Most gases present one or more hazards, at least in pure form. They may for example be suffocating, flammable, oxidizing, or poisonous. Hazard diamonds (also called ADR symbols) or orange square shaped symbols ( also called EC symbols) are used to identify the hazards on each product.

Suffocating, Neutral or Inert Gases
These gases (e.g. argon, helium or nitrogen) do not support combustion and are not toxic. However, they represent a hazard of asphyxiation through the reduction of the air's oxygen content. There is a hazard when the level of oxygen in the air falls below 19%.

Precautionary measures:
Ventilation and measurement of oxygen concentration in the air. Never enter a potentially dangerous area without a stand-alone respirator or without first checking the oxygen level. Mark potentially dangerous areas with risk of asphyxiation. Inert gases cannot be detected by the human senses and they will be inhaled just like air. Never enter any tank, pit or other confined area where an inert gas is present without first checking the oxygen content.

Flammable and Combustible Gases, Fuels
These gases (e.g. hydrocarbons, hydrogen or carbon monoxide) burn in air or any other oxidizing gas, and when mixed in some ratio with air create an explosive atmosphere. They can be at high pressure, toxic and reactive, and can displace breathing oxygen from the air. Each flammable gas has a lower flammability limit and an upper flammability limit between which it burns in air. Any concentration of gas mixture between these limits can cause fire to spread. Close to these limits, fire usually spreads at low speed (deflagration). Between these limits there is an interval in which the spreading speed is very high (detonation with significant impact). A change in temperature, pressure or oxidant concentration may vary the flammablility range considerably. Only a small amount of energy is needed to start a reaction; this amount depends on the gas.

Precautionary measures:
All possible sources of ignition must be eliminated through proper design of facilities and restrictions on smoking and naked flames. It is important to have (and know how to use) a fire extinguisher in areas where flammable gases are used and stored, together with a flammable gas detector that can determine when such an area is created.

Oxidising or Combustive Gases
These gases activate combustion. Oxygen is the most common, but others include nitrous oxide, nitrogen dioxide, and nitric oxide. They do not burn but are essential to support combustion. They can be at high pressure, toxic and reactive, and can displace breathing oxygen from the air (except 02 itself).

Precautionary measures:
Eliminate all risks of over-oxygenation of working atmospheres: the maximum oxygen concentration is 25%. Above this limit combustion occurs rapidly. Exposed persons should not smoke or wear clothes made of combustible textiles. To prevent the risk of spontaneous fire, do not allow fat, oil or other combustible materials to come into contact with oxidizing gases (skimming equipment, especially at high pressure). When liquid oxygen is spilled or vented, a white cloud of gas results. Standing in or near this cloud will saturate clothing with oxygen, making it extremely flammable.

Poisonous or Toxic Gases
These gases (e.g. arsine, phosgene, hydrogen phosphide, nitric oxide, nitrogen dioxide) present serious potential hazards to personnel and therefore require special handling. Toxicity is defined by an average value of exposure (occupational exposure limit), which is the average admissible concentration to which a worker may be exposed over a defined period (generally 8 hours). These gases can also be flammable.

Precautionary measures:
As a general rule, avoid all exposure and check concentration levels if necessary.

Corrosive Gases
These gases (e.g. hydrogen chloride, fluorine, nitric oxide, or sulphur dioxide) attack as a result of a chemical reaction with many products: metals, clothes, etc. Some corrosive gases damage human tissue and burn the skin. This burning may not visible immediately. In most cases, corrosive gases are toxic.

Precautionary measures:
Wear full protective clothing and wash abundantly with water in the event of splashing. Persons who are accidentally exposed to such gases should receive prompt attention from a physician. It is important to ensure that not only the Material Safety Datasheet is followed but that storage areas also comply with current guidelines. This includes appropriate segregation of products by their hazard type. It is also possible to mitigate hazards by consideration of the application and the gas selection. A classic example of hazard mitigation is the use of ethylene for fruit ripening. Use of pure ethylene creates a significant flammability hazard. However for the application only a few hundred ppm of ethylene is required. By supplying the ethylene at a maximum of 5 percent concentration in nitrogen, the product no longer represents a flammability hazard, because as the nitrogen/ethylene mixture dilutes into air then it can never enter the flammability zone.