Flammability and combustion characteristics of breathing filters

Status:

completed 10/1997

Aims:

Breathing filters eliminate noxious gases and vapours by means of specially treated activated carbon which absorbs the hazardous substances. Hot flames or welding spatter acting upon filters of this type may ignite the filter carbon. Should this happen, potentially fatally high concentrations of combustion gases (carbon monoxide, carbon dioxide) are produced very quickly. Studies into the flammability and combustion characteristics of breathing filters were prompted by an accident which occurred under operational conditions. The project was initated by the expert commitee personal protective equipment.

Activities/Methods:

The possibility of gas and combined filters igniting was studied under simulated breathing in consideration of the sources of ignition most commonly encountered in practice (propane and welding torch flames, welding spatter, grinding sparks, cigarettes). Both filters for powered respirators (70 to 100 l/min) and filters for use with non-powered respirators (30 l/min) were included in the study.

Results:

Activated carbon gas/combined filters may be ignited during welding work by propane and welding torch flames or by welding spatter. The nature and duration of action of the source of ignition, the design of the filter, the impregnation of the activated carbon and the breathing mode are of decisive significance. Under conditions of continuous flow carbon monoxide concentrations of 10 to 30% (v/v) and carbon dioxide concentrations of 10 to 15% (v/v) arise within only 90 to 180 seconds of the filter igniting. Respiratory gas temperatures of up to 200 °C are reached after some 150 seconds. Particularly critical is the fact that the increase in breathing gas temperature lags the increase in carbon monoxide, which is odourless and, in these concentrations, highly toxic; the wearer of the device does not therefore receive a timely warning. Under identical test conditions, gas filters are more easily ignited than combined filters. Filters in powered respirators ignite more easily than those in non-powered respirators. Owing to the results of the study, the "Respiratory Protection" working group of the "Personal Protective Equipment" committee of experts issued the following recommendation: during work involving exposed flames or activities in which welding spatter may occur, particular attention must be paid to the potential hazard posed by ignition of the filters when breathing filter apparatus is employed, in particular apparatus in which the gas or combined filters are not situated directly at the respiratory interface.

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