Occupational Health & Safety Hub.com » Exposure Limit

Combined Toxic Effects of Chemicals

admin — Thu, 21 May 2009 01:32:20 +0000

Below 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

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

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

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:

when exposures to the components are not clearly hazardous when considered singly, but potentially
hazardous due to additivity or interactions when considered together
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
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.

EPA

Consistent with the ASTDR approach

Join the forum discussion on this post - (1) Posts

Adjusting Occupational Exposure Limits for Extended Work Shifts

admin — Tue, 19 May 2009 21:25:19 +0000

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.

Join the forum discussion on this post - (1) Posts

PEL Adjustments for Extended Work Shifts – Noise

admin — Mon, 20 Apr 2009 22:33:42 +0000

The 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.

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.

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

Join the forum discussion on this post - (1) Posts