With modern explosives (ANFO and/or emulsion [or various mixtures of the two]), the amount of toxic gases or fumes produced from blasting varies according to the explosive mix, the degree of confinement in the hole, the amount of water in the hole and the sleep time of the explosive, among other factors.
It is always difficult to predict how much blasting fumes will be produced, and also how long it will take for the fumes to “clear”.
CO, CO2, NO2 and NO are common gases that need to be monitored however there could also be potential for SO2 and H2S when explosives are used in Sulphide-containing rocks. NH3 could also be a factor if lime is present due to cement.
Many Australian Mines now use dedicated, trained re-entry crews to check blast affected areas for fumes. These crews are usually either development charge-up or production blasting personnel and usually operate as “pairs” and operate to strict procedures. They usually have two persons per crew for safety, although often using only one gas monitor providing it is maintained, operated and calibrated strictly in accordance with the manufacturer’s requirements.