A confined space or confined enclosure is one that has reduced entrance openings, unfavorable natural ventilation and is not designed to remain inside.

Therefore, it may present an unbreathable atmosphere and harbor toxic or flammable gases, vapors or particles.

An example would be a closed tank, with a deficient oxygen concentration, which is accessed for maintenance.[1]​

[2]​

[3]​.

Although the precise definition may vary depending on local regulations or the sector, it is generally considered that:.

Manholes, sewers, service galleries, manholes, ship holds, cisterns, wells, septic tanks, underground transformer rooms, tanks, reactors, boilers, furnaces, ducts, transport tanks, silos and wind turbine blades are examples of confined spaces.

Although the term is usually associated with closed spaces, we should not forget those partially or totally open areas, in which the same risks are identified, such as waste ponds, slurry, WWTP sludge, landfills..., which have atmospheres deficient in oxygen and with gases produced by anaerobic reactions: methane, carbon dioxide, carbon monoxide, hydrogen, hydrogen sulfide, ammonia, benzene...[3]​.

In addition to their geometric characteristics (open or closed), confined spaces are usually classified into three classes and categories based respectively on their risks or work procedures:[4]​.

Accidents in confined spaces are especially relevant in the field of Industrial Safety given the exposure to very serious risks on the part of both the injured party and the rescue team.[1]​

In the industrial sector, the second most frequent form of fatal accidents is drowning and burial (12.4% of accidents in Spain in 2013).[5]​.

NIOSH (The National Institute for Occupational Safety and Health) estimates that two-thirds of fatal accidents in confined spaces are due to hazardous atmospheres, which in 70% of cases were present before work began. Furthermore, the fact that many accidents occur during the performance of unscheduled or unforeseen work (e.g. troubleshooting, emergency care...), which usually accompanies insufficient planning (work permits, instructions, qualification, etc.), significantly aggravates the risks inherent to a confined space.[4]​.