Exposure Limit

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MAK Collection for Occupational Health and Safety Now Available Online – FREE

The MAK Collection for Occupational Health and Safety now is available on-line and free.  It provides the German recommended exposure limits with documentation for chemicals used in Germany and many other countries.  The MAK-values are daily 8-hour time-weighed average values and apply to healthy adults. Substance-specific acceptable peak concentrations, including the highest possible duration of such peaks, are defined. If the substance can be taken up through the skin, this is indicated.  The MAKs have been collected and evaluated by the Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area – part of the DFG, the German Research Foundation.  The MAK values are similar to the ACGIH TLVs and documentation.

You can find the publication HERE

Source: Wiley Online Library

Combined Toxic Effects of Chemicals

cumulativeBelow is a summary of some of the very similar methods and rationale used by agencies to assess the combined toxic effects of chemicals.

ACGIH
Source: TLV’s and BEI’s Publication

  • Recommends an additive (combined) approach for two or more substances that affect the same target organ/system
  • The ratio of the exposure concentrations are summed together
  • {C_1}/{t_1} + {C_2}/{t_2} + cdots {C_n}/{t_n}

    where: C = observed concentration, t = TLV

  • If the sum exceeds one, the TLV for the mixture is considered to have been exceeded
  • Additive formula applies to simultaneous exposures for hazardous substances with TWA’s, STEL’s, Excursion Limits, and/or Ceiling Limits. Bases (TWA’s, STEL’s, etc.) should be kept consistent, as feasible
  • Exceptions are made when it is believed the major effects of the chemicals are not additive or possibly when the mixtures contain carcinogens
  • Synergistic effects should be carefully considered

OSHA
Source: 29 CFR 1910.1000

  • Recommends an additive (combined) approach for two or more substances that affect the same target organ/system
  • The ratio of the exposure concentrations are summed together
  • {C_1}/{t_1} + {C_2}/{t_2} + cdots {C_n}/{t_n}

    where: C = observed concentration, t = TLV

  • If the sum exceeds one, the TLV for the mixture is considered to have been exceeded
  • Approaches to chemicals with similar effects are not restricted

NIOSH
Source: NIOSH Methylene Chloride Intelligence Bulletin

  • Recommends an additive (combined) approach for two or more substances that affect the same target organ/system
  • The ratio of the exposure concentrations are summed together
  • {C_1}/{t_1} + {C_2}/{t_2} + cdots {C_n}/{t_n}

    where: C = observed concentration, t = TLV

  • If the sum exceeds one, the TLV for the mixture is considered to have been exceeded
  • Specifically addressed methylene chloride in the presence of carbon monoxide due to the formation of carboxyhemoglobin

Other Agencies

    ATSDR

  • Typically a semi-quantitative screening process using flow-chart methodology
  • Useful:
    1. when exposures to the components are not clearly hazardous when considered singly, but potentially
      hazardous due to additivity or interactions when considered together
    2. when the community-specific health outcome data indicated that the site might have an adverse
      impact on human health, but the exposure-based assessment of each separate component did not
    3. when the health outcome data were ambiguous or did not indicate an adverse impact on human
      health, but the exposure-based assessment identified a potential hazard from one or more of the
      components.

Adjusting Occupational Exposure Limits for Extended Work Shifts

factory-workers-bw


OSHA currently has two standards in which the PEL is adjusted based on the length of the work shift, both of which are lead standards:

All other PELs are based upon 8-hour time weighted average (TWA), a short term exposure limit (STEL), or a ceiling limit (C).

The lead PEL of 50 micrograms per cubic meter (ug/m3) is adjusted in accordance with the following formula:

New PEL = 400/hours worked during shift

e.g. If an employee works a 10 hour shift, the PEL is reduced from 50 ug/m3 to 40 ug/m3

Additionally, as discussed in OHShub.com‘s post PEL Adjustments for Extended Work Shifts – Noise, the Action Level for noise must be reduced for extended work shifts based on the following formula:

New AL (dBA) = 90 + 16.61 x log (50 / (12.5 x number of hours worked))

e.g. If an employee works a 10 hour shift, the PEL is reduced to 83 dBA

From a recommended practice standpoint, the Brief and Scala model provides a easy method of reducing exposure limits and is noted ACGIH’s TLVs and BEIs documentation. The Brief and Scala model provides a reduction factor via the following formula:

Reduction Factor = (8 x hours worked in shift) x ((24 – hours worked in shift) / 16)

It should be noted that the Brief and Scala model should not be used to justify very high exposures for short durations.

e.g. If an employee works a 10 hour shift, the reduction factor is 0.7. Multiply 0.7 times to the Exposure Limit to determine the recommended limit using the Brief and Scala model.


Additionally, it should be noted that OSHA does have PEL adjustments for cotton dust, but only when employees are required to wear respirators to reduce their respective exposures.

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PEL Adjustments for Extended Work Shifts – Noise

osha-interpretationsThe purpose of this memorandum is to resolve issues concerning adjustments of the PEL during extended work shifts. This applies for exposures to the noise levels of Table G-16 of 29 CFR 1910.95 or substances found in Subpart Z. Current OSHA policy requires only the action level to be reduced for exposures to noise during extended work shifts; the PEL is not required to be adjusted. The only standards which require PEL adjustments are the lead standards in construction and general industry. These standards have a specific provision which requires work shift adjustments. Existing policy for Occupational Exposure to Cotton Dust also has a requirement to adjust extended work shifts when employees are required to wear respirators for a portion of the work shift to reduce their level of exposure, as set forth in the Federal Register, Vol. 45, No. 251, pp. 85736-85739. The contribution that the extended work hours adds to employee exposure must be included in calculating the required time respirators must be worn during the shift.

As stated in a previous memorandum dated November 8, 1996:

Compliance officers can choose one of two approaches for employees who work extended work shifts beyond 8 hours. The choice taken will depend on the nature of the hazardous chemical.

  1. The first approach is to sample what the compliance officer believes to be the worst continuous 8-hour work period of the entire extended work shift.
  2. The second approach is to collect multiple samples over the entire work shift. Sampling is done such that multiple personal samples are collected during the first 8-hour work period and additional samples are collected for the extended work shift. Unless a compliance officer is dealing with lead, the PEL in this approach is calculated based upon the worst 8 hours of exposure during the entire work shift.

We hope you find this clarification helpful. If you have any questions or if we can be of any further assistance, please contact the Office of Health Enforcement at (202) 693-2190.

A link to the memo can be found HERE

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