What are the GHS Hazard categories for acute toxicity and how do they differ from WHMIS?

                                                                                         

Both Canada's WHMIS and the Globally Harmonized System have a classification system for acute toxicity. They differ in the fact that GHS has 5 hazard categories for acute toxicity, numbered 1 through 5, while WHMIS only has 2 - D1A and D1B.

Unfortunately the GHS and WHMIS categories are not synchronized between oral toxicity, dermal toxicity and inhalation (gas) toxicity, so there is not one 'rule of thumb' which translates GHS categories to WHMIS categories (or vice versa).

Oral toxicity categories are determined by Oral LD50 per mg/kg bodyweight. For GHS, Category 1 Acute-Toxicity Oral is for LD50 values under 5 mg/kg and Category 2 is for LD50 values between 5mg/kg and 50 mg/kg. These two categories combined make up the WHMIS D1A Oral Toxicity Category of 0 to 50 mg/kg. Things become confusing after this, with GHS Category 3 being reserved for values between 50 and 300, Category 4 for values between 300 and 2000 and Category 5 for values between 2000 and 5000. However, WHMIS category D1B is reserved for values between 50 and 500; this includes all of Category 3 of GHS and part of the GHS Category 4.

Dermal toxicity categories are determined by Dermal LD50 per mg/kg bodyweight. For GHS, Category 1 Acute-Toxicity Dermal is for LD50 values under 50 mg/kg and Category 2 is for LD50 values between 50 mg/kg and 200 mg/kg. These two categories combined make up the WHMIS D1A Dermal Toxicity Category of 0 to 200 mg/kg. Unlike oral toxicity, the situation remains more straight-forward, with GHS Category 3 and D1B under WHMIS both have LD50 Dermal values between 200 and 1000. At this point there are no more WHMIS classifications, while GHS has a Category 4 for values between 1000 and 2000, and a Category 5 for values between 2000 and 5000.

Finally, for Inhalation (gas), toxicity categories are determined by LC50 ppm. For GHS, Category 1 Inhalation (gas) is for LC50 values under 100pm, Category 2 is for LC50 values between 100 and 500ppm and Category 3 is for LC50 values between 500 and 2500ppm. These three categories combined make up the WHMIS D1A Oral Toxicity Category of 0 to 2500ppm. There is no D1B Oral Toxicity under WHMIS, but there is a Category 4 under GHS for values between 2500 and 20000ppm. Finally, there is a Category 5 which is reserved for an "LC50 in the equivalent range of the oral and dermal LD50".

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