Threshold Limit Values for Chemical Substances

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Threshold Limit Values for Chemical Substances

• Adopted by ACGIH with Intended Changes

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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 .

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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.

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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.

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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.

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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.

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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.

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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.

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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
• Aminodiphenyl.
• Butoxyethanol.
• Catechol.
• Cresol.
• HexaChlorobenzene.

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Mixtures

• 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/t1c2/t2....cn/tn
• exceeds unity, then the Threshold Limit of
  the mixture is considered to be exceeded.
• where,
• c1,c2. cn are concentrations of
  different
  substances in a mixture, and
• t1,t2. tn are TLVs of corresponding
  substances.

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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).

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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 lt 1 crystalline
  silica.
• For otherwise non-toxic dusts
• Inhalable particulate
  -TLV-TWA 10mg/m³.
• Respirable particulate -
  TLV-TWA 3 mg/m³.

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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.

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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.

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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.

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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.

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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).

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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.