desired temperature which is a function of the chemical composition and method of preparation of the pellet in one embodiment the bushing 150 is part of the ignition unit and couples to the canister the bushing 150 includes the primer and ignition mix and may be shipped separately from the canister and assembled when ready to use in one embodiment the pellet 143 is formed of a composition comprising potassium nitrate 67 72 dicyandiamide 9 16 phenolformaldehyde resin 8 12 and potassium benzoate bicarbonate or hexacyanoferrate 4 12 in various percentages by mass as indicated in parentheses various other compositions may be used some of which are described in u s pat nos 6 042 664 and 6 264 772 the size of the canister may be varied significantly to provide different amounts of aerosol producing material in one embodiment the mating threaded portions where the canister and ignition unit attach are the same size for the various sizes of canisters thus a canister designed for inside a cabinet may be fairly small such as smaller than a can of soda canisters designed for larger applications such as retarding fires in a room may be very large all the canisters may use the same size ignition unit provided they are designed to attach to each other through the use of mating threaded portions or other physical coupling mechanisms a top view of the ignition unit 115 is shown in fig2 at 200 several grooves may be cut into the top portion of the ignition unit as indicated at 205 to reduce the amount of material in the ignition unit 115 and thereby increase the responsiveness of the ignition unit to temperature changes fig2 also better illustrates a slot 210 for retaining clip 177.