Permissible and Threshold Limits of Exposure and Dosage

Various limits, values or dosages are assigned to indicate permissible or lethal limit etc.

(1) Permissible Exposure Limits (PELs)

  • Set by OSHA, 29 CFR 1910.1000, and 1910.1001 through 1910.1450.
  • Specifies the maximum amount or concentration of a chemical to which a worker may lie exposed.
  • Generally defined in three different ways.

A.Ceiling Limit (C): the concentration that must not be exceeded at any art of the workday

B.Short-Term Exposure Limit (TEL): the maximum concentration to which workers may be exposed for a short period of time (15 minutes)

    C.Time-Weighted Average (1 WA): the average concentration to which workers may be exposed for a normal, 8-hour workday

    (2) Threshold Limit Values (TLVs)

    • Prepared by ACGIH volunteer scientists
    • Generally can be denned as ceiling limits, short term exposure limits, and or time weighted averages
    • Usually equivalent to PELs
    • Denotes the level of expositive that nearly all workers can experience without an unreasonable risk of disease or injury
    • An adviso4r limit; not enforceable by law

    (3) Excursion Limit – ACGIH

    • This limit is applicable to those substances Which have no TV-STELs.
    • Excursion in worker exposure levels may exceed than a total of 30 minutes during a workday, and un exceed 5 times the TLVJI‘WA, provided that the TLVTWA is not exceeded.

    (4) Recommended Exposure Limits (RELS)

    • Recommended by NIOSH
    •  Indicates the concentration of a substance to which a ‘worker can be exposed tor UP 10 a 10hour workday during a 40-hour work week without adverse effects, however, sometimes based on technical feasibility
    • Based on animal and human studies
    • Generally expressed as a ceiling limit, short-term exposure limit, or a time-weighted average
    • Often more conservative than PELs and. TLVs

    (5)Workplace Environmental Exposure Limits (WEELs)

    • Developed by AIHA volunteers
    • Advisor limits; not enforceable by law
    • Typically developed for chemicals that are not widely used or for which little toxicity information is available

    (6) Company-Developed Limits

    Developed by company scientists

    • Advisor limits; not enforceable by law
    • Usually’ based on only short-term studies of animals
    • Generally intended for internal company use and sometimes for the customers TLV may differ in different countries as shown.

    Aniline  TLV in India 2 ppm  and TLV in Sweeden is 1 ppm

    Benzene TLV in India 10 ppm and TLV Sweeden is 5 ppm


      (6) Tentative Biological Exposure Limits; and Health Based Limits:

      In fact the human organism itself may be regarded as a kind of sampling service. A worker’s body represents his own-individual collector, register and monitor of his personal exposure. T o arrive at an accurate evaluation of toxic exposure effect, many additional physiological data are nevertheless required, such as rate of inhalation and quantity of. Inhaled air, percentage absorption by the skin and (occasionally) by the intestinal. Tract, retention rate of metabolism ‘ and excretion etc. Therefore the biological exposure (biological monitoring) has been gaining increasing attention recently. The tentative biological exposure limits for the most important toxic substances present in industry have been developed.

      The most modern approach is to consider the integral exposure resulting from all modes of entry (inhalation, ingestion, skin absorption) including exposure in the living environment.  Adopting this approach ‘WHO (World Health Organisation) study group recently published’ health based limits’ for occupational exposure to some common heavy metals.

      (7) Immediately Dangerous to Life or Health (IDLH)

      Concentration immediately dangerous to life or health from which a worker could escape without any escape impairing symptom or any irreversible health effect  (NIOSH/OSHA). It is a Concentration at which a person can escape without the use of a respirator within 30 minutes. This is used in selecting type of respirator etc.

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