In some industries, particularly the petroleum or oil/ gas industry, these are intentionally flooded with an inert gas to ensure the vapour space is too low to support combustion. Some areas of the world will carry standards and regulations which cover this work, particularly the USA, and the UK/ Europe, and many other countries have adopted these best practices which have been derived through experience in the workplace.

Normal minimum levels for safe entry into confined spaces are 19.5% Oxygen and less than 10% Lower Explosive Limit of the flammable gas present, although these may vary slightly depending on region and the site’s specific procedures.

In the case of inerted atmospheres, a space which must be entered, the Lower Explosive Limit should be maximum of 10%, and a maximum of 4% Oxygen to prevent fire or explosion. Due to the low oxygen levels there are special requirements on this work which require additional safe practice. If after entry into this environment the oxygen level
increases to 5% then the workers should be removed from the inerted space. These should support the local regulatory requirements and not conflict.

An inerted confined space should always be classed as requiring Permit to Work procedures, and Immediately Dangerous to Life and Health (IDLH). However due to the lack of oxygen this environment is considerably more dangerous than other confined space which are classed as IDLH.

The loss of respiratory protection in these conditions can result in rapid asphyxiation. Exposure can result in immediate incapacitation of normal consciousness, rendering selfrescue impossible. For this reason, extra redundancy should be built into the respiratory protective system with normally a minimum of three air sources of supply to each user.

Consideration should be given to the area where the gases are exhausted from the inerted confined space, as there is the potential for exposure in these areas of contamination of nearby plant areas.

On refineries, there is the potential if Carbon Monoxide enters the inert gas stream for the CO to combine with nickel from catalyst or stainless steel and to form gaseous Nickel Carbonyl which is extremely toxic, with concentrations of a few ppm for short durations causing severe acute symptoms and more than 30ppm for 30 minutes can be lethal. It creates a ‘damp cellar’ odour which is normally detected at 1-3ppm.