The invention provides an apparatus for increasing the size of gas-entrained particles in order to render the gas-entrained particles detectable by a particle detector, the apparatus comprising an evaporation chamber (2) and a condenser (7); the evaporation chamber (2) having an inlet (1) for admitting gas into the evaporation chamber and an outlet through which vapour-laden gas may leave the evaporation chamber; the evaporation chamber (2) having disposed therein a heating element (3) and a porous support (6), the heating element being in direct contact with the porous support, wherein the porous support (6) carries thereon a vaporisable substance and the heating element (3) is heatable to vaporise the vaporisable substance to form vapour within the evaporation chamber (2); the condenser (2) being in fluid communication with the outlet of the evaporation chamber, and the condenser (7) having an outlet for connection to the particle detector. the apparatus being configured so that vapour-laden gas from the evaporation chamber can flow into the condenser and condensation of the vaporisable substance onto gas-entrained particles in the condenser takes place to increase the size of the particles so that they are capable of being detected by a particle detector.