Accidental fires create emissions of many different toxic substances. Most important for acute toxicity is carbon monoxide (CO). About 95 % of fatal incidents are generated exclusively by CO. Many chronically toxic substances are created as well such as dioxins, polycyclic aromatic hydrocarbons (PAH) etc. These are mainly adsorbed onto soot and their bioavailability is much reduced due to this. Nevertheless they could constittute a danger and are occasionally responsible for hisher cleaning cost. In this work different substances (clioxins, PAH) are compaed regarding their carcinogenic potential. Unit risk factors for inhalative risk are used together with concentrations measured in soot from different fires. Dioxins are about 20 times more ＇＇carcinogenic＇＇ than PAH; but PAH occur at 10 000 times higher concentrations. Altogether the carcinogenic potential of soot from accidental fires is some 500 times higher because of PAH compared with dioxins. PAH can be found in similar amounts in soot from all burning material even in forest fires. This also applies to combustion experiments in tunnels using different automobiles or railway carriages. We also find that the carcinogenic potential of dioxins is not increased much by brominated dioxins or PCB＇s, even ii high amounts of brominated substances or PCB＇s have been burnt (such as in the Dusseldorf Airport Fire, 1996). Other carcinogenic PAH＇s increase the carcinogenic potential of benzo(a)pyrene (BaP) alone by a factor as much as 2. Alltogether, the carcinogenic potential of soot from fires is dominated by PAH and not by dioxins. Additional cost for security measures in cleaning operations is therefore caused predominantly by PAH and not by dioxins.