The TWA (Mean Exposure Value) measurement system, otherwise known as dosimetric colorimetric tube measurement, involves measuring the time-weighted average of Formaldehyde concentrations in the ambient environment. The dosimetric tube can be placed in its holder and hung at respiratory tract level (shirt or blouse collar, for example), enabling a person's exposure to be monitored during the working day. Using this system, it is easy to obtain daily gas concentrations and assess the working environment by comparing them with the recommended exposure limits. This dosing method can only be used to detect contamination. The use of this method does not exempt the employer from checking compliance with the TLVs for carcinogens, mutagens or agents toxic to reproduction present in the workplace atmosphere. In the case of a restrictive VLEP such as that for Formaldehyde, the test is carried out annually by an accredited outside body.
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