A humidifier (10) for use with tobacco products comprises a gel body (12) comprising a continuous porous network of particles supporting a volume of moisturising liquid, the gel body configured to release the liquid by evaporation at a predetermined rate, and a permeable layer (14) enclosing the gel body through which the evaporated liquid may pass. The permeable layer comprises a semi-permeable one-way membrane (18) that transmits evaporated liquid from the gel body but substantially blocks the passage of water vapour or liquid to the gel body. Examples of useful gels include alginates and pectins. Additional modifying substances such as salts can also be included for further tailoring of the evaporation rate.
