Humidity, temperature and membrane prestretch influence the dielectric breakdown strength of elastomer membranes. These three factors thus also influence the maximum force and strain that dielectric elastomer actuators (DEAs) can generate, as the Maxwell pressure is proportional to the applied voltage squared, and limited by the breakdown field. We compared several commercial silicones, 10 to 25 mu m thick under different equibiaxial prestretch conditions, for temperatures between 20 degrees C and 80 degrees C and relative humidity from 10% to 90%, measuring both breakdown fields and mechanical properties in order to compute two figures of merit for DEAs. The silicone films all have breakdown strengths of order 100 V mu m(-1). Higher humidity (90% RH) leads to decreased dielectric breakdown strengths compared to lower humidity (10% RH): up to 43% reduction for prestretches of 1.3 and 1.5, but only 2% to 10% reduction for prestretch of 1.1. Higher prestretch leads to up to 50% higher breakdown field, but also leads to higher effective Young's modulus due to strain stiffening. Higher temperatures (80 degrees C) lead to up to 30% lower breakdown voltages compared to lower temperatures (20 degrees C). Higher prestretch generally enhances the maximum Maxwell stress because of higher dielectric breakdown strength, but reduces the strain figure of merit owing to increased Young's moduli at high prestretch. For silicone elastomers, Sylgard 184 shows the highest stress figures of merit and LSR 4305 the highest strain figures of merit. Data for VHB is also presented for comparison. This work allows identifying elastomers better suited to harsh environments, and to selecting operating voltages that enable safe operation for a wide range of environmental conditions.