power plants. Regulating or limiting the level of continuous exposure to the sources of these particulates will help protect workers.
A short-term threshold limit value is the length of time a worker can be subjected to a maximum exposure limit. This exposure to higher-than-normal concentrations is offset by a much shorter continuous time limit, usually 15 minutes, followed by a long break. Such exposures usually can be done no more than four times a day. The ceiling threshold limit value is the highest concentration to which a worker can be subjected at any time, regardless of the circumstances.