Editing Threshold limit value (section)

== Definitions of TLV ==
The TLV for chemical substances is defined as a concentration in air, typically for inhalation or skin exposure. Its units are in [[parts per million]] (ppm) for [[gas]]es and in milligrams per cubic meter (mg/m<sup>3</sup>) for [[particulate]]s such as [[dust]], [[smoke]] and [[mist]]. The basic formula for converting between ppm and mg/m<sup>3</sup> for gases is ppm = (mg/m^3) * 24.45 / [[molecular weight]]. This formula is not applicable to airborne particles.

The four categories of TLVs for chemical substances are defined:
# Threshold limit value − [[Time-weighted average (PEL)|time-weighted average]] (TLV-TWA): The average exposure on the basis of a 8 hours per day, 40 hours per week work schedule.
# Threshold limit value − [[short-term exposure limit]] (TLV-STEL): A 15-minute TWA exposure that should not be exceeded at any time during a workday, even if the 8-hour TWA is within the TLV-TWA.
# Threshold limit value - surface limit (TLV-SL): Supplements airborne TLVs by establishing a concentration on a surface that is not likely to cause an adverse effect due to direct or indirect contact.
# Threshold limit value − [[ceiling limit]] (TLV-C): An absolute exposure limit that should not be exceeded at any time.

There are TLVs for physical agents as well as chemical substances. TLVs for physical agents include those for [[noise]] exposure, [[vibration]], [[ionizing radiation|ionizing]] and [[non-ionizing radiation]] exposure and [[heat]] and [[cold]] stress.