The safe usual concentration of oxygen in the air is approximately 20.9% If a workplace doesn’t have adequate ventilation the level can fall very quickly due to combustion, dilution form other gases or simply from breathing. Oxygen detectors have two alarms, a low oxygen level in which an alarm will sound when levels drop below 18 or 19% as levels below 17% can begin to effect the behaviour of a person. Levels as low as 10-13% can cause unconsciousness extremely quickly thus it is very important to check levels of oxygen BEFORE entering a confined space (confined gas detection kits are available which consist of a pump that is used to sample the air of a space before it is entered).
The second alarm is for when the oxygen levels get too high; usually 24%. When oxygen levels are high things become much more combustible so items such as clothes which may normally smoulder would just burst into flames.
Burning is a simple chemical reaction by which oxygen from the atmosphere combines with a substance to produce heat. Most organic compounds will burn; the simplest of organic compounds are the hydrocarbons. These compounds are made up of carbon and hydrogen, e.g. Methane CH4.
Other hydrocarbons include:
The physical properties of the compounds change with a changing number of carbon molecules in the chain, as the carbon chain increases the compound becomes more viscous and solid.
When hydrocarbons react with oxygen in the atmosphere they burn to produce Carbon Dioxide and water (if incomplete combustion occurs due to low levels of oxygen carbon monoxide will be produced which is highly toxic).
More complex hydrocarbons contain elements such as oxygen, bromine, nitrogen, sulphur & chlorine. If these burn not only will they produce CO2 and water but other compounds may also occur which may be harmful, e.g. Sulphur containing compounds will produce sulphur Dioxide which can be the cause of acid rain.
There is an explosion risk in most industrial environments due to the presence of flammable gasses and vapours, thus it is important when buying gas detectors that you buy something that is ATEX approved and will therefore not put you at risk of explosion.