To evaluate the explosion hazard of ITER-relevant dusts, a standard method of 20-1-sphere was used to measure the explosion indices of fine graphite and tungsten dusts and their mixtures. The effect of dust particle size was studied on the maximum overpressures, maximum rates of pressure rise, and lower explosive concentrations of graphite dusts in the range 4 mu m to 45 mu m. The explosion indices of I Pm tungsten dust and its mixtures with 4 mu m graphite dust were measured. The explosibility of these dusts and mixtures were evaluated. The dusts tested were ranked as St1 class. Dust particle size was shown to be very important for explosion properties. The finest graphite dust appeared to have the lowest minimum explosion concentration and be able to explode with 2 U ignition energy. (c) 2005 Elsevier Ltd. All rights reserved.