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OSHA GHS Guidance: Using 'Weight of Evidence' to Classify Hazards

Posted on 2/19/2016 by Roger Marks

Professionals in the chemical industry face new challenges now that OSHA’s adoption of the Globally Harmonized System (GHS) is in full effect. Of these challenges, one of the most prominent is adjusting to new hazard classification criteria and procedures that have changed to harmonize US rules with international standards.

To classify a chemical in the workplace, chemical manufacturers, importers, and employers rely on the evidence and information available to help determine if the chemical is hazardous and, if so, how hazardous. 

Physical Hazards vs. Health Hazards
Classifying physical hazards is typically straightforward: A chemical is flammable, for instance, if it has a measured flash point below a certain value. Classification of chronic health risks, though, can be more complex.

In some cases, a classification can be reached by evaluating the quantitative test data (e.g., LD50 for toxicity). Other times, however, the available data may be too complex or conflicting to result in a single classification. When this happens, the manufacturer, importer, or employer should use an approach known as “weight of evidence” (WoE) to determine the level of hazard.

What Is the GHS Weight of Evidence Approach?
The “Weight of Evidence” approach refers to the practice of combining and analyzing all of the available information that could affect the hazard classification—in vitro test results, relevant animal data, epidemiological and clinical studies, case reports, etc.

In many cases, the scientific information alone may be insufficient to make a sure-fire hazard determination. To classify these chemicals, employers should use “weight of information”—in essence, assigning more “weight” to the most reliable information.

Here is a step by step approach for evaluating weight of evidence, as found in OSHA's WoE draft guidance: 
Common steps to classify chemicals based on WoE
OSHA Seeks Comments on Latest GHS Guidance
To help employers classify chemical hazards using the WoE approach, OSHA has created and shared an expansive “Weight of Evidence” guidance document. OSHA is requesting public comments on this document. The document includes specific hierarchies of information and how employers should use the data available to reach a determination.

The new guidance is a complement to a previous OSHA document, the Hazard Classification Guidance for Manufacturers, Importers, and Employers.

GHS Training for Managers and Employees
Make sure your employees are prepared to protect themselves under OSHA’s HCS with the Hazard Communication Online Course, updated to reflect the new GHS standards in effect since June 1, 2015. For EHS managers who oversee their site’s HazCom program, Lion offers the Managing Hazard Communication Online Course.

Tags: GHS, osha

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