Safety minimum oxygen concentration

Acetylene is recognized internationally as a simple asphyxiant. The American Conference of Governmental Industrial Hygienists recommends that atmospheres containing acetylene have a minimum oxygen concentration of 18%. The National Institute of Occupational Safety and Health has a recommended exposure limit for acetylene of 2500 ppm as a ceiling. 

Injuries and fatalities from asphyxiation are often associated with personnel entry into inerted equipment or enclosures. Guidance on safe procedures for confined space access are provided by OSHA (OSHA, 29 CFR 1910.146, Confined Space Entry Standard, 2000), the American National Standards Institute (ANSI, Z117.1, Safety Requirements for Confined Spaces, 2003), Hodson (Hodson, Safe Entry into Confined Spaces, Handbook of Chemical Health and Safety, American Chemical Society, 2001), and BP (BP, Hazards of Nitrogen and Catalyst Handling, 2003). OSHA has established 19.5 vol % as the minimum safe oxygen concentration for confined space entry without supplemental oxygen supply (see Table 23-18). Note that OSHA imposes a safe upper limit on 02 concentration of 23.5 vol % to protect against the enhanced flammability hazards associated with 02-enriched atmospheres. 

Training of personnel is particularly important with toxic materials. Some safety precautions and first aid measures may be quite specific. Among the specific precautions is the use of the right gas mask ftat is, one equipped with the right adsorbent cartridge. Masks are acceptable only at toxic gas concentrations within the rating of the mask and at oxygen concentrations above the minimum of 19.5-20%. 

The minimum (lower) and maximum (upper) concentration of vapor or gas in air or oxygen below or above which explosion or propagation of flame does not occur in the presence of a source of ignition. The explosive or flammable limits are usually expressed in terms of percentage by volume of vapor or gas in air. In reality, explosive limits for a material vary since they depend on many factors such as air temperature. Therefore, the values given on a Material Safety Data Sheet (MSDS) are approximate. The difference between the lower and upper flammable (explosive) limits is the range, expressed in terms of percentage by volume of vapor or gas in air. See also Lower Explosive Limit (LEL) Upper Explosive Limit (UEL). 

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