Aerosols injected into the stratosphere by large volcanic eruptions may induce ozone destruction through processes including heterogeneous chemical reactions. The effect of ozone reductions on surface ultraviolet irradiation is not obvious, however, because aerosols also increase the reflection of sunlight. Here we use a radiative transfer model to estimate the changes in biologically effective ultraviolet radiation (UV-BE) at the Earth's surface produced by the El Chichon (1982) and Mount Pinatubo (1991) eruptions. We find that in both cases surface UV-BE intensity can increase because the effect of ozone depletion outweighs the increased scattering. Español