Compressed gases can have one or more hazardous properties. Researchers should consult the appropriate SDS for detailed information on the active chemical ingredients and composition in the gas cylinder, and associated physical and health hazards. Specific handling and storage information is outlined in section 7 of the SDS. Also, gas specific SDSs describe appropriate exposure controls and personal protective measures in section 8 of SDS.
High Pressure Hazards
All compressed gases are hazardous because of the high pressures inside the cylinders. Even at a relatively low pressure, gas can flow rapidly from an open or leaking cylinder. Damaged cylinders can become rockets/missiles or pinwheels resulting in severe injury and property damage. An unsecured or uncapped cylinder can become a cause of a major accident in the lab. If an unsecured cylinder is knocked over and the cylinder valve breaks, the gas can escape at a velocity of a missile. A poorly controlled release of a compressed gas in chemical reactors can also cause vessels to burst, create leaks in equipment or piping, or produce run-away reactions.
The pressure of the gas or gas mixture in the cylinder is indicated on the pressure gauge both in psi and kPa. Researchers should ensure not to empty the cylinder completely, but maintain at least 25 psi residual pressure in the tank.
Hazards of Refrigerated Liquefied Gases - Cryogenic Temperature
The DOT requires the Division 2.2 label for nonflammable and nonpoisonous refrigerated liquefied gases such as liquid helium, liquid nitrogen and liquid argon.
- Individuals should be properly trained in the safe handling of refrigerated liquefied gases such as liquid helium, liquid nitrogen and liquid argon.
- It is strongly recommended that the first time users in the lab should be adequately trained in the transfer of liquefied gases from one container to another under the direct supervision and instruction of a senior staff experienced in the handling of liquefied gases.
- Thermal gloves should be used to protect against cryogenic liquid temperature and the gloves should be loose enough so they can be quickly removed during an emergency to prevent tissue damage.
Even brief contact with a cryogenic liquid is capable of causing tissue damage and blisters similar to thermal burns. Prolonged contact may result in blood clots and serious personal injury. Also, surfaces cooled by cryogenic liquids can cause severe damage to the skin. Solvents that are cooled using dry ice (isopropanol) or dry ice-acetone mixtures can easily reach -70° C or lower, require the use of proper thermal insulated gloves to protect against cold frost and skin blisters.
It is important to work in a well-ventilated area when transferring liquid nitrogen to cool the vacuum trap because oxygen from the ambient air can condense in the cryogenic trap and lead to a potentially explosive condition. Similarly, do not use liquid nitrogen or liquid air to cool a flammable liquid or solvent mixture in the presence of air because oxygen from the air can condense in the flask and lead to a potentially explosive condition.
Small amounts of refrigerated liquefied gases can evaporate into large volumes of gas. For example, one liter of liquid nitrogen can vaporize and expand to 695 liters of nitrogen gas at ambient temperature and pressure. Even if the refrigerated liquefied gas is not toxic, it will displace the air in the storage area. Oxygen deficiency is a serious health hazard in enclosed or confined spaces. As a result of oxygen deficiency, asphyxiation can occur.
Liquid Dewars, cylinders and other vessels used for the storage and handling of refrigerated liquefied gases should not be filled to more than 80% capacity, to prevent the possibility of thermal expansion and bursting of the vessel. Use appropriate impact-resistant containers that are designed to withstand extremely low temperatures.
The DOT requires a red flammable gas pictogram and a Division 2.1 gas label. Flammable gases include acetylene (dissolved gas), butane, ethylene, hydrogen, methylamine, and vinyl chloride.
Each flammable gas has a specific flammable range. For a gas to be flammable its concentration in air should be between its lower flammable limit (LFL) and its upper flammable limit (UFL). For example, hydrogen’s LFL in air at atmospheric pressure and temperature is 4% and its UFL is 75%. This means that hydrogen can be ignited when its concentration in air is between 4% and 75%. However, the flammable range of any gas is widened in the presence of oxidizing gases such as oxygen or chlorine and by higher temperatures and pressures.
For a flammable gas within its flammable range in air or an oxidizing agent to ignite, an ignition source usually needs to be present. Ignition sources can include such things as open flames, sparks, and hot surfaces. However, some gases can also auto-ignite without any obvious ignition sources and others can ignite spontaneously in air – always review the SDS for proper storage and handling!
Flashback can occur with flammable compressed gases that are heavier than air. If a cylinder leaks in a poorly ventilated area, such gases can settle and collect in sewers, pits, trenches, or low areas in workspaces. The gas trail can spread very far from the cylinder and if it contacts an ignition source, the fire produced can flash back to the cylinder.
Oxidizing gases are labeled by the DOT as Division 2.2 and Division 5.1 (Oxidizer). Oxidizing gases include any gases containing oxygen at higher than atmospheric concentrations (23-25%), nitrogen oxides, and halogen gases such as chlorine and fluorine.
Fires in atmospheres enriched with oxidizing gases are very hard to extinguish and can spread rapidly.
Never use oxygen in place of compressed air or nitrogen to purge gas lines
The flammable gases can react rapidly and violently with combustible materials resulting in fire or explosion. Such combustible materials include oils, greases, plastics, fabrics; finely divided metals, hydrazine, hydrogen, hydrides, sulfur compounds, silicon, and ammonia or ammonium compounds.
Researchers should receive special cleaning instructions from the specific gas supplier (such as Praxair) for any equipment that uses oxidizing gases. Gaskets should be made of noncombustible materials. No part of the cylinder or fittings should be handled with bare hands contaminated with grease or oil. Similarly, rags and gloves that are contaminated with grease or oil should be kept away from oxidizing gas operations. Use only lubricants and connection or joint sealants recommended by the gas cylinder manufacturer or supplier.
Some vigorously reactive gases include acetylene, 1,3-butadiene, vinyl chloride, and vinyl methyl ether.
Some high purity compressed gases are chemically unstable. Vinyl gases, if exposed to elevated temperature, pressure, or mechanical shock, can undergo polymerization or decomposition. Uncontrolled polymeric reactions can become violent, resulting in fire or explosion. Some reactive gases are mixed with polymerization inhibitors to prevent the hazardous reactions.
Corrosive gases are labeled by the DOT as Division 2.3 and Division 8 (Corrosive). Corrosive gases in this group include ammonia, hydrogen chloride, chlorine, and methylamine. Corrosive compressed gases can burn and destroy body tissues on contact. Corrosive gases can also attack and corrode metals.
Toxic (Poison) Hazards
Toxic gases have the potential to cause adverse health effects depending on the specific gas, its concentration, the length of exposure, and the route of exposure (inhalation, eye or skin contact).
- The fume hood sash should be lowered below the breathing zone of researchers. Researchers should wash their hands with soap and water immediately after handling toxic compressed gases.
- DOT requires THE WHITE POISON LABEL or the label identification shows Division 2.3 because toxic gases under this division are known to be toxic to humans.
DOT Classification of Toxicity of Gases
The toxic gases are labeled by the DOT as Division 6.1A, 6.1B, 6.1C. Gases having inhalation LC50 (lethal concentration, 50%) up to 2500 ppmv (parts per million by volume) are classified as acutely toxic gases but with differing severity such as A, B, or C as shown below.
Toxic gases in research laboratories include arsine, boron trifluoride, chlorine, carbon monoxide, cyanogen, fluorine, hydrogen cyanide, hydrogen fluoride, hydrogen sulfide, phosphine, and phosgene. DOT Divisions for specific hazardous gases are summarized below.
NFPA Classification of Toxicity of Gases
Gases are described by NFPA as ACUTELY TOXIC if:
(1) NFPA health rating is 4 and inhalation LC50 is zero to <1000 ppmv (e.g., arsine, hydrogen selenide, and stibine)
(2) Or NFPA health rating is 3 and inhalation LC50 is ? 1000 ppmv, but < 3000 ppmv (e.g., boron trifluoride, and hydrogen fluoride)
Gases are described by NFPA as TOXIC if NFPA health rating is 2 and inhalation LC50 is greater than 3000 ppmv and up to 5000 ppmv
For accurate NFPA hazard rating of pure gases and mixtures, researchers are highly encouraged to review the product specific SDSs supplied by the gas supplier.
The GHS Classification of Toxicity Health Hazard for Gases
The GHS classifies the compressed gases by categorizing them as shown below. Detailed GHS classification of general and specialized gases used in research laboratories are summarized in Appendix A. This is not an exhaustive list.
Special Procedures for Hazardous Gases
It is highly recommended that appropriate engineering controls, neutralizing traps or charcoal based gas adsorbers be installed for toxic gases in consultation with the toxic gas suppler prior to purchase and use in research labs.
All toxic gases, including acutely toxic gases, should be used in a functioning fume hood or other vented engineering controls, or vented gas cabinet with appropriate scrubber or gas absorber, and exhausted outside through the fume hood duct work. Larger sized reactive gas cylinders such as sulfur dioxide, diborane, silicon tetrafluoride, should be stored in a vented gas cabinet with appropriate gas adsorbing scrubbers as recommended by the gas manufacturers and exhausted outside through the fume hood duct work. Certain charcoal based gas adsorbers for volatile gases can be purchased through Thermo Fisher, Sigma-Aldrich or Matheson TRIGAS companies
Laboratory experiments with low flow reactive gases can be safely controlled and neutralized using a simple scrubber train and exhausted to fume hood duct work as shown below.
Simple Asphyxiant (Inert Gases) Hazards
Typical examples of inert gases include argon, helium, neon and nitrogen and are labeled by the DOT as Division 2.2.
Inert gases are nonflammable, not oxidizing and nontoxic. However, inert gases can cause severe health injury or death at high concentrations by displacing oxygen in the air. If the oxygen level falls too low, individuals in the area or entering the area can lose consciousness or die from asphyxiation. Always use inert gases in well-ventilated areas.