The colour of the smoke is often considered as well, but there is no connection between the colour of the smoke and the risk of flashovers. Traditionally, black, dense smoke was considered particularly dangerous, but history shows this to be an unreliable indicator. For example, there was a fire in a rubber mattress factory in London in 1975 which produced white smoke. The white smoke was not considered dangerous, so firefighters decided to ventilate, which caused a smoke explosion and killed two firefighters. The white smoke from the pyrolysis of the rubber turned out to be extremely flammable.