Gas Hazard Definitions and Data

The detection of gases in plant environments has a critical and wide-ranging role in the chemical process industries (CPI). Among the major applications of gas detection are limiting personnel exposure to hazardous chemicals, preventing explosive atmospheres, protecting the environment and identifying leaks in process equipment. Table 1 summarizes the main reasons for gas monitoring.

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All CPI workers should be conversant with gas-detection terms to promote safety, health and environmental quality. The following is a collection of terms and data involving hazardous gases (Table 2).
PEL (permissible exposure limit). Set by OSHA to limit workers’ exposure to an airborne substance, PELs are based on an eight-hour time-weighted average. PELs are enforceable legal limits.
TLV (threshold limit value). Established by the American Conference of Governmental Industrial Hygienists, TLVs are based on known toxicity of chemicals in humans or animals, and are recommendations, rather than legal limits.
IDLH (immediately dangerous to life and health). Defined in the U.S. by the National Institute for Industrial Safety and Health (Part of the Centers for Disease Control and Prevention) as a level of exposure that is likely to cause death or immediate or delayed permanent adverse health effects
LC₅₀ (median lethal concentration). A measure of the toxicity of a surrounding medium that will kill half of a sample population of test animals in a specified period through exposure via inhalation.

Oxygen deficiency

Normal ambient air contains 20.8 vol.% oxygen. When oxygen concentration dips below 19.5 vol.% of the total atmosphere, the area is considered oxygen deficient. Oxygen deficiency may result from O₂ being displaced by other gases, such as carbon dioxide, and can also be caused by rust, corrosion, fermentation or other forms of oxidation that consume oxygen. Table 3 outlines the physiological effects of oxygen deficiency by concentration.
If oxygen concentrations in the air rise above 20.8%, the atmosphere is said to be oxygen-enriched. Higher oxygen levels can increase the likelihood and severity of a flash fire or explosion, because the oxygen-enriched atmosphere tends to be less stable than air.

Table 1. summary of the main reasons for gas monitoring.
Type of monitoring	Purpose	Hazard	Possible source of hazard
Personal protection	Worker safety	Toxic gases	Leaks, fugitive emissions, industrial process defects
Explosive	Worker safety and facility safety	Explosions	Presence of combustible gases and vapors due to leaks or process defects
Environmental	Environmental safety	Environmental degradation	Acid gas emissions
Industrial process	Process control	Process malfunction	Process errors
Source: MSA

Table 2. Exposure data for selected hazardous gases
Chemical and formula	Properties	OSHA PEL (ppm)	IDLH (ppm)	LC₅₀ (ppm)
Ammonia (NH₃)	Corrosive, flammable	50	300	4,000
Boron trifluoride (BF₃)	Toxic	1	25	806
Bromine (Br₂)	Highly toxic, corrosive, oxidizer	0.1	3	113
Carbon monoxide (CO)	flammable	50	1,200	3,760
Carbon dioxide (CO₂)		5,000	40,000	Not available
Chlorine (Cl₂)	Toxic, corrosive, oxidizer	1	10	293
Chlorine dioxide (ClO₂)	Toxic, oxidizer	0.1	5	250
Ethylene oxide (C₂H₄O)	Flammable	1	800	4,350
Hydrogen chloride (HCl)	corrosive	5	50	2,810
Hydrogen sulfide (H₂S)	Toxic, flammable	20	100	712
Methyl isocyanate (CH₃NCO)	Highly toxic, flammable	0.02	3	22
Nitrogen dioxide (NO₂)	Highly toxic, oxidizer	5	20	115
Phosphine (PH₃)	Highly toxic, pyrophoric	0.3	50	20
Sulfur dioxide (SO₂)	Corrosive	5	100	2,520

Table 3. Physiological effects of oxygen deficiency by degree
Concentration of O₂ in atmosphere, vol. %	Physiological effect
19.5 to 16	No visible effect
16 to 12	Increased breathing rate; accelerated heartbeat; Impaired attention, thinking and coordination
14 to 10	Faulty judgment and poor muscular coordination; Muscular exertion, causing rapid fatigue; Intermittent respiration
10 to 6	Nausea and vomiting; Inability to perform vigorous movement or loss of the ability to move; Unconsciousness, followed by death
Below 6	Difficulty breathing; convulsive movements; death in minutes
Source: MSA

Combustible atmospheres

Vapor and gas. Although these two terms are sometimes used interchangeably, they are not identical. Vapor refers to a substance that, though present in the gaseous phase, generally exists as a liquid or solid at ambient temperatures. Gas refers to a substance that generally exists as a gas at room temperature.
Vapor pressure and boiling point. Vapor pressure can be defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed or solid form. Vapor pressure is directly related to temperature, and along with boiling point, determines how much of a chemical is likely to become airborne. Substances with low vapor pressures generally present less of a hazard because there are fewer molecules of the substance to ignite, but they may require higher-sensitivity instrumentation to detect.
Vapor density. Vapor density is the weight ratio of a volume of vapor compared to an equal volume of air. Most flammable vapors are heavier than air, so they may settle in low areas.
Explosive limits. To produce a flame, a sufficient amount of gas or vapor must exist. But too much gas can displace the oxygen in an area, making it unable to support combustion. Therefore, there is a window of concentrations for flammable gas concentrations where combustion can occur. The lower explosive limit (LEL) indicates the lowest quantity of gas required for combustion, while the upper explosive limit (UEL) indicates the maximum quantity of gas (Table 4). Gas LELs and UELs can be found in NFPA 325. LELs are typically 1.4 to 5 vol.%. As temperature increases, less energy is required to ignite a fire and the percent gas by volume required to reach the LEL decreases, increasing the hazard. An environment with enriched oxygen levels raises the UEL of a gas, and the rate of flame propagation. Mixtures of multiple gases add complexity, so their exact LEL must be determined by testing.

References

1. U.S. Dept. of Labor, Occupational Safety & Health Administration (OSHA), 29 CFR 1910.1000 Table Z-1.

2. U.S. Centers for Disease Control and Prevention. National Institute for Occupational Safety and Health (NIOSH). NIOSH Pocket Guide to Chemical Hazards. www.cdc.gov/niosh/npg. Accessed March 2013.