Earlier work on the ignition of flammable atmospheres due to loose agglomerates of fine fibres being irradiated by a CW carbon dioxide laser beam is reviewed. An assessment of which fuels are likely to present the greatest hazard as regards the use of optical sensing in flammable atmospheres leads to the development of new apparatus. Results for diethyl ether, carbon disulphide and hydrogen in stoichiometric mixtures with air are presented in the form of minimum igniting radiation flux as a function of the time taken to ignite. Carbon disulphide proves particularly hazardous, leading to ignition at radiation fluxes as low as 22 kW m-2. It is shown that the hazard cannot be correlated simply with any of the obvious combustion parameters, though the correlation with autoignition temperature is much poorer than that with the critical minimum ignition energy of the mixtures. Lean hydrogen mixtures manifest incandescence due to local reaction under conditions when the flame does not spread to the rest of the volume.