They’re inherently dangerous and often alarmingly colorless, yet toxic gases and gas mixtures are found in close proximity to many people who could, from direct exposure, suffer adverse health effects, such as eye and skin irritation, headaches, dizziness, and organ damage, and even death. However, the reasoning for their abundant presence is clear: whether it is for the electronics specialty of nitrogen trifluoride, the production of engine coolant constituents from ethylene oxide, or the role as a food preservative by sulfur dioxide, the industrial applications and consumer uses of toxic gases are legion.
Toxic designations are well known, but the understanding of the exact severity of toxicity can vary depending on the system or regulation used to define the gas. For gaseous mixtures, CGA P-20-2017 – Standard for Classification of Toxic Gas Mixtures – fifth edition can prove highly beneficial in toxicity classifications.
CGA P-20-2017 applies to users, manufacturers, and transporters who must interact with or are otherwise affected by label requirements of gas mixtures, and it addresses classifications for these mixtures that contain toxic components. The standard’s primary purpose is to categorize gas mixtures under the transportation regulations that establish a toxicity threshold criterion based on LC50 rat 1 hr values.
A LC50 rat 1 hr value, simply put, is the lethal does of a toxin. LC50 stands for lethal concentration fifty, the concentration of a substance in air for which exposure is expected to cause the death of fifty percent of the entire defined experimental animal population over a specified length of time. A common exposure time length is one hour, and the general test subjects are rats, hence, LC50 rat 1 hr.
For the purposes of CGA P-20-2017, LC50 rat 1 hr is established by a procedure in which ten albino rats are exposed to a test atmosphere for one hour and are then observed for fourteen days. If five of the animals die within the 14-day observation period, the concentration level of the test atmosphere is the LC50.
The U.S. Department of Transportation (DOT) requires, in its Hazardous Materials Regulations, that toxic gases or mixtures of toxic gases be categorized into one of four hazard zones. These are based on LC50 rat 1 hr values, and include:
- Hazard zone A—gases (mixtures) with an LC50 rat 1 hr that are less than or equal to 200 ppm
- Hazard zone B—gases (mixtures) with an LC50 rat 1 hr that are greater than 200 ppm but less than or equal to 1000 ppm
- Hazard zone C—gases (mixtures) with an LC50 rat 1 hr that are greater than 1000 ppm but less than or equal to 3000 ppm
- Hazard zone D—gases (mixtures) with an LC50 rat 1 hr that are greater than 3000 ppm but less than or equal to 5000 ppm
The United Nations (UN) Globally Harmonized System of Classification and Labeling of Chemicals (GHS) also categorizes toxic gas mixtures into four categories but bases the toxicity categories on LC50 rat 4 hr values. The acute toxicity categories under GHS are:
- Acute toxicity category 1—gases (mixtures) with an LC50 rat 4 hr less than or equal to 100 ppm
- Acute toxicity category 2—gases (mixtures) with an LC50 rat 4 hr greater than 100 ppm but less than or equal to 500 ppm
- Acute toxicity category 3—gases (mixtures) with an LC50 rat 4 hr greater than 500 ppm but less than or equal to 2500 ppm
- Acute toxicity category 4—gases (mixtures) with an LC50 rat 4 hr greater than 2500 ppm but less than or equal to 20000 ppm
Both the DOT hazard zones and the GHG acute toxicity categories are important aspects of the CGA P-20-2017 standard, and in the tables at the end of the document, the toxicity limits for different mixture types under each hazard zone and acute toxicity category are listed. The calculations used for the determination of these values are given as well. In addition to this guidance, the standard specifies calculations for classifying the toxicity of a gas mixture and related principles.
CGA P-20-2017 is the fifth edition of the standard for the classification of toxic gas mixtures, and, as such, has undergone a plethora of revisions. To ease compliance for any past user of the standard, every technical change is underlined in the document.
CGA P-20-2017 was written by the Compressed Gas Association (CGA), an ANSI-accredited standards developing organization. Standards by CGA are related to the equipment and supply chain used in the compressed gas industry.
CGA P-20-2017 – Standard for Classification of Toxic Gas Mixtures – fifth edition is available on the ANSI Webstore.