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Threshold Limit Values for Chemical Substances. Adopted by ACGIH with Intended Changes. Policy Statement. The TLVs (Threshold Limit Values) are designed as guidelines to have a control on health hazards.

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threshold limit values for chemical substances

Threshold Limit Values for Chemical Substances

Adopted by ACGIH with Intended Changes

policy statement
Policy Statement
  • The TLVs (Threshold Limit Values) are designed as guidelines to have a control on health hazards.
  • These are not developed for use as legal guidelines and ACGIH does not advocate their use as such.
  • No individual other than someone trained in this discipline can apply these guidelines .
points to remember
Points to remember
  • TLVs represent conditions to which all workers may be repeatedly exposed day after day without adverse health effects.
  • Persons previously having health problems might be affected seriously.
  • The latest TLV documentation should be referred to get complete information of a chemical as TLV is revised often.
threshold limit value time weighted average tlv twa
Threshold limit value-Time weighted average (TLV-TWA)

This is the concentration averaged over an 8 hr. workday and a 40 hr. workweek, to which the worker may be xeposed day after day without adverse affect.

  • TLV values (2000)

Acetophenone - 10 ppm.

Carbon monoxide - 25 ppm.

Benzene - 10 ppm.

Cobalt carbony - 0.1 ppm.

Formic acid - 5 ppm.

threshold limit value short term exposure limit
Threshold limit value-Short-term Exposure limit

The concentration to which workers can be exposed continuously for a short period of time without suffering from

  • Irritation
  • Chronic or irreversible tissue damage
  • Narcosis of sufficient degree to increase the likelihood of accidental injury, impair self-rescue or materially reduce work efficiency
  • and provided that the daily TLV-TWA is not exceeded.
  • STEL values (2000)
  • Acrolein - 0.3 ppm.
  • Ammonia - 35 ppm.
  • Chlorine dioxide -0.3 ppm.
  • Decaborane -0.15 ppm.
threshold limit value ceiling tlv c
Threshold limit value- ceiling (TLV-C)
  • This is the concentration that should not be exceeded during any part of the working exposure.
  • If instantaneous monitoring is not feasible, then the TLV-C can be assessed by sampling over a 15 minute period.
  • For some substances only TLV-C may be relevant.
  • For some substances more than one categories may be relevant depending on their physiologic condition. If any of the TLVs is exceeded a potential hazard is presumed to exist.
  • TLV-C values (2000)
  • n-Butanol - 152 ppm.
  • tert-Butyl chromate - 0.1 ppm.
  • Ehtylene Glycol vapor and myst -50 ppm.
  • Glutaraldehyde -0.2 ppm.
how does twa and c differs
How does TWA and C differs?
  • Ceiling limit is best suited for chemicals that are fast acting and do not have a good control by the use of TWA.
  • TWA requires an explicit limit to the excursions that are permissible above the limited TLVs, whereas the ceiling limit places a definite boundary.
excursion limits
Excursion limits
  • Even if the 8 hr. TLV-TWA is within limits excursions above the TLV-TWA should be controlled
  • The maximum recommended excursion should be related to variability generally observed in actual industrial processes.
  • During a workday, excursions may exceed 3 times the TLV-TWA for no more than a total of 30 minutes and under no circumstances should they exceed 5 times the TLV-TWA; and under no case the TLV-TWA should be exceeded.

* where : TLV-TWA is threshold limit value-Time weighted average.

skin notation
“Skin” notation
  • Listed substances followed by the designation “skin” refer to the potential significant contribution to the overall exposure by the cutaneous route, including mucous membranes and the eyes, either by contact with vapors or, of probable greater significance, by direct skin contact with the substance.
  • Materials having a relatively low dermal LD50 would be given a skin notation based on acute animal toxicity data.
  • It alerts that air sampling alone is insufficient to accuaratey measure quantitative exposure to prevent cutaneous absorption.
  • Skin Chemicals






  • Special consideration should be given also to the application of the TLVs in assessing the health hazards that may be associated with exposure to mixtures of two or more substances.
  • TLV of a mixture:

If, TLV = c1/t1+c2/

exceeds unity, then the Threshold Limit of the mixture is considered to be exceeded.


c1,c2…. cn are concentrations of different substances in a mixture, and

t1,t2…. tn are TLVs of corresponding substances.

respirable and inhalable dust
Respirable and Inhalable dust
  • TLVs are expressed in terms of inhalable dust for solid substances and liquefied mists, except where the term “respirable dust” is used.

The particle size-selective TLVs (PPS-TLVs) are expressed in 3 forms:

  • Inhalable Particulate Mass TLVs (IPM-TLVs).
  • Thoracic Particulate Mass TLVs (TPM-TLVs).
  • Respirable Particulate Mass TLVs (RPM-TLVs).
particles not otherwise classified pnoc
Particles not otherwise classified (PNOC)
  • Many substances are not on the list for which there are no evidence of toxic effects.
  • TLV committee recommends the use of the term “PNOC” to emphasize that these materials are potentially toxic.
  • Particulates under PNOC heading are those containing no asbestos and < 1% crystalline silica.
  • For otherwise non-toxic dusts:

Inhalable particulate -TLV-TWA 10mg/m³.

Respirable particulate - TLV-TWA 3 mg/m³.

simple asphyxiants inert gases or vapors
Simple Asphyxiants-”Inert” gases or vapors
  • The limiting factor being the available Oxygen,TLV may not be recommended for each simple Asphyxiant.
  • The minimal Oxygen content should be 18% by volume under normal atmospheric pressure.
  • Account should be taken for several Asphyxiants which present an explosion hazard.
physical factors
Physical Factors

Physical factors which act adversely to increase the toxic response of a substance are :

  • Heat
  • Ultra violet rays
  • Ionizing radiation
  • Humidity
  • Abnormal pressure (altitude)
  • Light

These may place added stress on body, TLVs have built-in safety factors to guard against adverse effects to moderate deviations but not gross deviations.

  • Examples: Continuous work at temperature above 32ºC might be considered gross deviation. Judgment must then be exercised in proper adjustment of the TLV.
unlisted substances
Unlisted Substances
  • TLV is not a complete list of all hazardous substances.
  • Substances that do not appear on the TLV list might be harmful or harmless.
  • While using Unlisted Substances, the medical and scientific literature should be reviewed.
  • It is also necessary to conduct preliminary toxicity studies.
varying work schedules
Varying work schedules
  • Particular judgment to provide same safety to the workers working for a schedule different from the conventional 8 hr. day and 40 hr. week.
  • Field hygienists are referred to the “Brief and Scala model”:
    • It reduces the TLV proportionately for both increased exposure time and reduced recovery time.
    • Example: exposure to 8 times the TLV-TWA for 1 hour and zero exposure during the remainder of the shift.
conversion of tlvs in ppm to mg m
Conversion of TLVs in ppm to mg/m³
  • The conversion is based on 760 torr barometric pressure at 25ºC(77ºF), and molar volume of 24.45 liters
  • TLV in mg/m³ =

(TLV in ppm)*(gram molecular weight of substance) /24.45

    • TLV in ppm =

(TLV in mg/m³)*(24.45)/(gram molecular weight of substance).

biologically derived airborne contaminants
Biologically derived airborne contaminants
  • Biologically derived airborne contaminants include bioaerosals and volatile oraganic compounds released from living organisms.
  • Guidelines for the Assessment of Bioaerosals given in the Indoor environment, ACGIH, 1989.
  • The guidelines rely on medical assessment of symptoms, evaluation of building performance, and professional judgment.
  • There are no TLVs that allow ready interpretation of Bioaerosal data.
  • Examples
  • Bacteria.
  • Fungi.
  • Aspergillus fumigatus.
  • Legionella pneumophila.
  • Endotoxin.