Understanding the Inhalation of Gases Hazard Categories

We have been using OSHA’s Hazard Categories from the Globally Harmonized System of Classification and Labelling of Chemicals (GHS) since 2012. I have written a lot about the flammable categories, but I tend to get a lot of questions about the Health Hazard Categories in my HAZCOM and HAZMAT training so I thought I would explain in real-world terms how toxic some chemicals are using the new GHS categories. In this article, I will cover the most common route of entry: Inhalation of GASES

Here is OSHA’s table showing the inhalation categories.

Table A.1.1—Acute Toxicity Hazard Categories and Acute Toxicity Estimate (ATE) Values Defining the Respective Categories

Exposure route	Category 1	Category 2	Category 3	Category 4
Inhalation—Gases (ppmV)	LC₅₀≤ 100	LC₅₀ > 100 and ≤ 500	LC₅₀ > 500 and ≤ 2,500	LC₅₀ > 2,500 and ≤ 20,000
Inhalation—Gases (ppmV)	DANGER Skull and crossbones H330: Fatal if inhaled	DANGER Skull and crossbones H330: Fatal if inhaled	DANGER Skull and crossbones H331: Toxic if inhaled	WARNING Exclamation point H332: Harmful if inhaled

NOTE: under the GHS there are actually five (5) categories, but OSHA only adopted 1-4.

So what this table tells us that when a chemical has an inhalation hazard less than 100 ppm, it will be a Category 1 ACUTE TOXICITY (inhalation). They use Lethal Concentration-Fifty (LC50) to establish these levels (as I have depicted in the revised OSHA table above.

We can also use this information when we are using the International Fire Code (IFC) and we have “Toxic” and “Highly Toxic” materials.

TOXIC

A chemical falling within any of the following categories:
1. A chemical that has a median lethal dose (LD50) of more than 50 milligrams per kilogram, but not more than 500 milligrams per kilogram of body weight when administered orally to albino rats weighing between 200 and 300 grams each.
2. A chemical that has a median lethal dose (LD50) of more than 200 milligrams per kilogram but not more than 1,000 milligrams per kilogram of body weight when administered by continuous contact for 24 hours (or less if death occurs within 24 hours) with the bare skin of albino rabbits weighing between 2 and 3 kilograms each.
3. A chemical that has a median lethal concentration (LC50) in air of more than 200 parts per million but not more than 2,000 parts per million by volume of gas or vapor, or more than 2 milligrams per liter but not more than 20 milligrams per liter of mist, fume or dust, when administered by continuous inhalation for 1 hour (or less if death occurs within 1 hour) to albino rats weighing between 200 and 300 grams each.

HIGHLY TOXIC

A material which produces a lethal dose or lethal concentration which falls within any of the following categories:
1. A chemical that has a median lethal dose (LD50) of 50 milligrams or less per kilogram of body weight when administered orally to albino rats weighing between 200 and 300 grams each.
2. A chemical that has a median lethal dose (LD50) of 200 milligrams or less per kilogram of body weight when administered by continuous contact for 24 hours (or less if death occurs within 24 hours) with the bare skin of albino rabbits weighing between 2 and 3 kilograms each.
3. A chemical that has a median lethal concentration (LC50) in air of 200 parts per million by volume or less of gas or vapor, or 2 milligrams per liter or less of mist, fume or dust, when administered by continuous inhalation for one hour (or less if death occurs within 1 hour) to albino rats weighing between 200 and 300 grams each.

Here are some gases that would fall into the CAT 1 (what I commonly refer to as “Lethal Service”)

Chemical name	Chemical formula	CAS number	LC50 ppm
Arsenic pentafluoride	AsF₅	7784-36-3	20
Arsine	AsH₃	7784-42-1	20
Diborane	B₂H₆	19287-45-7	80
Dichloroacetylene	C₂Cl₂	7572-29-4	45.6
Hydrogen cyanide	HCN	74-90-8	40
Hydrogen selenide	H₂Se	7783-07-5	2
Oxygen difluoride	OF₂	7783-41-7	2.6
Perfluoroisobutylene	C₄F₈	382-21-8	1.2
Phosgene	CCl₂O	75-44-5	5
Phosphine	PH₃	7803-51-2	22
Selenium hexafluoride	SeF₆	7783-79-1	50
Stibine	H₃Sb	7803-52-3	20
Sulfur tetrafluoride	SF₄	7783-60-0	40
Tellurium hexafluoride	TeF₆	7783-80-4	25

You may notice that the two most common chemicals I write about often did not make the cut: Chlorine and Ammonia. And Chlorine and Ammonia are not slouches in the Hazard Categories – they just do not rise to the level of those in CAT 1.

Chlorine rates dangerous enough to be a CAT 2 ACUTE TOXICITY (inhalation) with an LC50 of 293 ppm, but some may be surprised to learn that Anhydrous Ammonia is only a CAT 4 ACUTE TOXICITY (inhalation) because it has an LC50 of 7,338 ppm.

This can best be seen in their IDLH values, where Chlorine has an IDLH of 10 ppm and Ammonia’s is 300 ppm.

But look what NH3 and Cl2 have for Skin and Eye hazards:

SKIN CORROSION – Category 1
SERIOUS EYE DAMAGE – Category 1

Chlorine

SKIN CORROSION/IRRITATION – Category 1
SERIOUS EYE DAMAGE – Category 1

For those wondering, if NH3 should be respected because it is a CAT 4 ACUTE TOXICITY (inhalation) – heck yes, it will kick some serious a_s!

One item that may be of interest to those in the process safety arena:

One of the chemicals that brought about OSHA’s PSM and EPA’s RMP standards and well as dozens of more PS standards around the world is Methyl Isocyanate (MIC). Which Category would MIC fall into?

MIC has an LC50 of 6.1 ppm, making it a CAT 1 ACUTE TOXICITY (inhalation)

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