Aerosol precursors are gaseous substances that, as a result of various physical or chemical reactions, may lead to the formation of aerosols... This unit represents levels that are so low that multiples are normally used: GBq (gigabecquerel) or TBq (terabecquerel*.) *Tritium, a hydrogen isotope, has a low level of radioactivity and is produced in the primary circuit coolant of nuclear reactors.