The fume class of an explosive is the measure of the amount of toxic gases produced in the detonation process. Carbon monoxide and oxides of nitrogen are the primary gases that are considered in the fume class ratings. Although most commercial blasting agents are near oxygen-balanced to minimize fumes and optimize energy release, fumes will occur and the blaster should be aware of their production. In underground mining or construction applications, the problems which can result from producing fumes with inadequate ventilation is obvious.
It should be pointed out that in surface operations, especially in deep cuts or trenches, fume production and retention can be hazardous to the personnel on the job. Certain blasting conditions may produce toxic fumes even when the explosive is oxygen balanced. Some conditions which can cause toxic fume production are insufficient charge diameter, inadequate water resistance, inadequate priming and premature loss of confinement.
The Institute of Makers of Explosives have adopted a method of rating fumes. The test is conducted by the Bichel Gauge method. The number of cubic feet of poisonous gases released per 200 grams of explosives is measured. If less than 0.16 cubic feet of toxic fumes are produced per 200 grams of explosives, the fume class rating would be 1. If 0.16 to 0.33 cubic feet of poisonous gases are produced, the fume class rating is 2, and if 0.33 to 0.67 cubic feet of poisonous gases are produced, the fume class rating is 3.
Strictly speaking, carbon dioxide is not a fume since it is not a toxic gas in its own right, however, many deaths have occurred over the years due to the generation of large amounts of carbon dioxide during blasting in confined areas. Although carbon dioxide is not poisonous, it is produced in large quantities in most blasts and it has the effect of causing the involuntary muscles of the body to stop working. In other words, the heart and lungs would stop working if one was placed in high concentrations of carbon dioxide. If concentrations are 18% or higher in volume, death can occur by suffocation. An additional problem with carbon dioxide is that it has a density of 1.53 as compared to air and it would tend to pocket in low places in the excavation or where there is little movement. A simple solution to the problem is to use compressed air to dilute any possible high concentrations in depressions of trenches.
Credits to U.S. Department of Transportation, Federal Highway Administration.
