Sometimes all it takes is a spark: In potentially explosive atmospheres, a mixture of air and flammable substances creates a high risk of explosion. Equipment used in such areas must therefore be specifically tested and certified. Respiratory protection devices, for example, which do not represent a potential source of ignition, are marked with an EX symbol. However, detonation is not the only hazard in such areas: Inhalation of explosive hazardous substances can also cause damage to health or death. Care must therefore be taken when using respiratory equipment with an EX mark: The fact that the equipment is not a potential source of ignition does not mean that it will provide adequate protection against the inhalation of hazardous substances in all Ex zones. This applies in particular to powered air-purifying respirators. Although these often receive ATEX certification, they have lower operational limits than other respiratory protective equipment.
Explosion limits for gas mixtures
Two conditions must be met for the atmosphere to ignite in a given area: First, there must be a sufficiently high concentration of the explosive substance in the surrounding atmosphere. These can be gases, vapours and/or dusts. Secondly, there must be enough oxygen to ignite the mixture
Experts refer to this as the explosion limit of the mixture. Below the lower explosion limit (LEL), the mixture is too lean. Put simply, there are too few flammable substances in the air for the entire surrounding atmosphere to ignite. Conversely, a mixture can also be too rich: Above the upper explosion limit, the mixture is so enriched that an explosion cannot occur because there is not enough oxygen.
These values are usually given in volume percent, i.e. the proportion of a substance in the volume of a mixture. These values can be converted for gases into the more common unit of measurement ppm or parts per million (0.1% by volume = 1,000 ppm). A problem quickly becomes apparent: The lower explosion limit for almost all explosive hazardous substances is many times higher than the maximum permissible operating limit for filtering respirators. If the concentration of a hazardous substance in the ambient air is so high that it can ignite, respiratory protective devices can usually no longer filter it out safely.[1]
Example calculation for powered air-purifying respirator and paraffin
The problem described above can be easily illustrated with an example using paraffin, a highly flammable substance found in many industries, such as petrochemicals and aviation.The lower explosion limit of paraffin is 6,000 ppm, which is well above the maximum permissible limit for filter units with Class 1 (up to 1,000 ppm) or Class 2 (up to 5,000 ppm) respiratory filters. In the case of a fan filter unit, the difference is even greater. Depending on the filter class, the LEL even exceeds the maximum application limit here by a factor of 6 to 12. The capacity of the filters is exceeded in such an explosive atmosphere long before the LEL is reached, so that protection for the user is no longer guaranteed.[1]
This calculation leads to the same result for almost all hazardous substances: The lower explosion limit is usually well above the maximum operating limit for filtering respirators. Also, the alarm thresholds of gas detectors, which are usually set to values between 10% and 40% of the LELs when monitoring explosive gases, are already far above the max. application limits for filtering respirators in most cases and warn the user much too late.[2]
Flammable gases | LEL in Vol.-% | LEL in ppm |
Carbon monoxide | 10,9 | 109.000 |
Methane | 4,4 | 44.000 |
Hydrogen | 4 | 40.000 |
Ethyne | 2,5 | 25.000 |
Propane | 17.000 | 17.000 |
