Abstract
Data from ground-level radiation monitors and cosmogenic nuclides are combined to a give a probability distribution for severe radiation events related to the well quantified event of 23 February 1956. Particle fluxes, single event effects rates and dose rates are calculated for ground-level and aerospace systems. The event of February 1956 would provide a challenge to air safety while more extreme events seen in historical records would challenge safety-critical ground systems. A new space weather hazard scale based on this event could be used to give rapid assessment of the radiation hazard using high latitude neutron monitor data.